Here I would like to return to an old topic with a fresh approach, partly inspired by 'Nobody,' a regular contributor to this site.
As early as August 10th 1969 Professor D.C.B. Marsh of the American Cryptogram Association (ACA) was commenting on the Zodiac ciphers. In the San Francisco Sunday Examiner & Chronicle he praised Donald Harden's deciphering skills and called the code "complicated and obviously drawn by somebody who knows his business". 
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The Zodiac Killer had already responded to prompts by Vallejo Police Chief Jack Stiltz to send more details to prove the letter writer and killer were one and the same. The more the Zodiac Killer wrote, not only lessened his time for killing, but may have provided additional clues to detectives regarding his identity. This tactic was likely adopted by Professor D.C.B. Marsh in the October 22nd 1969 San Francisco Examiner newspaper article by Will Stevens, which laid down a challenge to the Zodiac Killer to reveal his name. The newspaper stated "Dr Marsh told the Examiner today: "The killer wouldn't dare, as he claimed in letters to the newspapers, to reveal his name in the cipher to established cryptogram experts. He knows, to quote Edgar Allen Poe, that any cipher created by man can be solved by man. Zodiac has not told the truth in his cipher messages to the Examiner, the Chronicle and the Vallejo Times-Herald. Zodiac has not done this, because to tell the complete truth in relation to his name -in cipher code - would lead to his capture. I invite Zodiac to send The American Cryptogram Association a cipher code - however complicated - which will truly and honestly include his name." 

Clearly, Dr. Marsh and the detectives would have considered it highly unlikely for the Zodiac Killer to respond by giving us his name, but while he was constructing ciphers he wasn't murdering innocent civilians. There also remained an outside chance that Zodiac would rise to the challenge and create a code, "however complicated", that Dr Marsh and his fellow cryptographers could unpick. This article, rather than a straightforward challenge to Zodiac, could have been deliberately manufactured with the assistance of detectives, to bait the Zodiac Killer into using a particular methodology in his cipher construction. If the police knew the source of Zodiac's inspiration, then it could unearth the cipher technique he employed when designing his next code. 

'Nobody' considered the idea that the Zodiac Killer may have used a foreign language in the cipher, or possibly a rotating key that switches at certain intervals. By employing such a technique, the Zodiac Killer could make the cipher virtually "crackproof". 
The article in the October 22nd 1969 San Francisco Examiner may have been specifically designed to lead the Zodiac Killer down a particular path, making his next cipher easier to crack. Dropping innocuous sections of text into an article such as "to quote Edgar Allen Poe, that any cipher created by man can be solved by man", can be specifically designed to guide the Bay Area murderer into using one of Edgar Allan Poe's cipher techniques or a model suggested by him. We know the Zodiac Killer was an avid reader of the newspapers and may have took this bait of using a publication by Edgar Allan Poe, thinking he was getting one over on Dr Marsh and the police. The hope from the standpoint of law enforcement, was that the Zodiac Killer would bite the hook and get reeled in. 

Sixteen days later the Zodiac Killer mailed the 340 cipher revealing, not his name, but his identity, carefully positioned at the base of his cipher. This catapulted the idea that the Zodiac Killer had indeed responded to the article by Dr Marsh, but wasn't quite ready to reveal his name to authorities. But it is my belief that the Zodiac Killer did create the 340 and 13 Symbol ciphers in tandem, with a view to inextricably link his identity to his name using the word "Zodiac" on the final row of the 340 cipher, with the three 8's as the conduit between the two. See here.

The Zodiac Killer, however, would hold back his '13 Symbol' cipher for approximately six months. Zodiac researcher Michael Cole stated "Clearly, the Zodiac did not send the cryptogram to Marsh, as had been requested (this non-public way of satisfying the challenge was apparently unacceptable to the attention-craving serial killer). Neither did he respond in a time frame that most would have expected. But these details matter little. In fact, the timing itself provides yet another reason to conclude that what we have here is a specific instance of cause and effect. As mentioned, this article was published in the Examiner on October 22, 1969.  Six months to the day later, April 22, 1970, the people of the San Francisco Bay Area were reading about the My Name Is cipher in the Chronicle."

But there may have been a much more cunning reason why the Zodiac Killer delayed his mailing of the April 20th 1970 communication, and it has everything to do with Edgar Allan Poe. Did the Zodiac Killer pick up on the wording of Dr. D.C.B. Marsh in the article, create and mail his '13 Symbol' cipher based on something in the writings of Edgar Allen Poe? 

We can see in the Edgar Allan Poe essay 'A Few Words on Secret Writing,' at the very beginning he shows the alphabet being split into two halves of 13 letters, A to M, and N to Z.
"Were two individuals, totally unpractised in cryptography, desirous of holding by letter a correspondence which should be unintelligible to all but themselves, it is most probable that they would at once think of a peculiar alphabet, to which each should have a key. At first it would, perhaps, be arranged that a should stand for z, b for y, c for x, d for w, &c. &c.; that is to say, the order of the letters would be reversed. Upon second thoughts, this arrangement appearing too obvious, a more complex mode would be adopted. The first thirteen letters might be written beneath the last thirteen, thus:
n o p q r s t u v w x y z
a b c d e f g h i j k I m; and, so placed, a might stand for n and n for a, o for b and b for a, &c. &c. This, again, having an air of regularity which might be fathomed, the key alphabet might be constructed absolutely at random." ​​

The Zodiac Killer not only created the April 20th 1970 cipher with 13 characters, but it too began with an A and ended with an M, just like the suggested encryption technique featured by Edgar Allan Poe. Notice the way the cipher is organized symmetrically using the alphabetical characters, the three 8's and the two symbols. The same symmetry was observed when placing half the alphabet alongside the 13 character cipher. See here.  

The Dr. Marsh newspaper article opened with "A Challenge perhaps unique in the annals of American crime was injected today into the bizarre case of the killer who begins his letters to newspapers 'This is the Zodiac Speaking'."
The article continued, and bemoaned the fact that Zodiac hadn't revealed his true identity: "Zodiac has not done this, because to tell the complete truth in relation to his name - in cipher code - would lead to his capture. I invite Zodiac to send The American Cryptogram Association a cipher code - however complicated - which will truly and honestly include his name." 

'A Few Words on Secret Writing' stated "This challenge has elicited but a single response, which is embraced in the following letter. The only quarrel we have with the epistle, is that its writer has declined giving us his name in full. We beg that he will take an early opportunity of doing this, and thus relieve us of the chance of that suspicion which was attached to the cryptography of the weekly journal above-mentioned–the suspicion of inditing ciphers to ourselves. The postmark of the letter is Stonington, Conn." ​link. 

If the Zodiac Killer was basing the '13 Symbol' cipher on the Dr. Marsh article and secret writing of Edgar Allan Poe, why did he wait approximately six months to mail it to the San Francisco Chronicle?
It may not have been the six month anniversary that was important to him, but the only date mentioned in the Edgar Allan Poe literature, 'A Few Words on Secret Writing'. The date was April 21st. 
Had the Zodiac Killer deliberately waited six months to ensure his '13 Symbol' cipher letter would be postmarked April 21st, or be published on April 21st, which unfortunately for him never materialized? He was slightly early on the mailing, resulting in the postmark of April 20th, and the San Francisco Chronicle published his cipher in the April 22nd edition, effectively thwarting any intentions to recreate the Connecticut letter.

However, the possibility remains, that the Zodiac Killer could have used one of the cipher techniques employed or suggested by Edgar Allan Poe in 'A Few Words on Secret Writing', and the idea proposed by 'Nobody' regarding the use of a second language as the key, a much more viable proposition than initially considered.
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 "It is not to be supposed that Cryptography, as a serious thing, as the means of imparting important information, has gone out of use at the present day. It is still commonly practised in diplomacy; and there are individuals, even now, holding office in the eye of various foreign governments, whose real business is that of deciphering. We have already said that a peculiar mental action is called into play in the solution of cryptographical problems, at least in those of the higher order. Good cryptographists are rare indeed; and thus their services, although seldom required, are necessarily well requited. An instance of the modern employment cipher is mentioned in a work lately published by Messieurs Lea & Blanchard, of this city–“Sketches of Conspicuous Living Characters of France.” In a notice of Berryer, it is said that a letter being addressed by the Duchess de Berri to the legitimists of Paris, to inform them of her arrival, it was accompanied by a long note in cipher, the key of which she had forgotten to give. “The penetrating mind of Berrver,” says the biographer, “soon discovered it. It was this phrase substituted for the twenty-four letters of the alphabet–Le,gouvernement provisoire.” 'A Few Words on Secret Writing' by Edgar Allan Poe.

 Beach Street runs from Pier 39 to the Maritime Museum, but despite what Google maps indicates, there is no 56 Beach Street present today. Thanks to 'Nobody':"56 Beach St is the old block numbering from the original planning maps, which comes out as 1654/1656 on the street numbering. 1654/56 Beach St, San Francisco, CA is a condo home that contains 2,141 sq ft and was built in 1938." 

"During the 1960s, California was the site of many outdoor art fairs, which nurtured a culture of independent artists and craftspersons. At this time there was an effort to sell crafts on the sidewalks of the liberal Haight-Ashbury neighborhood of San Francisco. Artists and street performers who illegally set up in public areas were frequently harassed and arrested by the police. In the 700 block of Beach Street adjacent to Victorian Park and near Fisherman's Wharf, between 15 and 25 artists would set up their displays and use lookouts to alert them to the arrival of the police." Wikipedia.

Sergeant Lynch couldn't locate a 56 Beach Street in 1969, so considered Beechwood Street starting in the 700 block. This location was actually Beechwood Drive, Daly City, CA 94015. It is 8.6 miles from Beach Street, San Francisco.
This clearly indicates that Sergeant Lynch was considering the letter from the standpoint of San Francisco, due to its proximity to San Mateo. 

The question that arises from this letter, is were the "concerned citizen" and "good citizen" communications designed by the same author, or linked in any way? The first supplied a code key relating to the 408 cipher, solved by Donald Gene and Bettye June Harden of Salinas, California. The second referred to "code letters". Both correspondences were mailed to Sergeant John Lynch. Bearing in mind the "concerned citizen" card wasn't mentioned in any newspapers, does this increase the significance of the October 7th 1969 mailing?
U.S. Route 101, or U.S. Highway 101 links Salinas, San Mateo and Beach Street.​ Paul Stine picked up the Zodiac in the proximity of Union Square on October 11th 1969. The "good citizen" letter, mailed only four days earlier, mentioned ESP and Beach Street - this location is only 1.4 miles from Union Square.   

Without knowing more about this communication and its exact contents, this analysis must be treated with caution. 

# "Zener cards are cards used to conduct experiments for extrasensory perception (ESP) or clairvoyance. Perceptual psychologist Karl Zener (1903–1964) designed the cards in the early 1930s for experiments conducted with his colleague, parapsychologist J. B. Rhine (1895–1980). The original series of experiments have been discredited and replication has proved elusive. The Zener cards are a deck of twenty five cards, five of each symbol. The five symbols are: a hollow circle, a plus sign, three vertical wavy lines, a hollow square, and a hollow five-pointed star. They are used to test for ESP." Wikipedia.

As explained in the previous article, there appears little doubt that the author of the "concerned citizen" card had read the August 9th 1969 San Francisco Chronicle article entitled 'A Murder Code is Broken'. The article clearly stated that Donald Harden's worksheets had been forwarded from the Chronicle to the Vallejo Police Department (which would obviously contain the cipher key and method of decryption). It stated "Vallejo Police Sergeant John Lynch, in charge of the investigation of the murders and of the cipher letter writer, asked The Chronicle to send Harden's code breaking worksheets to him for further checking; which was done." The wording "cipher letter writer" is an unusual combination of words, that apparently didn't go unnoticed by the "concerned citizen" author, who used the same sequence in his opening gambit: "I hope the enclosed "key" will prove to be beneficial to you in connection with the cipher letter writer."  
This should be proof enough that the "concerned citizen" had read the Chronicle article, knew the code had been broken, and knew Donald Harden's worksheets had already been sent to the Vallejo Police Department and Sergeant John Lynch. Therefore, do you have to question the integrity of the "concerned citizen" regarding the motivation behind his August 10th 1969 mailing?    

​The Vallejo Times-Herald also broke the story on August 9th 1969, stating "Lynch talked to Harden and said, "I'm convinced he has it solved". He said he would talk to him again today. Meanwhile Lynch has Harden's worksheets and is studying and comparing them with the three-part cryptogram and the translation." 

So, having presumably read all this, was the "concerned citizen" just a helpful member of the public attempting to corroborate the Harden's solution, or did the Zodiac Killer, knowing his cipher had been broken quickly, begin playing a game of 'cat and mouse'. The author of the "concerned citizen" card didn't want to give the Vallejo Police Department his name for fear it would be published in the newspapers, just like Zodiac stated in the 408 - but did the anonymous card reveal his identity? 

The "concerned citizen" card only contained 69 words, so why did the sender bother to type the message rather than write a handwritten message or simply capitalize the text.
When he wrote the cipher key he capitalized the alphabetical letters to mirror the 408, so what purpose was behind him choosing not to do this in his message?

The sender would certainly not have given away his identity by using upper case lettering in the message (as he had already done it in the key), nevertheless, he chose to type it anyway. If the author was the Zodiac Killer, did he believe, that by using capitals in his message rather than handwriting it may have been construed as suspicious, so opted to use a typewriter for a more professional approach and give the card a level of credibility. Opting to write the message in his normal handwriting would obviously have given the game away. He also may have thought it easier to type the message rather than make the effort to disguise his handwriting. The cipher key was obviously written rather than typed because of the unusual characters present within the 408 cipher not being available on a standard typewriter. 

This is all very speculative, however, with very little to indicate one way or another as to the identity of the "concerned citizen", feeding on scraps may be the only avenue. Was the sender of the August 10th correspondence simply being a helpful citizen wanting to corroborate the Harden's solution, or Zodiac attempting to regain a semblance of lost pride by surreptitiously offering a lending hand to the Vallejo Police Department, declaring "I do not wish to have my name in the papers".
This may have been the Zodiac Killer's idea of a joke, after having previously stated "I want you to print this cipher on the front page of your paper. In this cipher is my idenity."
This contradiction certainly wouldn't have been lost on a killer who craved attention at every turn.
A killer who was prepared to confuse with "name" and "identity" in his first communications with the newspapers, would not be averse to creating similar parallels with "signature" or "name" in the "concerned citizen" card just ten days later. 

THE CONCERNED CITIZEN CARD [PT1]

Having not covered this topic previously, I thought I would add the "Concerned Citizen' card to this site, mailed from San Francisco and postmarked August 10th 1969, one day after the article 'A Murder Code is Broken' featured in the San Francisco Chronicle newspaper. In this article "Vallejo Police Sergeant John Lynch, in charge of the investigation of the murders and of the cipher letter writer, asked The Chronicle to send Harden's code breaking worksheets to him for further checking; which was done." (see foot of article for newspaper cutting).     

Many have asked the identity of the "concerned citizen."  Was it Donald Harden, the Zodiac Killer or a third party who broke the code independently? The latter having not been aware of the code being broken, as published in the newspaper.
The Chronicle, according to the August 9th 1969 newspaper article had "already done" sending Sergeant John Lynch the worksheets, so why would Donald Harden be mailing the cipher key through the mail, when clearly he was already in contact with police. Therefore, this option can be summarily dismissed as the least likely option. 

The Bay Area murderer, as of August 4th 1969, had already given us his pseudonym with the dramatic introduction of "This is the Zodiac Speaking", so why would he now be acting coy in giving us the cipher key under the guise of a "concerned citizen". This tactic may have surfaced during the 1974 communications, but to implement it so early, just after threatening to "cruse around all weekend killing lone people in the night then move on to kill again, until I end up with a dozen people over the weekend", seemed an immediate backpedal. From "ruthless killer" to "concerned citizen" in a matter of days.
On the other hand, this may have been the start of his game with both the newspapers and law enforcement alike. 

The card reads:
Dear Sergeant Lynch.
   I hope the enclosed "key" will prove to be beneficial to you in connection with the cipher letter writer.
  Working puzzles criptograms and word puzzles is one of my pleasures. Please forgive the absence of my signature or name as I do not wish to have my name in the papers and it could be mentiond by a slip of the tongue.
      With best wishes.
                                     concerned citizen.

Although it has been stated on many forums that the author of the "concerned citizen" card may have mailed it, unaware of the code being broken and published in the San Francisco Chronicle on August 9th 1969, this seems highly unlikely. The author used the wording "cipher letter writer", exactly as published in the newspaper. This, from a grammatical standpoint, is highly likely to have been copied from the newspaper, indicating that the person who mailed in the cipher key already knew it had been broken. This could indicate it was Zodiac after all.

Zodiac researcher David Oranchak has comprehensively covered all aspects of the Zodiac ciphers on his excellent ZodiacKillerCiphers website, including two articles on the topic covered in this article. The first is a 'Detailed Analysis of the 408 Solution'  and the second is entitled 'Who is the Concerned Citizen'. The latter will show you the code key mailed by the "concerned citizen" for handwriting comparison purposes. 
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THE CONCERNED CITIZEN CARD [PT2]

Vallejo Times-Herald - Cryptogram Deciphered by Teacher August 9th 1969 

In a recent article David Oranchak posed the question "Are the Ciphers Gibberish", analyzing the pros and cons to the legitimacy of the Zodiac 340 cipher. There is, however, another legitimate argument to be discussed when searching for a solution to the Zodiac Killer's second cipher. Is it worth trying to solve at all? If it does contain a message, it will likely be a collection of inane ramblings akin to the solved 408 cipher. There will of course be the immense notoriety attached to your name, having been the cryptographer who broke the infamous Zodiac code, from which numerous television appearances and a book deal will inevitably follow. In the midst of tragedy, your name will be up in lights. But here is the reality check. Even if you do achieve the seemingly insurmountable task of unequivocally solving the 340 cipher, then comes the next insurmountable hurdle - convincing people you have the correct solution. In a Zodiac community wearily grazing on skepticism and fueled by a heavy dose of déjà vu syndrome, your perfectly good solution will likely drift on down the river with barely a ripple from the esteemed adjudicators.   

Even if it does get snagged by an overhanging branch for a fleeting moment, attempting to prove that the "key" used to decipher it, is the one Zodiac intended, will again create a hill you can never climb.

There are so many claimed solutions to the 340 cipher haystack, that if one really existed, it will eventually be as hard to find as a solution to the cipher itself. David Oranchak stated here:   
​"The 340 cipher has 63 unique symbols. If one of 26 plaintext letters can be assigned to each symbol, then the number of possible solutions is 26 raised to the 63rd power. So, how many of those solutions would computers need to examine PER SECOND to finish within 100 years? The answer is about 4.4 x 10^80 solutions, or 440 million billion billion billion billion billion billion billion billion. This is LIGHT YEARS beyond what current computers are capable of.
According to Top500.org, the world's fastest supercomputer as of November, 2011, achieved a computational power of 10.51 Petaflop/s, or 10.51 quadrillion floating point operations per second. If you were somehow able to check a decryption solution of the 340 cipher using a single floating point operation, then this supercomputer could go through all of the possible solutions to the 340 cipher in about 1.3×10^57 years. By comparison, our humble universe is estimated to be only 13.7 x 10^9 years old!"

With so many variables that can be applied to the cipher, or more importantly, the one conjured up by the warped and dissociative brain of Zodiac, the probability is, that a supercomputer designed by logic and born from reality will probably be heading course on the wrong superhighway. Throw in a handful of Zodiac errors into the mix, could mean proving your solution an impossible task to the baying hordes of furrowed experts, armed with statistical probabilities. Solving the 340 cipher may yet prove to be the easy part, however, convincing somebody else you have, could take a quadrillion years you haven't got.

David Oranchak stated "When Z408 was cracked, the papers talked about how the solvers took advantage of patterns found in the cipher text. Zodiac could have focused on somehow removing those patterns from Z340. Or maybe he found some obscure idea in a book or one of those pulp fiction detective stories".
​Herein lies part of the problem. If you discover an obscure idea that answers key elements of the 340 cipher, or provides a readable message (possibly using a rotating key code), then it could be argued that your obscure idea is so obscure, that it isn't worth considering as a credible solution and thereby roundly dismissed as lunacy. The more crazy the solution, the more unlikely it is, despite being crafted by the perfectly sane Zodiac.

Alternatively, how do you prove the cipher gibberish while it still remains unsolved in the eyes of most observers. By extension, if it's unsolved, then it still has a possible solution. But be warned - if you do find the real solution to the 340 cipher, you will have to work far harder than the Harden's did in 1969 to convince the world you are correct. And my guess is, you never will.    

Robert Michael Salem's body was discovered by friends on the evening of Sunday April 19th 1970 at his 745 Stevenson Street, San Francisco residence. The autopsy concluded he had died on Wednesday April 15th. The article below was published in the San Francisco Chronicle on the morning of April 20th 1970. This is the same day that Zodiac's '13 Symbol' cipher communication was postmarked, proclaiming "I hope you have fun trying to figgure out who I killed."​​​
​
Bearing in mind this article was considering a Zodiac connection, did the Bay Area murderer respond by creating this letter and mailing it almost immediately. The killer hadn't written since the 'Melvin Belli' letter on December 20th 1969, exactly 122 days ago. Therefore, do we conclude that [1] the Zodiac Killer just happened to independently write the April 20th 1970 communication, which accidentally coincided with the April 20th 1970 San Francisco Chronicle article, but he wasn't the murderer of Robert Salem. [2] He was the murderer of Robert Salem and had prepared the letter from the time of his death on the 15th, before mailing it on the 20th, or [3] He immediately sprang into action on April 20th 1970 after reading the article, and created the '13 Symbol' cipher in double quick time, designed his 'photoelectric switched' bus bomb diagram, and recalled the murder of San Francisco police officer Brian McDonnell on February 16th 1970, all in time to have this letter postmarked on the same day. To answer this question we have to go back six months. 

Michael Cole wrote an excellent article entitled "My Name Is" Cipher Motivation that suggested the Zodiac Killer likely drew his inspiration for the '13 Symbol' cipher from an October 22nd 1969 newspaper article by Will Stevens, which laid down a challenge to the Zodiac Killer from the president of the American Cryptogram Association (ACA), Professor D.C.B. Marsh, to reveal his name. Apparently, several months later the killer duly obliged by opening his correspondence with the line "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...."

Michael Cole wrote "Clearly, the Zodiac did not send the cryptogram to Marsh, as had been requested (this non-public way of satisfying the challenge was apparently unacceptable to the attention-craving serial killer). Neither did he respond in a time frame that most would have expected. But these details matter little. In fact, the timing itself provides yet another reason to conclude that what we have here is a specific instance of cause and effect. As mentioned, this article was published in the Examiner on October 22, 1969.  Six months to the day later, April 22, 1970, the people of the San Francisco Bay Area were reading about the My Name Is cipher in the Chronicle. This relationship between the Examiner article and the My Name Is cipher may well mean there is an even higher likelihood that the cryptogram does indeed encipher some form of the killer's name"

If the April 20th 1970 letter was referring to the Robert Salem murder, by teasing us with the statement "I hope you have fun trying to figgure out who I killed"​​​, then any manufactured correlation between the dates by the Zodiac Killer would rely on the Zodiac Killer committing the murder just shy of the six month anniversary and then mailing the letter. This would mean the Zodiac Killer could prepare the letter in the days between the murder and the mailing date of April 19th or 20th, to enable it to be postmarked on the 20th. If he didn't kill Robert Salem, and was simply responding to the San Francisco Chronicle with an unprepared letter, then his code and bus bomb diagram must have been designed with little forethought, and mailed in time to be franked on the same day. The other possibility, is he had the code and bus bomb diagram already prepared in advance and just added the text around these designs. However, there is one major problem. The photograph of the crime scene wasn't shown until a follow up article by Paul Avery in the San Francisco Chronicle on April 21st 1970. By which time, the Zodiac Killer had already mailed the letter.               

It has previously been argued that the Zodiac Killer created the '13 Symbol' code in tandem with the 340 cipher, a matter of weeks after the October 22nd 1969 challenge by the American Cryptogram Association (ACA) and Professor D.C.B. Marsh. 
The reason behind this claim was the correlation between eight characters on the final row of the 340 cipher and eight characters from the '13 Symbol' cipher (six being the circled eights).   

On the left we have lined up the alphabet over the top of the cipher. Now calculate numerically the difference in each column. ie: C to N produces a difference of 11, whereas B and E produces a difference of 3. Non alphabetical symbols cannot produce a difference, so a zero is inserted. The resulting numbers produced have been color coded. The red section is before the first circled 8, the green section is after the last circled 8 and the blue section spans the first and last circled 8. Notice the perfect symmetry. The red and green numbers are identical read forwards or backwards, and the blue numbers again read identically in either direction, producing a mirror image. Is this a coincidence or is it by design. 
​
The 340 Cipher was the Zodiac Killer's unsolved masterpiece, and being a man with an inflated ego, driven by self publicity, what better way to drop us a hint by placing his signature or pseudonym on the final line of the 340 Cipher. You will notice he placed the Zodiac crosshair symbol, followed by what could be interpreted as Zodiac and then an upside down Aries symbol, with a line on the top and a dot to the left. So what we shall do is place the correct spelling of Zodiac alongside the version he placed on the 20th line of the 340 Cipher and use the identical technique we used to create the symmetry on the '13 Symbol' cipher. The alphabetical difference of ZOD produces three zero's, the IAC produces 888. These two sets of figures incorporated gives us the three circled 8's found on his next cipher, as though he was carrying them forward. It is also apparent on the '13 Symbol' cipher that either side of the three circled 8's are the Zodiac crosshairs and an inverted Aries symbol, identical to the final line of the 340 Cipher. So, the author has effectively carried eight symbols from one cipher to another.

"In Christian numerology, the number 888 represents Jesus, or sometimes more specifically Christ the Redeemer. This representation may be justified either through gematria, by counting the letter values of the Greek transliteration of Jesus' name, or as an opposing value to 666, the number of the beast." Wikipedia.

The ZOD and IAC are separated. The first produces the 000. The second produces the 888. The two are then combined, to create the transition from the 340 cipher to the '13 Symbol' cipher. This was presumably designed by the Zodiac Killer when he created the 340 cipher sometime before November 8th 1969, with a view to his next cipher. 

When the Zodiac Killer mailed the April 20th 1970 letter and code (assuming he wasn't the killer), he hadn't seen the way Zodiac was written on the wall of Robert Salem's apartment. Therefore, did the killer of Robert Salem just accidentally separate the ZOD and IAC of ZODIAC, thereby mimicking the Zodiac Killer's 340 cipher creation, when he had ample room on the wall to write it normally, or was the Zodiac Killer present at the 745 Stevenson Street, San Francisco residence on April 15th 1970, giving him the opportunity to create the design, and at least five days to prepare his letter? The April 20th 1970 code being the confirmation of his involvement in the murder. 
Was this the reason for the wording "I hope you have fun trying to figgure out who I killed"?

The crucified image on the wall of Robert Salem's wall may well be the Egyptian ankh. The ankh appears frequently in Egyptian tomb paintings and other art, often at the fingertips of a god or goddess in images that represent the deities of the afterlife conferring the gift of life on the dead person's mummy; this is thought to symbolize the act of conception. 
​Wikipedia.

The Zodiac Killer's primary focus was attention from the media, who themselves also benefited from the extensive reporting of his murderous campaign, letter writing and extortion, in a form of symbiotic relationship. The murderer would use this to full advantage during and after his confirmed attacks dried up, often claiming or insinuating his involvement in certain crimes. The August 1969 Debra Gaye Furlong and Kathie Ann Snoozy stabbings, along with the 1970 murders of Robert Michael Salem, Officer Richard Radetich and Donna Ann Lass come to mind. But the killer was also prepared to mention other crimes, such as the murder of San Francisco police officer Brian McDonnell on February 16th 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." The Zodiac Killer wasn't averse to rubbing their "noze in their booboos" when adversity came their way.
The following is extremely speculative and probably way off base, but since it has never been suggested before (as far as I am aware), it is worth putting forward as a possible example of Zodiac piggybacking a crime unrelated to his murderous exploits. 

At approximately 8:10 pm on October 19th 1970 Lieutenant Shaw and Sergeant Noren entered the Sosquel area of Santa Cruz, California responding to a fire on a hill between Rodeo Gulch Road and San Jose Road. The horror of what ultimately lay at 999 North Rodeo Gulch Road was yet to be unearthed.

The bodies of Dr. Victor Ohta (45) and his wife, Virginia (43), along with their two sons, Victor Taggart (11) and Richard Derrick (12), and secretary, Mrs Dorothy Cadwallader (38) were found murdered in the swimming pool of the residence. They had been callously executed.

Dr. Ohta had been shot in the back and under the arm, while the others had been shot in the back of the neck. It was literally a bloodbath. The victims had been bound and blindfolded with scarves before being murdered. The Ohta residence was then set on fire. When the fire crews arrived at the isolated estate they were hampered by Dr. Victor Ohta's Rolls Royce and Mrs. Dorothy Cadwallader's Lincoln Continental blocking the entrance.

Later that day Dr. Ohta's green Oldsmobile station wagon was discovered burnt out on railway tracks near Felton, approximately 18 miles northwest of the crime scene. Although not definitively linked to the other five murders, Thomas DeCecco (19), a service station attendant was found bound and shot in the head near Saratoga only hours after the initial crime, possibly bringing the victim count to six.    

"KPIX News footage from October 20th 1970 with reporter Pat O'Brien featuring scenes from a house in Soquel, CA, in the aftermath of the mass murder of eye surgeon Dr. Victor Ohta, his wife Virginia, two sons Taggart and Derrick and secretary Dorothy Cadwallader. Includes aerial views of the Ohta home, the swimming pool in which their bodies were found." link.

A typewritten note was discovered under the windshield wipers of Dr. Ohta's Rolls Royce. It stated "Halloween...1970. Today World War III will begin as brought to you by the people of the free universe. I and my comrades from this day forth will fight until death for freedom, against anything or anyone who does not support natural life on this planet. Materialism must die or mankind will." Signed "Knight of Wands, Knight of Cups, Knight of Pentacles, Knight of Swords."

John Linley Frazier was arrested shortly after the murders and was sentenced to death on November 29, 1971. However, in 1972 the death penalty was abolished in California and his sentence was commuted to life imprisonment. He committed suicide by hanging  on August 13th 2009.

One week after the Ohta murders and typewritten note stating "Halloween...1970", the Zodiac Killer mailed the 'Halloween' card on October 27th 1970, featuring two skeletons, with twelve eyes added to its design. One eye peered from the knothole of a tree, surrounded by the words "peek-a-boo you are doomed". Why did the killer add twelve eyes to this card?
​Dr. Victor Ohta was an eye surgeon (ophthalmologist), and six possible victims were reported in the newspapers (6 pairs of eyes). The skeletons on the card outer and inner also had the eyes highlighted in red, one in the form of a blindfold, in similar fashion to the victims thrown into the pool. 

Zodiac researcher Tom Voigt 'floated' this idea on his message board in 2012: "I received an interesting observation of the Halloween card via e-mail recently -- it suggested the skeletons were meant to depict a water scenario, with the top skeleton being a diver (my guess would be the pumpkin representing swim trunks?) and the bottom skeleton floating under water, I'm guessing then the many eyes possibly representing bubbles?" What if the water was from the swimming pool at 999 North Rodeo Gulch Road.

The 'Halloween' card would emphasize four methods of death: By Fire, By Gun, By Rope and By Knife, three of which would feature heavily in the Ohta murders.

Had the Zodiac Killer read the coverage of the brutal murders at Sosquel and beyond, and fearing that his relevance was dwindling in the newspapers, used the crime of John Linley Frazier to piggyback himself back into prominence? If this was the case, unfortunately for him, nobody understood the connection, and the meaning of the 'Halloween' card was lost in translation. As an explanation for the October 27th 1970 correspondence, this is probably wide of the mark. It does, however, give us a flavor of some of the radical views that permeated California in the 1960s and 1970s, and the beginning of the end for the Zodiac Killer.

I have recently read Zodiac researcher Mike Rodelli's new book 'The Hunt for Zodiac: The Inconceivable Double Life of a Notorious Serial Killer'. In it he tackles the payphone call from the Springs and Tuolumne intersection to the Vallejo Police Department in the early morning of July 5th 1969. He takes the widely perceived directions given by the killer to identify the location of St John's Mine Road along Columbus Parkway, from which investigators would be led 'one mile east' to the location of the crime scene. The Zodiac Killer chose this location because he was creating radian angles using the payphone as a vertex to St John's Mine Road and the Lake Herman Road murder site.

This theory relies entirely on the transcription of the message given by Zodiac being written and recorded correctly, despite the fact that police dispatcher Nancy Slover and the Zodiac Killer were talking over each other for part of the message. The police report clearly stated "Writer received call from male subject who did not identify himself. Substance of statement was as follows". The 'substance of statement' reference, indicates that this is not necessarily a verbatim transcription of the spoken message, just the overall substance of the call. The above theory also relies on these locations being predetermined to create the necessary radian angles, and supposes that the killer already had 'radians' in his mind one year in advance of his letter proclaiming the use of such a term. The killer has once again been elevated to a master criminal, creating a labyrinth of puzzles from the terrain of northern California. This is the truth - or there is a much simpler and far more mundane answer.
The idea that the Zodiac Killer was just a ruthless killer scouring the Bay Area for victims using 'hit and miss' tactics, isn't half as appealing as the Moriarty style puzzle merchant he has metamorphosed into in recent years.
What is the likelihood that a killer calling the Vallejo Police Department from a payphone near the police station, was directing the police to the crime scene from their location, rather than creating an elaborate puzzle from a predetermined position along Columbus Parkway to manufacture radian angles? This is for you to decide. 

Thomas Horan, author of 'The Great Zodiac Killer Hoax of 1969', admits that the Nancy Slover police report statement of the message is not a verbatim transcription, but goes on to say "Blue Rock Springs is not one mile east of Vallejo Police Department." Despite this, he then uses the transcription to identify a fire station one mile west of the crime scene in his following hypothesis. Both theories rely on the wording and context of the message having been recorded exactly as it was spoken by the killer.
The crime scene is neither 'one mile east' of the fire station or St John's Mine Road. A significant portion of the journey would be traveling closer to south than east. At best (as the crow flies), southeast would be the more accurate determination.  

Zodiac researcher Gian Quasar also tackled this problem on his website 'The Quester Files'.  He stated "The Zodiac does not know the name of the park. Nor was he in a state to notice the sign. Feigning ignorance would buy him nothing. Not knowing the name or just pretending he didn’t know the name would not indicate he was a stranger. Under normal circumstances a stranger would have noticed the park sign. When he calls the police, he actually gives them directions. But even in these he is quite wrong. He is really not aware of directions and distances.
Blue Rock Springs was neither 1 mile east of where he called, nor a mile east of the junction of Springs Road and Columbus Parkway. It is north of Springs Road by about 1 mile and 3 miles away from where he had called. Instead of simply saying the name of the park, which would tell all and sundry the location, The Zodiac gives instructions and doesn’t really have a grasp on distance and direction. He didn’t stalk his ground much at all. Nor was he in a state to notice the sign. He could not have been a Vallejoan."

​This statement makes the assumption that Nancy Slover recalled and recorded the spoken message of Zodiac 100% correct, while just assuming that Zodiac was incorrect with his payphone message. It doesn't consider the possibility that Zodiac was 100% correct and Nancy Slover recalled and recorded the message incorrectly. This same bias was not applied to either party at Presidio Heights, regarding the incorrect negro male adult description given to responding personnel. We will show later that the message by Zodiac was in all likelihood correct - and Nancy Slover also got the wording of the message correct, but with one crucial mistake. Reading into the message and claiming Zodiac was feigning ignorance by only mentioning the word 'park' is a stretch, and again the same argument could be applied to "The S.F. Police could have caught me last night if they had searched the park properly". Using the word 'park' is not proof of Zodiac knowing the area or not. 

Here is a newspaper article statement regarding the murder of Cheri Jo Bates in Riverside on October 30th 1966.
A Mexican-American student who noticed Cheri Jo Bates close to opening time, stated "he knew Cheri Jo Bates and had noticed her in the library the night in question. He said he saw the girl "writing something with a ball point pen in her blue spiral school notebook." The boy told us he was outside about 5:30 pm, waiting for the library to open at 6:00 pm, and it was then he saw the girl". This statement is confusing, as it can be read in two different ways - did he see Cheri at 5:30 pm or 6:00 pm. 

Let us consider the three teenagers at the corner of Washington and Cherry Streets, and assume they had a direct hotline to Officer Armond Pelissetti one mile west on Washington Street. They are viewing Zodiac heading up Cherry Street, so they call Officer Pelissetti and state "If you go one mile east, on Cherry Street you will see Zodiac." In the second instance they call Officer Pelissetti and state "If you go one mile east on Cherry Street you will see Zodiac." Both statements are composed of the same words, but mean two different things entirely. The only difference is intonation or the use of a slight pause or break, making the first statement correct, while the other giving the impression of directional unawareness. 

Applied to the Blue Rock Springs call, we go from "If you will go one mile east on Columbus Parkway to the public park, you will find the kids in a brown car" to "If you will go one mile east. On Columbus Parkway to the public park, you will find the kids in a brown car." The wording hasn't changed, but the meaning of the payphone call has changed entirely 
The Zodiac Killer is instructing police to head east to Columbus Parkway, from where they will find the stricken couple at the public park. All this is achieved by a simple full stop. Why would he be directing Vallejo police from a point on Columbus Parkway, when he is ringing the police station near to the payphone? 
Police traveling from the station along Solano Avenue to Springs Road, would then have to travel 1.34 miles east before reaching Columbus Parkway, on which they would head north towards the crime scene. 
Do you want this boring answer, or do you want something far more exciting? The choice is yours. 

Paul Avery, a reporter at the San Francisco Chronicle, covered many of the Zodiac stories during his reign of terror, and became the focus of the murderer's attention when the Zodiac Killer mailed the 'Halloween' card to the San Francisco Chronicle newspaper on October 27th 1970. The murderer responded to an article published on November 16th 1970 by Paul Avery, linking him to the Riverside murder of Cheri Jo Bates on October 30th 1966. This would be followed nearly three years later, when the 'Exorcist' letter was mailed on January 29th 1974, closely followed by the 'SLA' letter on February 14th 1974, regarding the Symbionese Liberation Army kidnapping of  Patty Hearst, a media heiress, on February 4th 1974.
"Paul Avery covered the Hearst case until the newspaper heiress-turned-bank robber was arrested in September 1975. Avery then holed up on his houseboat at Gate 5 in Sausalito with Boston writer Vin McLellan to write The Voices of Guns, a book on the SLA and the Hearst kidnapping." Wikipedia.

The 'Exorcist' letter has recently courted new attention with the publication of Mike Rodelli's new book 'The Hunt for Zodiac: The Inconceivable Double Life of a Notorious Serial Killer', pouring some doubt on the authenticity of this communication.
This letter, once again, appeared another response to a Paul Avery newspaper article, featuring the recently released Exorcist movie, an American horror film released in 1973, directed by William Friedkin and adapted for screenplay by William Peter Blatty, based on his 1971 book. The San Francisco Chronicle article entitled 'Weird Goings on at the Movies,' was published on January 11th 1974 and detailed the audience reaction to what they had just experienced. The opening lines of the article are extremely interesting when we consider the opening lines of the January 29th 1974 'Exorcist' letter, only 18 days later.     

The article began "If these crowds queuing up three times a night for "The Exorcist" prove anything, it's that a lot of people are willing to go to a lot of trouble to get their wits scared out of them.
Since the day after Christmas, some 60,000 persons have endured a harsh winter's elements standing in lines - sometimes for as long as two hours - to see the see the current cinema rage playing at the Northpoint Theater near Fisherman's Wharf.
It is an experience that produces reactions ranging from vomiting to fainting, to one apparently jaded patron's demand for his money back because it was about as scary as Snow White". 

The author of the 'Exorcist' letter apparently concurred with this disgruntled patron, beginning the letter with "I saw + think "The Exorcist" was the best satirical comedy that I have ever seen,"
The author also made sure to place "The Exorcist" in quotation marks, just like the article. If this letter was the Zodiac Killer, he would go on to use quotation marks around words in his following two correspondences - something he had never previously done in his communications from 1969 to 1971. 

The middle section of the 'Exorcist' letter is signed yours truley 'Ko-Ko Lord High Executioner' by means of Tit-Willow from The Mikado's On a tree by a river. The same can be said of the 'Little List' letter on July 26th 1970, which also featured Ko-Ko in A more humane Mikado.

Then we travel to the most interesting part of the 'Exorcist' letter at the bottom of the correspondence, and its comparison to the first letters mailed by the Zodiac Killer on July 31st 1969. The 'Exorcist' letter was mailed to the San Francisco Chronicle, so we will take a look at the July 31st 1969 letter mailed to the same paper. This is what the killer signed off with: "I want you to print this cipher on the front page of your paper. In this cipher is my idenity. If you do not print this cipher by the afternoon of Fry.1st of Aug 69, I will go on a kill ram-Page Fry. night. I will cruse around all weekend killing lone people in the night then move on to kill again, until I end up with a dozen people over the weekend." It begins with a demand to print his material in the paper, and finishes with the threat "to kill" again if his instructions are not followed.       

The foot of the 'Exorcist' letter begins with a demand to put this note in the paper, and finishes with a threat you know he is capable of. We know what the killer is capable of - it is "to kill" - as he threatened at the foot of the July 31st 1969 San Francisco Chronicle letter if his demands to print the cipher were not met. Even the decoded cipher rammed home the message "To kill something gives me the most thrilling experience". Therefore, by logical deduction, the strange characters at the foot of the 'Exorcist' letter should read "to kill". Thanks to the great work of Zodiac researcher Kevin Robert Brooks, this is exactly what he showed. Even more importantly, the beginning of each word began with an upper case letter, just like "The Exorcist" - forming the words "To Kill" with significant emphasis on the 'K' of "Kill".

This emphasis would be carried forward into the SLA letter, mailed on February 14th 1974, where he uses quotation marks to highlight the word "kill" once more. The murderer now appears to have slayed his demons, signing off the letter with "a friend" - albeit, a friend you wouldn't invite home for tea and scones with grandma.

His reformation now appeared complete in the May 8th 1974 'Citizen' letter, railing against the publication of advertisements for the "Badlands" movie starring Martin Sheen and Sissy Spacek, and based upon the real life events of spree killers Charles Raymond Starkweather and his girlfriend Caril Ann Fugate.

His final consternation would be reserved for Marc H Spinelli in the July 8th 1974 'Red Phantom' letter, calling for the cancellation of his Count Marco column. The murderer now seemingly devoid of the Zodiac persona after exorcising his demons subsequent to January 29th 1974, along with his inability to spell.

Had the Zodiac Killer returned after an absence of nearly three years, or was the return of the Bay Area murderer a demon that had to be resurrected for the sake of publicity in an ever dwindling case.

Zodiac researcher Mike Rodelli recalled a conversation with Alan Keel, Criminalist at the San Francisco Police Department, San Francisco, California from 1996 to 1999. He stated "In contrast (to earlier communications), Keel analyzes two other letters, one of which is the 1978 forgery, and finds that this letter and one of the 1974 letters are loaded with saliva and cells. He then easily extracts DNA from both of these letters using the more primitive DNA technology of that time and finds that the DNA matches between those two letters, thus proving that one person sent both." 

For these two communications to be matched definitively using DNA, "proving that one person sent both", they must have revealed full DNA profiles, otherwise, a partial DNA profile in one could not have proved conclusive. The DNA tests conducted by the San Francisco Police Department in the late 1990s, indicated that subsequent to 1974, only the January 29th 1974 'Exorcist' letter had been processed and "cells found". There was no mention of the other 1974 letters being processed. Therefore, one could conclude that the two letters discovered to match one another (according to Keel), were the 1974 'Exorcist' letter and the April 24th 1978 letter - and therefore written by the same individual.

The 1978 letter is listed in the report as "not authentic Zodiac letter." If the San Francisco Police Department concluded the 1978 letter was not an authentic Zodiac letter, then the 'Exorcist' letter would not be either. This brings forth some interesting questions.    

Michael Butterfield wrote an article entitled 'Zodiac DNA:The Magic Bullet' on his 'Zodiac Killer Facts' website, stating
"In the late 1990s, San Francisco police obtained DNA from a suspected Zodiac letter, and the DNA did not match Allen’s DNA".. 
The San Francisco Police Department surely wouldn't have used the DNA obtained from the April 24th 1978 letter for comparison, as they themselves had deemed it "not authentic Zodiac correspondence" and it had long been considered questionable material. Therefore, they must have obtained this DNA from another letter they "suspected" was a Zodiac letter, to compare to known suspects. But, if the 'Exorcist' letter (as Mike Rodelli suspects) and the 1978 letter matched in DNA, were written by the same individual, and by association were considered "not authentic Zodiac correspondence", then which "suspected Zodiac letter" did the San Francisco Police Department use to compare to Allen's DNA. 

In the San Francisco Police Department report, the only letters in which they stated "cells found", other than the 'Exorcist' letter, were the July 24th 1970 'Kathleen Johns' letter and July 26th 1970 'Button' letter. Every correspondence tested prior to July 24th 1970 were labelled 'Envelope processed for DNA - few cells'. The 'Exorcist' letter came under scrutiny, alongside the 1978 letter, when Inspector David Toschi fell under the microscope for forging one or both in 1978. So, would it be likely the 'Exorcist' or 1978 letter would be chosen by the San Francisco Police Department in the 1990s as a "suspected Zodiac letter" (and for the reasons given above), and used to rule out Arthur Leigh Allen's DNA? They surely would have turned to more credible sources. But they only had two other letters where "cells were found".

In 2002, Dr. Cydne Holt performed a new round of tests on some of the early 1969 Zodiac letters. Having failed to find suitable DNA on these letters (as suggested by the "few cells" findings of the 1990s), it has now come to light that the ABC Primetime documentary scoured the outside of the envelopes and stamps for DNA instead, thereby negating any findings as to the identity of the Zodiac Killer.

But why were they searching for Zodiac DNA, to compare to suspects such as Arthur Leigh Allen and Kjell Qvale, when: 
"in the late 1990s, San Francisco police obtained DNA from a suspected Zodiac letter, and the DNA did not match Allen’s DNA".
The new round of tests performed in 2002, would now suggest that the DNA they found in the 1990s from a "suspected Zodiac letter", was no longer believed to be from Zodiac. 

In 2018 "Vallejo police Detective Terry Poyser, who has worked the Zodiac case for four years, said his agency has submitted two envelopes that contained letters from the Zodiac Killer for a type of advanced DNA analysis that previously had not been available in the case. The department has three letters and two envelopes from the Zodiac, Poyser said. The envelopes each have a double stamp, which Poyser said was a trademark of the Zodiac. They originally were sent to the San Francisco Chronicle and the San Francisco Examiner, he said. The third letter was sent to the Vallejo Times-Herald, but does not have an envelope with it, he said." Sacramento Bee.

However, these are the July 31st 1969 envelopes and stamps already tested by Dr. Cydne Holt in 2002, which failed to produce any viable results. The hope is that more advanced DNA analysis may turn up something new. But what about the "suspected Zodiac letter" from the 1990s with "cells found." Couldn't this be examined using advanced DNA analysis? This letter has apparently already produced a DNA profile to rule out Allen, so why is it being ignored - or is it?

In a poll of 93 people conducted on this site, 85% believe the 'Exorcist' letter to be genuine Zodiac correspondence, whereas, in a poll of 94 people conducted on this site, only 54% believe the 1978 letter to be genuine Zodiac correspondence.
If Mike Rodelli's suspicions are true, then both the 'Exorcist' and 1978  letter are "not authentic Zodiac correspondence". If the San Francisco Police Department have long considered these two communications as not credible, then which "suspected Zodiac letter" was used to rule out Arthur Leigh Allen in the 1990s, and can it be used again for advanced DNA analysis?

We have long been led to believe the 'Exorcist' letter was the last confirmed communication by the Zodiac Killer. If the poll on this site were true of the entire Zodiac community, then 85% of people believe the 'Exorcist' letter to be genuine Zodiac correspondence. If the other 1974 letters were not tested or found to contain DNA (as the SFPD report suggests), then it is extremely likely a full DNA profile exists for the 'Exorcist' letter (shown at the top of this article).

For those who believe the January 29th 1974 'Exorcist' letter to be an authentic mailing from the Zodiac Killer with a full DNA profile, you should be asking "why hasn't this been run through GEDmatch."