In respect to the Mount Diablo code, the Zodiac Killer gave us two clues to where his bomb was to be set, in the form of two crosshairs on July 26th 1970. The first crosshairs contained a bold, black circle positioned close to 4 radians and magnetic north around its circumference (the Zodiac Killer's crude design was off by about 3 degrees). Next to this circle, the Zodiac Killer placed SFPD=0, indicating the target was a San Francisco Police Department. The second crosshairs were overwritten with the wording "P.S. The Mt. Diablo code concerns Radians + # inches along the radians". The hashtag (recognized as a number sign) was positioned above a pointing arrow, beckoning us to insert a number value before inches. The answer is five inches, because this is the only value that falls over San Francisco from Mount Diablo using the map scale of 6.4 miles to the inch. The bomb was to be set alongside Ingleside Police Station, on an embankment bordering the Southern Freeway. For further explanation in simplified form, please visit here, or for more extensive coverage read the article The Answer to the Mount Diablo Code.

Footnote: Ordinarily radians would be measured counter-clockwise, but the Zodiac Killer gave us crosshairs superimposed over Mount Diablo with an arrow pointer set in the north facing position, "to be set to Mag. N". He allocated it a zero position, with a 3, 6 and 9 in the form of a clock face around its circumference. This was to instruct us to travel clockwise around the circumference of the crosshairs in this instance. Which is why it was no fluke that the bold, black circle fell exactly at 4 radians and magnetic north.

The Blast Estimate of Zodiac (Part Two)

The Zodiac Killer often responded to the newspaper articles written about him, usually in close proximity to their release. The Dripping Pen card had likely been purchased subsequent to the morning of October 25th 1969 in response to the last San Francisco Chronicle article by Paul Avery. If the 340 character cipher had been specifically designed to compliment this card, then it too was crafted subsequent to October 25th 1969. As the title attributed to this communication suggests, this greeting card featured a dripping pen on its front face, accompanied by the wording "Sorry I haven't written, but I just washed my pen". The Avery article read "Zodiac struck last on October 11 when he gunned down cab-driver Paul Stine on Washington Street in Presidio Heights. He revealed himself the killer in a letter sent to the Chronicle three days later. Since then he has remained silent".

Bearing in mind the wording "since then he has remained silent", it could be argued that the chosen card of "Sorry I haven't written" was purposefully selected in response to this article - and therefore chosen by the Zodiac Killer after October 25th 1969. The paragraph in the newspaper article also referred to Washington Street, thereby completing the phrase on the card "but I just washed my pen". The last observation is probably a little far-fetched, and heavily reliant on the Zodiac Killer being able to find a greeting card apologizing for being silent, as well as containing a word related to Washington Street. Extremely unlikely, but not impossible.

Then, the day before the November 9th 1969 'Bus Bomb' letter, came the newspaper article detailing the attempted murder of Richmond school teacher, Daniel Williams. Somebody claiming to be the Zodiac Killer made sadistic phone calls spanning October 23rd 1969 and November 2nd 1969, and culminating in a soft drink in his refrigerator being laced with arsenic. This failed murder plot may have inspired the Zodiac Killer (if the poisoner) to insinuate his continued pursuit of Daniel Williams, by stating "I though you would nead a good laugh before you hear the bad news" and "up to the end of Oct I have killed 7 people". On November 12th 1969, the San Francisco Chronicle released information on the Dripping Pen card and Bus Bomb letter under the banner of I've Killed Seven, The Zodiac Claims.

This newspaper article featured the observations of an individual with respect to the X's on the Bus Bomb crosshairs (see right). This observation was suggested in the article The Promise of Murder on Line 5.

I believe the person who initially made this suggestion was almost certainly correct. However, when I floated this idea on November 23rd 2020, I was fully aware of the Mount Diablo map and crosshairs accompanying the Button letter, which was mailed to the San Francisco Chronicle on June 26th 1970. I positioned the Bus Bomb crosshairs over Mount Diablo and rotated them slightly clockwise, so that a line drawn from the peak of Mount Diablo through four of the five X's passed directly over Lake Berryessa, Vallejo (where two attacks occurred), San Francisco and San Jose.

When we consider that this person made the observation on November 12th 1969, seven-and-a-half months before the Mount Diablo map was mailed by the Zodiac Killer (on June 26th 1970), the interpretation by this individual is extremely intriguing.  

The five cross marks cannot directly pinpoint the five claimed attacks in Benicia, Vallejo, Lake Berryessa, Presidio Heights and San Jose by superimposing them over a map. If you arrange any two X's over Lake Herman Road and Blue Rock Springs, the other three X's will never fall over Lake Berryessa, Presidio Heights and San Jose. The only way it is possible, is by placing the crosshairs over Mount Diablo (rotated clockwise} and drawing lines through the X's on the circumference. But this still leaves the 9 o' clock line unaccounted for. This is where Daniel Williams, the Richmond school teacher comes in. The sadistic phone caller to Daniel Williams may have wanted to target schoolchildren at the Richmond school where he worked (Salesian High School falls over the line drawn through the 9 o' clock position}. This is covered in the article A Killer on Line Five.

The newspaper article stated "Zodiac's letter contained five cross marks which possibly refer to attacks. Comparing these markings against a map, one interpretation that can be drawn is that they pinpoint Lake Berryessa, Vallejo (where two attacks occurred}, San Francisco and San Jose". It was effectively saying that five cross marks could pinpoint four locations (identifying Vallejo as the location where two attacks occurred). Despite classifying Lake Herman Road as Vallejo, the tight grouping of the first two attacks on the map would allow the observer to reach the same conclusion as shown in the above map. After all, this person didn't say "Zodiac's letter contained five cross marks which possibly refer to five attacks".

The Zodiac Killer likely chose Mount Diablo because of its use when surveying land boundaries in Northern California, with the summit used as the "initial point". Was it possible that the person behind the interpretation on November 12th 1969 had correctly identified Mount Diablo as the "initial point" to center the crosshairs over, by using the same reasoning as Zodiac? The Zodiac Killer himself classified Lake Herman Road and Blue Rock Springs under the banner of Vallejo (evident on Bryan Hartnell's car door), so when he provided five cross marks to signify four general locations, he was deliberately leaving the 9 o' clock line as a puzzle to the location of his next attack. He employed exactly the same technique, when substituting the X around the crosshairs with a bold, black circle around the circumference of the crosshairs on July 26th 1970, again designed to pinpoint the location of his next attack. On this occasion, it would be a school bus traveling on the Southern Freeway by Ingleside Police Station, positioned at 4 radians and magnetic north.   

After watching the very educational and insightful presentation of Let's Crack Zodiac (Part Four) by Dave Oranchak for the National Cryptologic Museum, the section about the X's positioned around the Bus Bomb crosshairs sparked a fresh interest. Dave Oranchak is correct when he identifies the difficulty regarding scale when overlaying the crosshairs over a map. The Zodiac Killer overlayed his crosshairs over Mount Diablo as part of the Button letter on June 26th 1970, which he followed up with a further clue in the Little List letter on July 26th 1970. The Zodiac Killer explicitly stated that the crosshairs were to be set to Magnetic North (17 degrees in 1970). Therefore, it wasn't particularly surprising that the center of the black circle around the circumference of the first crosshairs on the Little List letter, fell exactly at 4 radians when set to Magnetic North, ultimately identifying Ingleside Police Station as the focus for the bomb.

Two overlays were used to great effect in the June 26th and July 26th communications. It is with little doubt that the Bus Bomb crosshairs were designed to pinpoint locations as well. However, as Dave Oranchak pointed out, the scaling of such an operation is likely to lead to inaccuracy. Therefore, we have to treat the crosshairs on the Bus Bomb letter as a guide. Only a day earlier, the Zodiac Killer had listed his murder count on the Dripping Pen card, mailed to the San Francisco Chronicle on November 8th 1969. He chronologically listed the murders he was claiming by use of months, writing "Des, July, Aug, Sept, Oct = 7".

We know the Zodiac Killer had murdered five victims at Lake Herman Road, Blue Rock Springs, Lake Berryessa and Presidio Heights, but he added in the murders of Kathy Snoozy and Deborah Furlong in San Jose on August 3rd 1969. To this point, the Zodiac Killer had listed his victims, but always left us dangling with the promise of more murders. Would it be all that unusual if the Zodiac Killer again pinpointed these seven victims the following day by use of X's around the circumference of his crosshairs, just like he pinpointed 4 radians and Magnetic North around the circumference of his crosshairs in the Little List letter on July 26th 1970 - with both crosshairs overlayed over Mount Diablo. But in true Zodiac style, the promise of another location in which he would soon murder. If this were the case, we would expect one X on the circumference of the Bus Bomb crosshairs to be unknown.              

To a reasonable degree of accuracy, line [1] falls over Lake Berryessa and the murder victim Cecelia Shepard. Line [2] passes directly over Lake Herman Road and Blue Rock Springs, accounting for David Faraday, Betty Lou Jensen and Darlene Ferrin. Line [3] passes over Presidio Heights and the murder of Paul Stine. Line [4] passes over San Jose and the murders of Kathy Snoozy & Deborah Furlong. These were the locations of his seven victims which he had claimed just a day earlier. One line remains unknown. This line passes over Walnut Creek (where Elaine Davis was abducted from on December 1st 1969, and eventually murdered), as well as Richmond and San Rafael. Had the Zodiac Killer deliberately created this X on the 9 o'clock position as a teaser to his next destination of murder? Next to the bottom right quadrant of his crosshairs on the Bus Bomb letter, he wrote "Be sure to print the part I marked out on page 3 or I shall do my thing". Considering this section of text was placed next to the crosshairs with the X's, was his "thing" going to occur somewhere in the vicinity of Walnut Creek, Richmond or San Rafael, along the unknown line? As far as we know, no murders can be connected to the Zodiac Killer over the next few months in these locations, making you wonder whether this was just another threat that didn't materialize. Or it was simply just a game he was playing.  

FOLLOW UP ARTICLE: A KILLER ON LINE FIVE  

The first three San Francisco Police officers killed in the line of duty in 1970 were Eric Zelms, murdered when he lost control of his firearm when investigating a pawn shop robbery on January 1st. The second was Brian McDonnell on February 16th 1970, who suffered devastating and sadly fatal injuries seven weeks later, when a bomb packed with fence staples exploded on an outside ledge of Park Police Station in the Upper Haight neighborhood. The third was Richard Radetich on June 19th 1970 at 5:25 am, who was gunned down by three shots from a .38 caliber revolver at point blank range through the driver side window of his vehicle while in the process of serving a parking ticket. He was sat in his police car near 643 Waller Street, San Francisco in the Lower Haight District.

The Zodiac Killer didn't personally name any of them, but referred to them indirectly when he stated "two cops pulled a goof", "I hope you do not think that I was the one who wiped out that blue meannie with a bomb at the cop station" and "I shot a man sitting in a parked car with a .38". Additonally, all three communications carried the message of his bomb-making. 

The April 20th 1970 communication not only threatened that there was "more glory in killing a cop", but provided us with imagery that showed he had already set aside a location for his school bus bomb. The Zodiac Killer had specifically chosen a bus route traveling from north to south with a steep embankment on the right and the sun rising to the east. When deciding where to place his bus bomb, one would think the killer would be drawing from experience with regards to the best location. The diagram of the bus bomb location in the April 20th 1970 letter would eventually be discovered to be the embankment directly opposite Ingleside Police Station and alongside the parking lot of the City College of San Francisco. The addition of two crosshairs on the July 26th 1970 "Little List" letter would expand on the Button letter (a month earlier), and show to a 98% accuracy that the target was the Southern Freeway embankment opposite Ingleside Police Station - which was also a bus route. This would effectively kill two birds with one stone for the Zodiac Killer. Was the Zodiac Killer familiar with the City College of San Francisco, having worked at this location or even studied there?   

The Zodiac Killer certainly wanted to paint a picture he was familiar with law enforcement practices, giving rise to the notion he may have worked at, or attended the City College of San Francisco, where he could possibly have crossed paths with Richard Radetich, who attended a criminology course prior to becoming a police officer with the San Francisco Police Department. Greg Corrales was a legendary San Francisco Police officer who knew Richard Radetich and stated that "Richard, who had recently been assigned to Traffic, regularly stopped at Ingleside Police Station to say hello to his former colleagues. He glowed when he talked about his new baby girl and his wife. He was an outstanding police officer and a devoted family man". It is clear that Richard Radetich took criminology classes at the City College of San Francisco before being assigned to duties at Ingleside Police Station just 850 feet east of the college. Richard Radetich had formerly been a student at Balboa High School, 585 meters to the east of Ingleside Police Station. This aligns Sgt. Richard Radetich at the City College of San Francisco (and likely parking lot) studying criminology, with the bus bomb designated for the eastern edge of the parking lot on an embankment, alongside a bus route traveling north to south (with the sun rising to the east), only 274 feet from Ingleside Police Station (where Radetich had worked) - and subsequently - the Mount Diablo code pinpointing this location. The Mount Diablo code letter would also indirectly reference the murder of Sgt. Richard Radetich on June 19th 1970.

It is highly unlikely that the Zodiac Killer was responsible for the murder of Sgt. Richard Radetich, with the previous observations just a simple case of unfortunate coincidences, but it still leaves pause for thought when you consider the location of the bomb had been earmarked in a letter two months in advance of his murder, on April 20th 1970. A letter that highlighted the bombing of Park Police Station and the murder of Brian McDonnell, with the claim that "It just wouldn't doo to move in on someone else's teritory". So, did the Zodiac Killer move to a territory slightly south of Park Police Station, the clue to which lay in the attached bus bomb diagram?

On October 12th 1970, Paul Avery of the San Francisco Chronicle wrote an article concerning the Zodiac Killer's latest two communications, notably the July 26th 1970 "Mikado" letter and the October 5th 1970 "13 Hole" postcard. The Zodiac Killer may have used much of this newspaper article to choose and fashion the October 27th 1970 Halloween card, including a continuance of The Mikado theme. The Zodiac Killer wasn't averse to plagiarising reading material for his communications, with the Halloween card fashioned not only from the Tim Holt comic book, but possibly crafted using the Paul Avery article as his inspiration.

The October 12th 1970 newspaper article was entitled Gilbert and Sullivan Clue to Zodiac, thereby giving the Bay Area murderer about two weeks to find the appropriate reply in the form of a Halloween card beginning "I feel it in my bones you ache to know my name. And so I'll clue you in". The newspaper article detailed "Sheriff's detective sergeant Kenneth Narlow who had fruitlessly followed up another 900 tips as to Zodiac's identity since Sept 27, 1969". So finding a Halloween card teasing us again with his name and offering to clue us in, appeared like the ideal card in response.

In the first column of the article, Paul Avery wrote "Homicide detectives who've been hunting Zodiac for nearly two years frankly admit he is much an enigma now as he was on Dec. 20, 1968, when he chalked up his first two victims". Those two victims were teenagers David Faraday and Betty Lou Jensen. The Zodiac Killer decided to chalk his victims on the Halloween card in white marker pen above the head of the inner skeleton and beneath its feet, in the form of 4-TEEN and what could be Zodiac Victims Fourteen or Zodiac Fourteen.

The author of the card could have written 14 (similar to the outer skeleton hand) or fourteen, but intentionally separated it to 4-TEEN. David Faraday and Betty Lou Jensen were two of Zodiac's confirmed victims, but he had also claimed Deborah Furlong and Kathy Snoozy by the addition of "Aug" in the Dripping Pen card on November 8th 1969, who were both teenagers. The Zodiac Killer had claimed 4-TEENS as victims, hence why he altered fourteen to 4-TEEN, to incorporate them into his running victim total. The addition of four dots (little marks) around ZVF or ZF possibly created for a similar reason. It can be noted that Paul Avery also referred to David Faraday and Betty Lou Jensen in the article as teenagers, but he hyphenated the word into teen-agers, just like Zodiac did with his running teenager total of 4-TEEN.

The majority of the newspaper article focused on the July 26th 1970 "Mikado" letter and the search for potential Ko-Ko's, The Lord High Executioner featured prominantly in the July 26th 1970 (Little List) letter. The Mikado Act One Part 5a  As some day it may happen was performed by Ko-Ko of the town of Titipu. This led investigators to consider the possibility of a Zodiac Killer who played Ko-Ko in his student days. But where was The Mikado and Ko-Ko in the Halloween card? Perhaps the investigators could have searched the knothole in the tree.

The original card came with one eye peeking from the knothole in the tree, in which the Zodiac Killer circled the words "Peek-a-Boo you are doomed". But notice how "Peek-a-Boo" and "you are doomed" are deliberately separated.

The Mikado is a two-part comic opera by Gilbert and Sullivan, which opened to the paying public on March 14th 1885 and was hugely successful, running for 672 performances at the Savoy Theatre in London. However, this wasn't the only version of the opera from that time period, In 1888, Ed J. Smith wrote a stage parody of The Mikado called The Capitalist; or, The City of Fort Worth to encourage capital investment in Fort Worth, Texas. In this version, Peek-A-Boo is listed as one of the three sisters and wards of Kokonut (The Lord High Executionist).

The Mikado original featured Peep-Bo: A ward of Ko-Ko and sister of Yum-Yum and Pitti-Sing. Peep-Bo is the British version of Peek-a-Boo. Peekaboo (also spelled peek-a-boo) is a form of play primarily played with an infant. To play, one player hides their face, pops back into the view of the other, and says Peekaboo!, sometimes followed by I see you! There are many variations: for example, where trees are involved, "Hiding behind that tree!" is sometimes added.

Peep-Bo can also be reversed to Bo-Peep, believed to originate from the 16th century practice of concealment, before reappearing to startle or surprise. See here. Little Bo Peep is a popular English nursery rhyme The earliest record of this rhyme is in a manuscript of around 1805, which contains only the first verse. There are references to a children's game called "Bo-Peep", from the 16th century, including one in Shakespeare's King Lear (Act I Scene iv), but little evidence that the rhyme existed. The additional verses are first recorded in the earliest printed version in a version of Gammer Gurton's Garland or The Nursery Parnassus in 1810.

The phrase "to play bo peep" was in use from the 14th century to refer to the punishment of being stood in a pillory. For example, in 1364, an ale-wife, Alice Causton, was convicted of giving short measure, for which crime she had to "play bo pepe thorowe a pillery". Andrew Boorde uses the same phrase in 1542, "And evyll bakers, the which doth nat make good breade of whete, but wyl myngle other corne with whete, or do nat order and seson hit, gyving good wegght, I would they myghte play bo pepe throwe a pyllery". Taken from Wikipedia. The skeleton on the card inner also appeared to be hanging in the form of punishment, as if placed in a pillory. I suppose the "punishment must fit the crime".

Bearing in mind the newspaper article of Paul Avery, was the Halloween card drawing on the search for Ko-Ko's by its design of inserting Peek-a-Boo around the knothole of the trunk, with the eye "hiding behind that tree"? Was the chalked-up victim count of 4-TEEN above the pilloried skeleton denoting the running total of fourteen, including four teen-agers? The newspaper article of Gilbert and Sullivan Clue to Zodiac may have spawned the choice of Halloween card with the introduction of "I feel it in my bones you ache to know my name. And so I'll clue you in". But did it inspire anything else?

The problem with solving the Zodiac Killer's 32 Symbol cipher and Phillips 66 Map code, is a failure to follow the instructions given by the killer in the June 26th 1970 letter and its follow up on July 26th 1970. The Zodiac Killer stated "The map coupled with this code will tell you where the bomb is set.  You have untill next Fall to dig it up". This tells us that the solution to the 32 symbol cipher will not supply us with the exact position of the supposed bomb. Any solution to this 32 character code pinpointing the exact position of the bomb will be incorrect, because this can only be realized by coupling it with the map. Therefore, the position of the bomb to be set, was a combination of both code and map.

In his following letter he stated "P.S. The Mt. Diablo code concerns Radians + # inches along the radians". This tells you that the code is about radians and inches but not necessarily the entire solution. Any solution to the position of the bomb that doesn't use radians and inches (as implicitly stated by the killer) will be wrong. The phrase "P.S. The Mt. Diablo code concerns Radians + # inches along the radians" would supply the inches value, hence the hashtag before inches, along with the arrow beneath, instructing us to insert a value here. This isn't rocket science - the hashtag is widely recognized as a number sign, so by placing this immediately before the word "inches", the Zodiac Killer is telling us to insert a number before inches. The only way to find a bomb in Northern California using his instructions, is to travel a set amount of radians around his crosshairs, and then subtend a set amount of inches from it. In this case, a number of radians around Mount Diablo and a number of inches from it. Placing a hashtag and arrow before the word "inches", is rather a giveaway that a number value should precede it. Even the map code was 6.4 miles to the inch. Any solution that gives us the answer in degrees or miles is wrong, because "The Mt. Diablo code concerns Radians + # inches along the radians". We need to follow his instructions. This is why any theory of crime scene locations being chosen because they are separated by a radian or near radian value, is wrong also. The whole premise of the June 26th 1970 letter was to discover the location of his bomb, not measure angles between crime scenes or any other object we choose to construct or imagine

The Zodiac Killer gave us crosshairs superimposed over Mount Diablo with an arrow pointer set in the north facing position, "to be set to Mag. N". He allocated it a zero position, with a 3, 6 and 9 in the form of a clock face around its circumference. This was to instruct us to travel clockwise around the circumference of the crosshairs, to be set at 17 degrees (which was the magnetic north value in 1970). We had to travel a set amount of radians around his crosshairs over Mount Diablo, in addition to this value of 17 degrees. So, we are looking for a set amount of inches and a set amount of radians. The Zodiac Killer was incapable of keeping secrets and resisting putting pen to paper, so just one month later he gave us the answer, when the July 26th 1970 letter arrived at the San Francisco Chronicle.          

Never done before or since by the Zodiac Killer, he drew a big fat zero around the circumference of his crosshairs with SFPD alongside it. This represented the San Francisco Police Department. This bold, black circle sat close to 246.2 degrees, which is 4 radians and magnetic north (the Zodiac Killer's crude design was off by about 3 degrees). This was the radian component of "The Mt. Diablo code concerns Radians + # inches along the radians". The answer was 4 radians. All we had to do now was find the "inches" value.

The Zodiac Killer mailed virtually every correspondence to the San Francisco Chronicle, describing the makeup of his bomb on November 9th 1969. He followed this up with a diagram of a school bus and photoelectric cells on April 20th 1970, referencing the murder of San Francisco police officer Brian McDonnell on February 16th 1970, who suffered devastating and sadly fatal injuries two months earlier, when a bomb packed with fence staples exploded on an outside ledge of Park Police Station in the Upper Haight neighborhood of San Francisco. The Zodiac wrote about the "blast" he was going to have when he mailed the Dragon card on April 28th 1970. All of these communications directed towards the San Francisco Chronicle indicating he was constructing a bomb, with the SFPD attribution totaling zero adorning many. Therefore, it's pretty obvious that placing SFPD around the circumference of his crosshairs, next to a big fat zero and set close to a value of 4 radians and magnetic north (4 X 57.3 + 17), indicates that the target for the bomb is San Francisco - or more specifically - the San Francisco Police Department. When the above crosshairs are set over Mount Diablo, we now have to travel 4 radians and magnetic north from the peak of the mountain. Let us now find the inches value to be inserted into "Radians and # inches along the radians".

We actually don't have to look hard to find the answer. The land mass and geography of San Francisco is such, that only 5 inches can fall over it. Using the map scale of 6.4 miles to the inch, anything less than 5 inches, and anything more than 5 inches, doesn't fall inside San Francisco. Therefore, there is only one answer. The same applies to the radians value. Anything more or less than 4 radians and magnetic north (such as 5 radians and magnetic north, or 3 radians and magnetic north), doesn't fall inside the geography of San Francisco. A bomb threat on San Francisco (hence SFPD=0), can only be 4 radians and magnetic north, and 5 inches along that radian value. The choice of words by the Zodiac Killer when he wrote "Radians and # inches along the radians". were chosen carefully. We were instructed to find "radians", and an amount of "inches" along the amount of "radians" we found. Once we found the radians value, we would travel 5 inches along that radian. The answer to the map was the magnetic north value of 17 degrees, to be coupled with four radians and five inches of the code. The two "coupled" together gave us the full answer. Druzer, an avid Zodiac researcher, came up with a solution to the 32 character code which I believe is correct, when he suggested Estimate: four radians and five inches. This solution not only satisfies the 32 characters, but correlates three plaintext letters to the only three repeating ciphertext characters in the code, thereby providing a perfectly valid key. See The Blast Estimate of Zodiac, The Blast Estimate of Zodiac (Part Two). and The Blast Estimate of Zodiac (Part Three).

The map scale is 6.4 miles to the inch. This gives us a distance of 6.4 multiplied by 5 (inches). Therefore, the bomb was to be set close to 4 radians and magnetic north, and 32 miles from the peak of Mount Diablo. The only San Francisco Police Department that comes anywhere close is Ingleside Police Station. This San Francisco Police Department sits at 30.96 miles from the peak of Mount Diablo (4.84 inches). It also sits at 251 degrees from the peak of Mount Diablo (4.08 radians).

The Zodiac Killer gave us a rough guide of 4 radians and magnetic north, and 5 inches (hence the word "estimate"). If he had given us the exact measurement, it would have been 4.08 radians and 4.84 inches, He was clearly giving us an approximate measurement. Since Ingleside Police Station was the only San Francisco Police Department anywhere close to these measurements, it's fairly obvious this was his target. The Zodiac Killer's distance in miles was 96.8% accurate. His angular accuracy in degrees was 98.0%. The Zodiac Killer was very unlikely to have selected a specific target that fell on an exact whole number of radians and inches.   

The Zodiac Killer was targeting Ingleside Police Station for his bomb, but clearly he wasn't going to start digging a hole on the lawn of Ingleside Police Station. The Zodiac Killer also threatened to blow up a school bus on April 20th 1970, in which he drew us a diagram with a steep embankment on the right, with the sun rising to the east. This would suggest we should be looking for a bus route traveling from north to south (where the sun rises to the east in the morning), with a steep embankment to the right of the bus, where the Zodiac Killer can lay his photoelectric cells and conceal his bomb. He also has to access that roadway from a position of anonymity, with his vehicle parked at the top of the embankment in relative seclusion. What are the odds of that location being the exact closest portion of road to Ingleside Police Station, meeting all the criteria above. This would satisfy the drawing on the April 20th 1970 letter and the target of SFPD (Ingleside Police Station). The Zodiac Killer could metaphorically kill two birds with one stone.

So let us travel just 274 feet from Ingleside Police Station to the nearest section of road heading north to south.

Again, what are the chances that the area highlighted by the red rectangle would mimic exactly his bomb diagram of a school bus and embankment, as drawn in the April 20th 1970 letter. Not only that, but this would have been a school bus route in 1970. Children from the northern and eastern areas of San Francisco would use the school bus to reach St Thomas More Catholic School at 50 Thomas More Way. There is even a bus route road sign 38 feet from the proposed location of the bomb (whether it existed in 1970 I don't know). So, let us drop down to the exact position of the red rectangle and take a closer look.

All the Zodiac Killer had to do to access the embankment, was to access the parking lot of the City College of San Francisco and drive to its eastern edge - and at night - in relative anonymity to the police station, dig the bomb into the embankment. It is highly unlikely that the Zodiac Killer ever intended to plant a bomb, but this location fits all the criteria of targeting both a San Francisco Police Department and a school bus at the same time. It achieves his objective of targeting school children for maximum terror, and achieves his objective of "rubbing the pigs nozes in their booboos". The location of the bomb not only matches the diagram from April 20th 1970, but satisfies the criteria of the Mount Diablo map and code to an overall accuracy of 97.5%. The 32 character cipher is also answered. The Zodiac Killer likely had no intention of laying this bomb, but it was the threat that mattered most of all. The Zodiac Killer knew that the clues he provided us with on June 26th 1970 were inadequate in cracking the bomb location and code, so he gave us these additional clues in the Little List letter on July 26th 1970, one month later.

# Radian values are normally measured in a counter-clockwise direction, but the positioning of the bold, black circle at 246 degrees (4 radians and magnetic north), along with the north facing arrow pointer, the addition of the clock face set at zero, and the instructions to "set to Mag. N.", appears to indicate the Zodiac Killer measured his radians from the north position clockwise. In effect, he was multiplying the radian value of 57.3 by a factor of 4, set to magnetic north of 17 degrees (246.2).   

It has been stated on this site and elsewhere, that the 32 character code is unsolvable using conventional cipher techniques without the killer supplying us with a key. There are 29 unique characters within the code, making any solution practically unprovable. The solutions offered by using the 408 and 340 keys have produced less than convincing results, particularly when we consider that this code contains an "anchor" and "omega" symbol, not used in the 408 and 340 ciphers. Attempting to correlate the letters used in the code to written text in both the Button letter (June 26th 1970) and Little List letter (July 26th 1970) also produces negative results. Therefore, it could be argued, that a code containing 29 unique characters must have an alternative meaning, in accompaniment to the code solution of "estimate:four radians and five inches". Here is one possibility. 

The Zodiac Killer gave us an estimate of 4 radians, but the exact figure is 4.084 radians. That is, 4.084 multiplied by one radian value (57.3 degrees) to equal 234 degrees. When we add in the magnetic north which Zodiac instructed us to do (17 degrees in 1970), we get 251 degrees and the precise location of the Southern Freeway embankment and Ingleside Police Station. Could the 32 character code contain the value of 234 degrees, which once set to magnetic north, now identified the exact location of Ingleside Police Station? If we resort to basic numerology where A=1, B=2, C=3 etc, and add up the values of the forward facing alphabetical characters in the Zodiac Killer's 32 character code, we arrive at a total of 234 (or 234 degrees). When set to magnetic north, it gives us the value of 251 degrees, which when subtended from the peak of Mount Diablo passes directly over Ingleside Police Station and the Southern Freeway. 

​    "The map (set to 17 degrees magnetic north) coupled with the code of 234 will tell you where the bomb is set".

On April 20th 1970 the Zodiac Killer mailed a letter to the San Francisco Chronicle with three key components. [1] The Zodiac Killer stated "I hope you do not think that I was the one who wiped out that blue meannie with a bomb at the cop station.  Even though I talked about killing school children with one. It just wouldn't doo to move in on someone else's teritory". The Zodiac Killer was referencing the murder of San Francisco police officer Brian McDonnell on February 16th 1970, who suffered devastating and sadly fatal injuries two months earlier, when a bomb packed with fence staples exploded on an outside ledge of Park Police Station in the Upper Haight neighborhood. He continued [2] "But there is more glory in killing a cop than a cid because a cop can shoot back". [3] The Bay Area murderer also included a bus bomb diagram, with a school bus traveling south in the morning and the sun rising to the east. The bomb was positioned on a steep embankment.

It therefore shouldn't be any surprise that his next letter on June 26th 1970 contained all three of these components in its design: [1] He claimed he had set a bomb by Ingleside Police Station, [2] claimed his "glory" by insinuating his involvement in the shooting death of Officer Richard Radetich and [3] his bus bomb diagram matched perfectly the bomb to be set by Ingleside Police Station. 

In the June 26th 1970 Button letter he stated "The Map coupled with the code will tell you where the bomb is set*. It is clear the bomb was to be set in San Francisco, the target for the majority of his communications and ire toward police. The code provided the inches and radians value, and the map was to provide the magnetic north value of 17 degrees in 1970 (to be set at Mag. N" - magnetic north - from his zero point clockwise). This was plainly evident when he provided the extra two clues in the following Little List letter on July 26th 1970.

The positioning of the bomb had to meet certain criteria. With a high degree of accuracy, Ingleside Police Station was situated 4 radians and magnetic north, and 5 inches from Mount Diablo. Even a line subtended from Mount Diablo passing over the neighborhood of Ingleside in San Francisco, when considered alongside the addition of SFPD=0, should have told us that the target for the bomb was likely the San Francisco Police Department in Ingleside. No other police station in San Francisco comes close to 4 radians and magnetic north, and 5 inches along the radians. The next task was looking for the exact location for the bomb to be buried. It most likely would have been set in the vicinity of the police station, just distant enough not to be observed.
 
The bus bomb diagram in the April 20th 1970 letter (shown above) depicted a school bus traveling from north to south in the morning, next to a steep embankment, with the sun rising to the east. This location is just 230 feet from Ingleside Police Station. School buses traveling from the northern and eastern edge of San Francisco to St.Thomas More Catholic School would travel the Southern Freeway, passing only 210 feet from Ingleside Police Station. From this location the bus would be traveling south in the morning to begin classes for the day, the sun would be rising from the east, and this location has a steep embankment mirroring exactly his bus bomb diagram of April 20th 1970. This portion of the Southern Freeway is flanked by the parking lot at the City College of San Francisco. This parking lot is the ideal location to access the embankment without parking on the freeway.        

Sgt Richard P. Radetich (25) was murdered on June 19th at 5:25 am, gunned down by three shots from a .38 caliber revolver at point blank range through the driver side window of his vehicle while in the process of serving a parking ticket. He was sat in his police car near 643 Waller Street, San Francisco in the Lower Haight District. Numerous magazine and newspaper articles covered his brutal execution and gave a background of the young officer's life. Officer Richard Radetich attended Balboa High School in San Francisco, before attending a criminology course at the City College of San Francisco with the express intention of joining the San Francisco Police Department. His first assignment was at Ingleside Police Station, before transferring to the Accident Investigation Bureau.  

The Zodiac Killer was an avid reader of the newspapers and had previously mentioned the murder of Officer Brian McDonnell on February 16th 1970, killed by a bomb placed on the ledge of Park Police Station. In the June 26th 1970 Button letter, the Zodiac Killer claimed his involvement in the murder of Richard Radetich, while giving us a location for the bomb that bisected Ingleside Police Station and the City College of San Francisco - two of the places Officer Richard Radetich attended prior to his murder, while serving in the Accident Investigation Bureau. Had the Zodiac Killer read about the murder of the young officer, and cynically positioned his claimed bomb on the Mount Diablo map at a location pertinent to Officer Radetich's life. Exactly between Ingleside Police Station and the City College of San Francisco? And the design of the bus bomb diagram was just a fortunate coincidence to this location.

The April 20th 1970 bus bomb diagram was penned two months before the murder of Officer Richard Radetich, so if this location was deliberately earmarked well in advance of his murder, it is unlikely the Zodiac Killer chose it because of the history of the young officer in the newspapers subsequent to June 19th 1970. But it wouldn't rule out the deliberate targeting of an officer connected in some fashion to Ingleside Police Station or an advance knowledge of their history, however implausible this may seem. Whatever the case, the primary focus of Ingleside Police Station and an attack on an officer, because "there is more glory in killing a cop" is a viable one. The location of a proposed bomb on the Southern Freeway by Ingleside Police Station, matches both the Mount Diablo map and code of 4 radians set to magnetic north, and 5 inches along the 4th radian. The location also matches perfectly the bus bomb diagram within the April 20th 1970 letter. These still stand, irrespective of any connection to Officer Richard Radetich, which may have no bearing whatsoever. 

The Ingleside Police District encompasses the area south of Cesar Chavez Street to the San Mateo County line and west from Highway 101 to Faxon Avenue. It is  home to the San Francisco City College campus and many diverse residential neighborhoods including Sherwood Forest, Mt. Davidson, Glen Park-Sunnyside, Diamond Heights, Bernal Heights, Upper Noe, Mission Corridor, the Excelsior and St. Francis Wood, St. Mary's Park, Visitacion Valley, Precita Park, Cayuga, Outer Mission and Holly Park.

This is another speculative article searching for the name of the Zodiac Killer within his communications - so caution must be applied to any conclusions that are reached in the following analysis.

After the challenge by Dr. Marsh on October 22nd 1969 requesting that the Zodiac Killer give us his real name in a cipher "however complicated", the notion of a killer hinting at his name in later communications is certainly plausible. Presented in a hidden format, the Zodiac Killer knew that it could never be used as evidence against him, unless the unearthed solution could be proven beyond doubt to be the correct one. In the article Return to Sender we explored the introduction and answer to the 13 symbol cipher of "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is....Fk, I'm crackproof". The Zodiac Killer effectively giving us his name, but in the shortened format. 

Three months later, the Zodiac Killer mailed the Little List letter on July 26th 1970 paraphrasing the recital of Groucho Marx's As some day it may happen from the Gilbert & Sullivan comedic opera, The Mikado. On October 12th 1970, the San Francisco Chronicle featured this letter under the title of Gilbert and Sullivan Clue to Zodiac, stating "A quiet search for onetime Ko-Ko's has turned up none that could be Zodiac. Obvious differences in physical description and handwriting comparisons have cleared all Ko-Ko's tracked down since the arrival of the July 27 letters". But what if the Zodiac Killer wasn't choosing the character of Ko-Ko because he played him in a production or even liked the theater, but the name Ko-Ko was somehow pertinent to his identity. Bearing in mind the phrase "My name is....Fk, I'm crackproof", could the Zodiac Killer's surname begin with Ko. The phrase "FK, I'm crackproof" is actually 14 letters long, but it was fashioned into a code of only 13 characters. Could this deliberate formatting indicate his initials were FK in a name comprising of 13 letters?

Although questionable Zodiac correspondence, the December 1990 Eureka card yet again featured Groucho Marx in imagery on the front of the card, promising us his name yet again. The card read "From your secret pal, can't guess who I am yet? Well, look inside and you'll find out" - and contained within was a xerox copy of two keys. This too triggered the notion of a surname beginning with K. The Exorcist letter mailed on January 29th 1974 also featured a verse recited by Groucho Marx, but more importantly, the verse of Tit-Willow by Ko-Ko was preceded by the line "Signed, yours truley", implying the character in the verse had something to do with his name. This wasn't all.

We argued that the Zodiac Killer gave us the answers to all his "unsolved" codes in his own words, and the Celebrity Cypher mailed on September 25th 1990 to the Vallejo Times-Herald may have followed a similar pattern. The name of the sender was hidden behind 5 and 8 characters totaling 13, placed on the address side of the postcard and at the foot of the communication. A Celebrity Cypher with a likely introduction of "My name is", exactly like 13 symbol cipher mailed on April 20th 1970. If the initials of the killer were given in that instance within the phrase "Fk, I'm crackproof", then there are reasonable grounds to believe the 5 and 8 letters of the name on the Celebrity Cypher begin with an F and K also. In the Return to Sender article we hypothesized the forename as "Frank", based upon the phrase "how much money you have on my head now" in reference to the stamp on the April 20th 1970 letter - and this fits nicely into the Celebrity Cypher solution. I have speculatively placed "Ko" at the beginning of the surname regarding The Mikado references, but will venture no further. There are possibilities based on the "keys" provided by the Eureka card to suggest his surname could end in "key" or "ki" such as "Kominski", however, there are plenty of other options pertaining to the xeroxed image.

Comparisons can be drawn between the April 20th 1970 and September 25th 1990 communications with respect to a name comprising of 13 letters (split into 5 and 8 characters) - and bearing in mind the Christmas card, likely mailed close to December 25th 1990 continuing the theme of promising us his name - can a link be forged between all three regarding the identity or name of the Zodiac Killer? 

THE 340 CIPHER WAS CRACKED ON DECEMBER 3RD 2020 BY DAVE ORANCHAK, SAM BLAKE AND JARL VAN EYCKE, SO THIS EARLIER ARTICLE SHOULD BE VIEWED IN RESPECT TO RECENT DEVELOPMENTS.

The following article will coalesce several pieces of work from 2019 into one cogent story regarding a handful of communications mailed by the Zodiac Killer in 1970. For some, the following will be repetitive, for which I apologize. It will hopefully make the case that the Zodiac Killer was winding down his operations and ultimately revealing the answers to his Z340, Z32 and Z13 ciphers and codes, to which he knew were practically unsolvable without such a prompt. The Zodiac Killer was a narcissistic braggart, who boasted of his exploits and threatened all and sundry with the possibility of more murders and mayhem if his demands were not met, including the claim he had killed far more than could be accounted for. If we applied the same model to his ciphers and codes, we would have expected him to be unable to resist giving us the answers - or at the very least - some clues to break open his puzzles. The notion of a killer keeping deathly silent on the solutions to his codes for up to 50 years, runs contrary to everything we know about the Bay Area murderer during his brief but deadly reign. That is why I don't believe he did.

The story begins with the final thing he wrote on the July 26th 1970 Little List Letter, and finishes just two communications later with the October 27th 1970 Halloween Card. All the answers to his codes were contained in just three short months. The end of the Little List Letter read "PS. The Mt. Diablo Code concerns Radians and # inches along the radians". The Z32 was the easiest code to crack because the Zodiac Killer just gave us the answer above - the code concerned "Radians and inches along the radians". Why make it any more complicated than it has to be? A code of 29 unique symbols within a total of 32 characters is simply not solvable without a key - and there is insufficient wording on the 13 Hole Postcard, Halloween Card or Los Angeles Letter written in plain English to formulate any such key. The Z32 is even worse than giving us the English alphabet of 26 letters, replacing M with A, W with B and H with C and saying "now solve it". There is no transposition key present in the remaining three communications from October 5th 1970 to March 13th 1971, so if the Zodiac Killer was to be consistent in his inability to resist revealing his deeds, then the only realistic alternative is he gave us all the answers instead.

You need look no further than the Paul Stine police report to notice that the hashtag symbol is used to represent the word "number". The Zodiac Killer, by use of an arrow pointer underneath the hashtag, was telling us to replace the hashtag with a number value. The Mount Diablo map scale was 6.4 miles to the inch, so it is probably unsurprising that the "Mt Diablo code would concern inches". In this instance, I have placed the number 5 into place, giving us the answer of "5 inches along the radians". The reason for 5 inches is self explanatory, because 4 or 6 inches from Mount Diablo to San Francisco would fall into the sea on the east and west side. The San Francisco element will be covered shortly. The Zodiac Killer also gave us a second set of crosshairs (shown below).

This was the first time the Zodiac Killer had ever placed an accentuated zero within his crosshairs alongside the acronym SFPD. This had to be the second clue to the location of the bomb. The hashtag and arrow signified the value of inches required, so this had to signify the radian value. By some massive stroke of luck (almost beyond belief), the bold, black accentuated circle fell over 246 degrees on a compass rose - which is 4 radians and magnetic north (17 degrees in 1970). Therefore, the target of his bomb must have been 4 radians and magnetic north, and 5 inches along the radians. 

All Zodiac's bomb threats were focused towards the San Francisco Chronicle and San Francisco Police Department, so placing this bomb location alongside the acronym SFPD should tell us that the threat was most likely focused toward a police station. By another massive stroke of luck, when we travel 4 radians and 5 inches from Mount Diablo, the target area for the bomb lies right by Ingleside Police Station. Allowing for a degree of flexibility of about 2%, by the Zodiac Killer rounding off his radians and inches, along with the black circle diameter, no other San Francisco police station comes close. However, we need to find a location where a school bus would be traveling from north to south (with the sun to the east), that has an embankment on the right side (as displayed on the April 20th 1970 bomb diagram), with easy and covert access to that embankment, and within a few hundred feet from Ingleside Police Station. The Zodiac Killer certainly wouldn't have wanted to attract attention by parking on the road itself, but by accessing the incline from a secondary location - likely at the top of the embankment - he could achieve this task in relative anonymity. Fortunately for us, that location sits approximately 230 feet from Ingleside Police Station. The perfect location for the Zodiac Killer to target a school bus and thumb his nose at the San Francisco Police Department, with covert access to the top of the embankment via the car park of San Francisco City College.             

Then came the 13 Hole Postcard punched in the style of a 'Dick Tracy' code with a "red crucifix decoder". What are the chances that if we arranged the 13 holes into the shape of a cross and place it over the midsection of the 340 cipher both horizontally and vertically, it would reveal the wording "paradice" and "slaves"? Then, what are the chances that the Zodiac Killer would write "paradice" and "slaves" in his next communication in a cross formation on the card, accompanied by "sorry no cipher" in a cross formation on the inside of the Halloween Card envelope? - totaling three cross formations - but supposedly, all created for absolutely no reason whatsoever in just two communications. Isn't it more believable they were created for purpose. The Zodiac Killer could easily have written "sorry no cipher" just the once, but deliberately fashioned it this way to tell us that the 340 character cipher was not a cipher after all. It was simply a design of "paradice" and "slaves" as exhibited by the Halloween Card. Throw in a few bigrams, pivots and 'corrected sixth line', and hopefully it will look convincing. 

All these clues from the back end of the Little List Letter to the Halloween Card giving us the literal answer to both the Z340 and Z32 in "paradice" and "slaves", and "Estimate: Four Radians and Five Inches". The 32 character code wasn't the complete solution, because the Zodiac Killer explicitly stated "the map coupled with the code will tell you where the bomb is set", which included magnetic north of 17 degrees in 1970. The code contained "four radians and five inches", to be coupled with the map which incorporated the magnetic north, giving us a total of 246.2 degrees.​ See The Blast Estimate of Zodiac and The Blast Estimate of Zodiac (Part Two) and The Blast Estimate of Zodiac (Part Three). Could either the 13 Hole Postcard or Halloween Card give us the answer to the Z13 as well? Again, totally unsolvable because of its limited number of characters, the Zodiac Killer could either keep quiet for up to fifty years or give us the answer.

From a previous article: On January 29th 1970, the Yellow Cab Company put up a thousand dollar reward for any information leading to the arrest of the Zodiac Killer. In addition, the Teamsters Union which represents the Yellow Cab Company were reported in the San Francisco Chronicle to be considering offering a further reward on February 8th 1970 in the case of Paul Stine and Charles Jarman (another taxicab murder victim). The April 20th 1970 letter gave us the code and was immediately followed by "I am mildly cerous as to how much money you have on my head now". The October 5th 1970 postcard gave us the possible solution of "Fk, I'm crackproof" and was immediately followed by "What is the price tag now". These are the only two communications in which the Zodiac Killer asked the police the price of his capture. These two communications not only had a code and an answer that fitted the code, but both were followed by the exact same question. When the Zodiac Killer was designing his 13 Hole Postcard, it is very likely he had at his disposal a copy of his original April 20th 1970 letter, or the San Francisco Chronicle newspaper cutting below, to which he was now supplying the answer. This may have been the trigger that influenced him to repeat the phrase "how much money you have on my head now" into the very similar "What is the price tag now". The Zodiac Killer, in essence, had inadvertently bound both communications together through phraseology, as well as code and solution.   

The Zodiac Killer introduction would now give us his name (albeit his initials) in absence of his full name. It would also read fluently as a sentence, as follows: "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you? My name is Fk, I'm crackproof". Nobody realistically believed he was going to supply his full name, but this way he satisfies the promise in a restricted format. Two communications mailed nearly six months apart, with a solution that satisfies the code, as well as a satisfactory introduction to the code - and both followed by the same question.    

The three 8's (representing Jesus Christ in gematria), along with the number 8 which symbolizes a new beginning, actually begins each element of the phrase Fk I'm crackproof. In addition, when we begin the phrase on the first 8, it fits perfectly around the existing K and M already present in the code and thereby creating a cyclical format, where the code ends and begins with the same letter. The Little List Letter provided the solution to the Button Letter and Mount Diablo code that preceded it. His next communication, the 13 Hole Postcard, provided the solution to the 13 Symbol Cipher before that, and finally the Halloween Card provided the solution to the 340 cipher. Three chronological solves to the three previous ciphers - and the only thing we had to add was the number "5", for which he inserted the hashtag and arrow. Why look for convoluted and excessively complicated solutions to these ciphers, when the Zodiac Killer gave us all the answers literally word for word.

This isn't a case of dragging wording from the entire Zodiac collection of cards and letters, but using just the two crosshairs of the Little List Letter, followed by the 13 Hole Postcard and Halloween Card. The bold, black circle and "PS. The Mt. Diablo Code concerns Radians and # inches along the radians" to answer the Z32. The 13 holes, red cross, "paradice" and "slaves", and "sorry no cipher" to answer the Z340. And finally, "Fk, I'm crackproof" to give us a shorthanded version of his name, while still retaining his anonymity. The alternative is that the codes are genuine, the designs on the 13 Hole Postcard and Halloween Card are devoid of any purposeful meaning, and the big-headed, braggart called Zodiac, kept his lips 'butoned' for however many years afterwards. 

This has been covered before, but I just wanted to expand in a little more detail without pinpointing a particular location.

How embarassing is the notion of a 50-year-old unsolved cipher, that we were given the answer to in just thirty one days? - and an answer that came straight from the horse's mouth. The 32 Symbol Cipher solution was served to us on a silver platter in the July 26th 1970 Little List Letter using the Zodiac crosshairs and six words. 

When the Button Letter was mailed on June 26th 1970, the Zodiac Killer gave us a map of San Francisco and Vicinity with his crosshairs centered over Mount Diablo, stating "the map coupled with this code will tell you where the bomb is set." If we were to find a perceived bomb, it is fairly obvious we have to travel a certain angle and distance from the top of Mount Diablo to find the location. Therefore, the 32 symbol code, if it served any purpose whatsoever, had to contain a measurement in the form of degrees and distance. The map scale was 6.4 miles to the inch - so that will be the distance the Zodiac Killer has to employ from Mount Diablo to make any sense. The degrees he used was radians, because he told us so in the July 26th 1970 Little List Letter. The Zodiac stated "P.S. The Mt. Diablo code concerns Radians + # inches along the radians". He is actually telling us that the 32 symbol code contains radians and inches in the solution.

"P.S. The Mt. Diablo code concerns (is about ) (has to do with) Radians + # inches along the radians". Any cipher expert such as David Oranchak, or quite frankly, any Joe Bloggs such as myself, will tell you that a 32 symbol code with only three repeating characters is unsolvable without a key. The Zodiac Killer was certainly no muppet - and clearly knew this when he concocted this jumble of random characters. But he gave us a large chunk of the key a mere 31 days later in the form of an arrow and a hashtag, writing "Radians and # inches along the radians" on a separate line to distinguish the very fact. We just had to key in the number. The symbol # is most commonly known as the number sign, The symbol is described as the "number" character in an 1853 treatise on bookkeeping, and its double meaning is described in a bookkeeping text from 1880. The instruction manual of the Blickensderfer model 5 typewriter (c. 1896) appears to refer to the symbol as the "number mark". Some early-20th-century U.S. sources refer to it as the "number sign", although this could also refer to the numero sign. A 1917 manual distinguishes between two uses of the sign: "number (written before a figure)"; and "pounds (written after a figure)". Mainstream use in the United States is as follows: when it prefixes a number, it is read as "number", as in "a #2 pencil" (indicating "a number-two pencil"). The one exception is with the # key on a phone, which is always referred to as the "pound key" or "pound". Thus instructions to dial an extension such as #77 are always read as "pound seven seven". Wikipedia.

All of Zodiac's bomb threats were mailed to the San Francisco Chronicle newspaper - even detailing the murder of San Francisco police officer Brian McDonnell on February 16th 1970, who suffered devastating and sadly fatal injuries when a bomb packed with fence staples exploded on an outside ledge of Park Police Station in the Upper Haight neighborhood of San Francisco. The Zodiac constantly signed off his communications with the barbed acronym SFPD = 0 and threatened "there is more glory in killing a cop than a cid because a cop can shoot back". If that wasn't enough, the Phillips 66 Road Map was labelled San Francisco and Vicinity.  Based on a map scale of 6.4 miles to the inch, only 5 inches would land in San Francisco (4 and 6 inches would fall into the sea, other than Treasure Island). Fortunately, the Zodiac Killer gave us the angle from Mount Diablo, courtesy of his bold, black circle on the Little List crosshairs. It fell 4 radians (and magnetic north) clockwise from his arrowed pointer, around the clock face he provided. Radians are usually measured counter-clockwise, but the Zodiac Killer placed the bold circle exactly 4 radians "set to Mag.N" clockwise.

The answer was now extremely simple. All we had to do was insert a number in place of the hashtag, shown by the big flashing arrow pointing us in the right direction. The hashtag conveniently preceded "inches", so it's here we will slot in the number 5 - and bingo - we have our answer to the inches component of "Radians + 5 inches along the radians" or "Radians and 5 inches along the radians". The full answer of "four radians and five inches", coupled with the magnetic north value of 17 degrees would be our guide.  As with all the best puzzles, the answer is usually hidden in plain sight. Druzer, an avid Zodiac researcher, came up with a solution to the 32 character code which I believe is correct, when he suggested Estimate: four radians and five inches. This solution not only satisfies the 32 characters, but correlates three plaintext letters to the only three repeating ciphertext characters in the code, thereby providing a perfectly valid key. See The Blast Estimate of Zodiac, The Blast Estimate of Zodiac (Part Two). and The Blast Estimate of Zodiac (Part Three). The map scale is 6.4 miles to the inch. This gives us a distance of 6.4 multiplied by 5 (inches). Therefore, the bomb was to be set at 4 radians and magnetic north, and 32 miles from the peak of Mount Diablo. The only San Francisco Police Department that comes anywhere close is Ingleside Police Station. This San Francisco Police Department sits at 30.96 miles from the peak of Mount Diablo (4.84 inches). It also sits at 251 degrees from the peak of Mount Diablo (4.08 radians).

The Zodiac Killer gave us a rough guide of 4 radians and magnetic north, and 5 inches (hence the word "estimate"). If he had given us the exact measurement, it would have been 4.08 radians and 4.84 inches, He was clearly giving us an approximate measurement. Since Ingleside Police Station was the only San Francisco Police Department anywhere close to these measurements, it's fairly obvious this was his target. The Zodiac Killer's distance in miles was 96.8% accurate. His angular accuracy in degrees was 98.0%. The Zodiac Killer was very unlikely to have selected a specific target that fell on an exact whole number of radians and inches.   

In the Little List communication it is apparent that Zodiac Killer gave us two prominent crosshairs: [1] with the accentuated black circle with SFPD=0 alongside, and [2] the extra large crosshairs with "P.S. The Mt. Diablo code concerns Radians + # inches along the radians" scrawled across it. Attaching these important additions was clearly done for purpose, thereby informing us of the radian value required, and the correlation between the crosshairs on the Phillips 66 Map and the measurements of radians and inches. With 29 unique characters in a 32 character code, any correlation between the characters and alphabetical solution was always going to be minimal, therefore it is encumbent upon us to look to other reasons for its design. The Blast Estimate of Zodiac (Part Three). 

In the final instalment of this series we will attempt to refine the previous conclusions gleaned from the Zodiac Killer letters.
The Zodiac Killer stated on April 20th 1970 "I hope you do not think that I was the one who wiped out that blue meannie with a bomb at the cop station. Even though I talked about killing school children with one. It just wouldn't doo to move in on someone else's teritory.  But there is more glory in killing a cop than a cid because a cop can shoot back.  I have killed ten people to date. It would have been a lot more except that my bus bomb was a dud.  I was swamped out by the rain we had a while back." In the correspondence, the Zodiac Killer was referring to the murder of San Francisco police officer Brian McDonnell on February 16th 1970, who suffered devastating and sadly fatal injuries two months earlier, when a bomb packed with fence staples exploded on an outside ledge of Park Police Station in the Upper Haight neighborhood. The wording in the letter suggested that the Upper Haight neighborhood was not somewhere he was prepared to venture, but the suggestion of a school bus bomb and police station, when we take into account the SFPD positioned strategically by the bold, black circle on the Little List crosshairs and indicative of a location point, brings forth the notion of a school bus bomb under the noses of the San Francisco Police Department. Targeting a school bus within sight of a police station would certainly have dealt law enforcement a mighty blow.

The Zodiac Killer may have had a location in mind, despite the fact it was never followed through on - and this location has tentatively been identified as four radians (+magnetic north) and five inches along the radian in two previous articles. The exact position he had in mind to set up his photoelectric cells to detonate the bomb didn't necessarily have to conform to these measurements exactly, just be close enough to be rounded off by the Zodiac Killer. However, the other clues he gave us in the April 20th 1970 and July 26th 1970 communications were replete with information. We narrowed down a section on Google Maps between Daly City and Ingleside in San Francisco (between 246 and 249 degrees).

To narrow down the location even futher, we should be looking for a bus route approaching a school from north to south (so the sun is rising from the east on the left side). The roadway must have sufficient embankment on its right side to elevate photoelectic switch B. It must be within the parameters of 4 radians and 5 inches (allowing for a small margin of error). The addition of the pronounced black circle within the crosshairs of the Little List Letter in the form of SFPD=0, is suggestive of a location close to a San Francisco Police Department. In other words, the Zodiac Killer may have intended his "blast" right under the noses of law enforcement: "Hey pig doesnt it rile you up to have your noze rubed in your booboos?" Despite not knowing the exact topography of this location in 1970, I have found a site just 230 feet from Ingleside Police Station alongside Southern Freeway. 

The Zodiac Killer mailed us three parts of one cipher on July 31st 1969, to the San Francisco Chronicle, San Francisco Examiner and Vallejo Times-Herald. In his communication to the Chronicle he stated "In this cipher is my idenity". Bearing in mind this is the only portion of cipher with unsolved characters, it could be argued that any identity the Zodiac was offering would be found at the signing off phase in this cipher. It didn't really matter, because only four days later he gave us his identity in the form of "This is the Zodiac Speaking". The Zodiac Killer clearly made a mistake when taking his draught message into his 408 cipher, accidentally omitting the word "people" from the final offering. If we therefore conclude his original identity to be left was actually six characters shorter, it would have reduced the 18 unsolved characters to 12. Using the first-person singular pronoun of "I", his identity at the base of the 408 cipher would read "I am the Zodiac", to become "I am the Zodiac Killer" upon realization of his mistake. The Bay Area murderer literally gave us the exact wording to his unsolved 18 characters through his introductions in the Examiner and Herald letters (both communications began with "I am the killer"). Once we knew the pseudonym or identity as Zodiac on August 4th 1969, the conversion to "I am the Zodiac Killer" was the next logical step. This would become a habit for the Zodiac Killer, who would ultimately give us the exact wording to solve his remaining three ciphers. Not only would he give us the solutions to the codes, but his practice of using 'filler' with no apparent correlation to the solution would continue.  

The Zodiac Killer was no master criminal in the style of Moriarty, but he was nobody's fool either. To make the false assumption he didn't understand ciphers or codes when creating his remaining three offerings to the San Francisco Chronicle, would be a misguided one. Even somebody with limited knowledge of coding, knows that giving us a 32 character code with 29 unique characters within its framework, is practically unsolvable without a key. You can literally fill in the blanks as you choose. Therefore, the only realistic solution to the two smaller codes can come in two forms: [1] The Zodiac Killer can give us a key to decrypt the code, or [2] He can simply give us the answer. The Zodiac Killer may well have chosen option two, and given us the entire solution to all his three unsolved codes in subsequent communications. In fact, the possibilty exists that he supplied us with all the answers in three consecutive communications from July 26th 1970 to October 27th 1970 - in the Little List Letter (07/26/70), 13 Hole Postcard (10/05/70) and Halloween Card (10/27/70). The Zodiac Killer was likely giving us all the solutions before slipping back into anonymity. The newspaper article on November 16th 1970 by Paul Avery, linking Zodiac to the Riverside murder of Cheri Jo Bates on October 30th 1966, likely drew one final breath from the Zodiac Killer through 1971, before his race was run.    

The "key" phrases for the four codes are *in this cipher is my identity", "the Mt. Diablo code concerns radians and inches",  "Fk, I'm crackproof" and "paradice and slaves". The 408 cipher we have examined, so we will now tackle the 32 symbol code.

The Little List Letter mailed on July 26th 1970, one month after the Mount Diablo code (06/26/70), told us the code "concerns radians and # inches along the radians". It was as simple as adding in two figures to complete the code (hence the hashtag symbol denoting a number). The Zodiac gave us the exact wording for the solution, detailed here in a previous article. As stated above, the 32 symbol code with 29 unique characters couldn't be solved without a key or the answer. The Mount Diablo map and crosshairs were the framework, onto which we "coupled" the code for the location of the bomb. The answer was likely "4 radians and 5 inches along the radian", thereby giving us the solution word for word. An alternative to this solution (where only the hashtag is replaced by the number 5), is "P.S. The Mt. Diablo code concerns Radians and 5 inches along the radians". The Zodiac didn't necessarily have to incorporate the number 4 because we already knew the radian value from the bold, black circle provided on the Little List Letter crosshairs.

The next solutions may have been given to us in the 13 Hole Postcard - that may ultimately become the most important communication of them all. By combining the "red crucifix" decoder and 13 punch-holes in true "Dick Tracy" style, the Paradice and Slaves formation would be revealed, bisecting the 340 cipher superficially along its mid-section both horizontally and vertically. See the article 'The Dick Tracy 340 Cipher'.  The Zodiac Killer wrote Paradice and Slaves on the Halloween card in the identical formation it could be found on the face of the 340 cipher. Three clues within the space of three weeks.

The Zodiac Killer also gave us the phrase "Fk, I'm crackproof" (upside down) on the 13 Hole Postcard. Apart from an expletive, the other viable option here, is the Zodiac Killer exclaiming that he is crackproof, preceded by his initials. The reason for this line of thinking, is the Zodiac Killer's wording before the 13 Symbol Cipher on April 20th 1970. The code is designed symmetrically and looks contrived, with three circled 8's fashioned at its center. The Zodiac Killer couldn't have supplied us with his name with any expectation it would be solved, because the code was simply too short and capable of providing us with thousands of solutions. However, he could provide us a solution to the code by giving us his initials. By doing it this way, he is in a roundabout way, giving us his name and a solution to the code that satisfies the introduction that preceded it: "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is Fk, I'm crackproof".

This solution not only fits into the 13 Symbol Cipher, but reads continuously if imagined in circular fashion. The Zodiac Killer gave us three 8's within a circle (the symbol of infinity, and regeneration and rebirth within Christianity), with the letters K and M positioned ideally in the code to fit "Fk, I'm crackproof" around. There are three elements to this phrase: Fk, I'm, and crackproof. When the entire phrase is slotted around the letters K and M, the number 8 begins each element of the phrase (as shown below).

The Zodiac Killer had effectively given us the exact wording of "I am the Zodiac Killer", "Paradice and Slaves", "Fk I'm crackproof" and "Radians and (5) inches along the radians" - all of which can be inserted untouched into all four ciphers and provide us with solutions that don't require a key (The "paradice and slaves" solution would only be present on the "canvas" of the 340 cipher, so this would only be a superficial observation). The idea of a killer who constantly responded to the newspapers at every turn, somehow resisting the temptation to ever disclose the workings of his codes and ciphers is a difficult story to sell. If we are of the belief that the 32 and 13 character codes are simply too short and too variable to be solved, then without a key the Zodiac Killer likely supplied an alternative. In the case presented above, the "key" and "answer" could be described as exactly the same thing - and It was just a case of transferring the wording from one communication to another in at least three of the examples.

John Rose used Dave Oranchak's webtoy to create features of the Halloween Card in just an 81 character block around the center of the 340 cipher. Some have stated that the variability of the characters in this area of the cipher have allowed the freedom to manufacture such a result. While that may be the case, a larger pattern of Paradice and Slaves is replicated in a 17 X 17 formation along both same midsections, effectively mirroring the John Rose presentation. The fact that both are even achievable in the same cipher, is worthy of mention. The Bay Area braggart must have been itching to throw us a few clues to his unbroken codes and ciphers - so the notion that he didn't give us any before he disappeared into the twilight - is somehow difficult to swallow.   

In a previous article we concluded that the 32 character code mailed to the San Francisco Chronicle on June 26th 1970 stated "P.S. The Mt. Diablo code concerns 4 Radians and 5 inches along the radian", based upon the clues the Zodiac Killer gave us in the Little List Letter on July 26th 1970. This led to an area somewhere between Ingleside and Daly City on the south edge of San Francisco. The Zodiac drew us a bus bomb diagram on April 20th 1970, suggesting he was going to place two photoelectric switches on an embankment or hillside on the right side of the road. He stated it was to be set up early in the morning when the sun was rising in the east. The images below show the El Camino Real by Outer Mission, heading south, with the April 20th 1970 'Bus Bomb' diagram superimposed over it. The hillside to the right is the type required to place his two photoelectric switches - one by the roadside and the other higher on the banking. Any school bus heading along this road for early morning assembly, would have the sun rising in the east (shining left to right in the image below).

If the Zodiac Killer had any intentions of blowing up a school bus (which is unlikely), then he would be targeting the schoolchildren on their way to school in the morning. If the children were being bussed in from other parts of San Francisco for this particular school (St. Thomas More Catholic School), then traveling from the north and east, the bus driver could be using Highway 101 and Interstate 280 in the direction shown by the red line below (recommended on Google maps). The blue circle is 4 radians (+ magnetic north), and 5 inches from Mount Diablo, satisfying the 32 character cipher and Zodiac's statement of "P.S. The Mt. Diablo code concerns Radians + # inches along the radians". The highway under the blue circle is traveling south with the sun to the east in the morning - and is heading toward St. Thomas More Catholic School (just 1.37 miles away).      

The Zodiac Killer stated on April 20th 1970 "I am mildly cerous as to how much money you have on my head now.  I hope you do not think that I was the one who wiped out that blue meannie with a bomb at the cop station.  Even though I talked about killing school children with one. It just wouldn't doo to move in on someone else's teritory.  But there is more glory in killing a cop than a cid because a cop can shoot back.  I have killed ten people to date. It would have been a lot more except that my bus bomb was a dud.  I was swamped out by the rain we had a while back. The new bomb is set up like this. Sun light in early morning. A + B are photoelectric swiches when sun beam is broken A closes circut. B opens circut, which makes B the cloudy day disconect so the bomb won't go off by accid. PS I hope you have fun trying to figgure out who I killed".

​​The choice of words the Zodiac used at the beginning of this communication were unfortunate, had his target been children from St. Thomas More Catholic School. The school was named after Thomas More, an English lawyer, social philosopher, author, statesman, and noted Renaissance humanist. He was also a Chancellor to Henry VIII, and Lord High Chancellor of England from October 1529 to 16 May 1532. He was sentenced under the Treasons Act 1534 and decapitated with an axe. Therefore, it was very unfortunate that the Zodiac Killer should state "I am mildly cerous as to how much money you have on my head now".

There are several other schools in this area between Ingleside and Daly City, but we must remember that it is the bus he was targeting, not the school. Therefore, we should prioritize the roadways in this area (shown by the red rectangle), from the perspective of a bus traveling south, with its left side facing east.

The Zodiac Killer mailed a bomb threat on June 26th 1970, stating "the map coupled with this code will tell you where the bomb is set." One month later, realizing the inadequate information he had provided, the Zodiac Killer mailed another communication stating "P.S. The Mt. Diablo code concerns Radians + # inches along the radians", while adding some crosshairs with a black circle and the addition of SFPD = 0 alongside it. The overwhelming majority of the Zodiac Killer's communications were mailed to the San Francisco Chronicle and his taunting of the police heavily focused on the San Francisco Police Department from 1969 to 1971. If a bomb was to be set, then the Zodiac Killer was likely targeting San Francisco. He almost certainly never followed through on any of these threats, but it would be interesting to discover where his intended focus was, and find the solution to the 32 character code supplied on June 26th 1970. This topic has been covered before, but this will iron out a few problems experienced with previous articles.

The Zodiac Killer couldn't resist responding to the newspapers when he was asked to supply more details about his crimes, as demonstrated when he delivered the August 4th 1969 'Debut of Zodiac' Letter. He responded in a seven-page letter to refute the claims of Captain Martin Lee of the San Francisco Police Department, when his October 13th 1969 letter was disbelieved. He also wasn't averse to claiming various murders, disappearances and abductions that had featured in the newspapers. Despite this, the overwhelming consensus exists of a killer who remained resolutely silent on the workings of his three unsolved cryptograms or codes. However, there is every chance he gave us the exact wording that permeated the 340 cipher, to which he fashioned into the Halloween Card one year later. This was explored in the article Zodiac Admitted 340 Not a Real Cipher. I also believe he gave us the exact wording in the July 26th 1970 'Little List' Letter to solve the 32 character cipher a month earlier.

A radian value is approximately 57.3 degrees, which travel counter-clockwise. However, the Zodiac Killer placed a form of numerical clock face around the circumference of his crosshairs on the Mount Diablo map and stated that it be set to Magnetic North (about 17 degrees in 1970) - so it was considered that he had formulated his radians to run clockwise. The only radian value that can fall near San Francisco when a line is subtended from Mount Diablo, is 4 radians + 17 degrees = 246.2 degrees. Three and five radians doesn't fall anywhere near San Francisco. Therefore, a bomb set in San Francisco would have to be around the 4 radian + Magnetic North value. The bold, black circle present on the 'Little List' Letter crosshairs, falls in the region of 246.2 degrees - bearing in mind it is just a rudimentary sketch. Shown on the map above, is 2 radians counter-clockwise from True North (which would land at 245.4 degrees clockwise from True North). Both examples landing in the same region of the map. Daly City Police Department sits around 246 degrees from Mount Diablo, but resides in San Mateo County (shown by the red line). Ingleside Police Station (shown by the green line) sits closer to 249/250 degrees, but resides in San Francisco. If the Zodiac Killer had chosen a target in this region, we might expect the radian value to be only a rough guide. But it is clear, that if he is referring to a radian value, then it has to be a whole number - and it can only be 4 radians. 

Likewise, the value of inches can only be 5, if we are referring to a bomb threat in San Francisco. The scale on the Mount Diablo map is 6.4 miles to the inch. Four and six inches (25.60 and 38.40 miles) both fall into the sea on the east and west coast of San Francisco. So, unless the Zodiac Killer was wearing scuba gear and was planning to blow up an underwater school bus, the value of inches had to be the whole number of 5. When 5 inches is subtended from Mount Diablo, it gives us a value of 32 miles (somewhere in the red box below, allowing for a small margin of error). This margin of error has also been applied vertically to encompass Ingleside Police Station, bearing in mind that SFPD = 0 threat. Whether the mention of SFPD in the crosshairs on the 'Little List' Letter implies a specific threat to San Francisco itself, or somewhere close to San Francisco is unknown - but the region of San Francisco seemed the primary focus of the Zodiac Killer's communicaions during this two-year period. 

Now that we know a threat in the San Francisco region has to be subtended using the whole numbers of 4 radians and 5 inches, let us use the Zodiac's own words to complete the 32 character cipher, just like the words Paradice and Slaves from the Halloween Card was used to fill in the 340 cipher. The Zodiac Killer wrote on July 26th 1970: "P.S. The Mt. Diablo code concerns Radians + # inches along the radians". But notice the hashtag.

The symbol # is most commonly known as the number sign, The symbol is described as the "number" character in an 1853 treatise on bookkeeping, and its double meaning is described in a bookkeeping text from 1880. The instruction manual of the Blickensderfer model 5 typewriter (c. 1896) appears to refer to the symbol as the "number mark". Some early-20th-century U.S. sources refer to it as the "number sign", although this could also refer to the numero sign. A 1917 manual distinguishes between two uses of the sign: "number (written before a figure)"; and "pounds (written after a figure)". Mainstream use in the United States is as follows: when it prefixes a number, it is read as "number", as in "a #2 pencil" (indicating "a number-two pencil"). The one exception is with the # key on a phone, which is always referred to as the "pound key" or "pound". Thus instructions to dial an extension such as #77 are always read as "pound seven seven". Wikipedia.

The Zodiac Killer could easily have left the phrase as "P.S. The Mt. Diablo code concerns Radians + inches along the radians", but deliberately forced the hashtag into place to denote a number - that number being 5. The phrase would now read "P.S. The Mt. Diablo code concerns Radians + 5 inches along the radians". We know that any threat on San Francisco had to fall in the 4 radians value, so we can complete the phrase: "P.S. The Mt. Diablo code concerns 4 Radians + 5 inches along the radians". But because we have now added a specific "5 inches" to be subtended along the line of the 4th radian, the correct phrase would read "P.S. The Mt. Diablo code concerns 4 Radians + 5 inches along the radian".
An alternative to this solution (where only the hashtag is replaced by the number 5), is "P.S. The Mt. Diablo code concerns Radians + 5 inches along the radians". The Zodiac didn't necessarily have to incorporate the number 4 because we already knew the radian value from the bold, black circle provided on the Little List Letter crosshairs.

There is no way this code could realistically be solved with 29 unique characters and no key. But fortunately the Zodiac Killer realized this and gave us the answer one month later. He did the same with Paradice and Slaves in the Halloween Card, to help our understanding of the 340 cipher. So, where is the answer to the '13 Symbol' cipher mailed on April 20th 1970?

THE LOCATION OF THE BOMB [PART TWO]

Question marks have always hung over the October 5th 1970 13-Hole Postcard as to whether the author was the Zodiac Killer or a copycat. This article will hopefully go a long way to convince people that the communication was indeed mailed by the Bay Area murderer. The Zodiac Killer wrote the lengthy Little List Letter on July 26th 1970, beginning the correspondence with his intentions for his slaves in paradise, followed by the torture methods he was going to apply, before finishing with the plagiarism of Gilbert & Sullivan's The Mikado, from the Act As Some Day it May Happen. The crucial thing to remember here, is that the Little List Letter was withheld from the newspapers and public alike, so by October 5th 1970 when this postcard was mailed, the author (if not Zodiac} was totally unaware to the contents of the Little List Letter. It is possible that the Zodiac Killer could have been reemphasizing his victim total of thirteen after his previous letter was unpublished, or a copycat could have been suggesting thirteen victims for the first time, unaware that the Little List Letter had ever been mailed. Therefore, this is no help in determining whether the author of the 13-Hole Postcard was the Zodiac Killer or not. We need to look at the wording on both communications back to back.

The first thing to notice on the 13-Hole Postcard is the crucifix and number 13 in close alliance - extremely relevant when we consider the Zodiac Killer using the phrase "I shall (on top of everything else) torture all 13 of my slaves that I have waiting for me in Paradice" in the Little List Letter. The crucifix, a symbol of Christianity and the progression from life on earth to the paradise of heaven, is further cemented by the October 27th 1970 Halloween Card where the Zodiac Killer actually combines all three elements. However, if you are also a skeptic of the Halloween Card, then we need to focus entirely on finding a link between the 13-Hole Postcard and the previously unpublished Little List Letter, both eventually released in tandem with one another on October 12th 1970 in a newspaper article by Paul Avery. Below I have separated the Little List Letter into its constituent parts - his promise of torture, his methods of torture and the search for victims.            

The Zodiac Killer refers to his victims in the plural and in the future tense, beginning his torture methods with "Some I shall tie over ant hills and watch them scream + twich and sqwirm. Others shall have pine splinters driven under their nails + then burned. Others shall be placed in cages + fed salt beef untill they are gorged then I shall listen to their pleass for water and I shall laugh at them". He uses the word "some" to begin his rhetoric, then continues with his Little List of potential victims, sourced from As Some Day it May Happen, beginning with "As some day it may hapen that a victom must be found". Therefore, is it any great surprise that the Zodiac Killer having noted his lengthy communication was not published in the newspapers, would begin the 13-Hole Postcard in the past tense, stating "The pace isn't any slower. In fact it's just one big thirteenth. Some of them fought it was horrible". The last sentence likely referring to the slaves he previously mentioned he was going to torture. 

The Zodiac Killer is effectively saying that he had now tortured his victims in paradice and "some of them had fought". He is telling Paul Avery and the San Francisco Chronicle newspaper he had made good on his promises. This strongly suggests that the author of the 13-Hole Postcard knew the contents of the July 26th 1970 Little List Letter, thereby cementing the notion that the Zodiac Killer was the author of both communications. The alternative, is that a copycat just threw together a random postcard that not only continued the theme of the previous letter accidentally, but was adopted by the Zodiac Killer in the October 27th 1970 Halloween Card and the following March 13th 1971 letter, where the Zodiac Killer stated "This is the Zodiac speaking Like I have allways said, I am crack proof". The copycat scenario proliferated by many, a sensational subplot to discredit certain Zodiac communications and attacks, where no merit for such claims exist. 

The other notable element of the 13-Hole Postcard is the Mon, Oct 5, 1970 attribution in the top right corner. The Zodiac Killer did recite the Gilbert & Sullivan Act As Some Day it May Happen. So was Monday, October 5th the day something happened - and the reason he added a specific date for the very first time?