On April 20th 1970, the Zodiac Killer mailed a 13-Symbol cipher to the San Francisco Chronicle, in addition to a bus bomb diagram threatening to blow up a school bus.
This letter, including the Zodiac Killer's coded reply to the newspapers, was very likely prompted by Professor Donald C. B. Marsh, who told the San Francisco Examiner on October 22nd 1969: "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 Allan 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". See article. The correspondence began; "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you? My name is...."
"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".
He also included the bus bomb diagram (see right). WORDING ON DIAGRAM "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 " which makes B the cloudy day discon--ect so the bomb won't go off by accid. PS I hope you have fun trying to fiygure out who I killed". This diagram detailed how the Zodiac Killer was prepared to blow up a school bus by remote control. It employs the use of photoelectric switches, so that when the bus passes by it blocks the light and detonates the explosives at window height. Experts who have examined the diagram have stated the bomb would function. However, its ability to recognize a school bus as opposed to any other tall vehicle would be problematic.
In the letter, 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 1 1/2 inch fence staples exploded on an outside ledge of Park Police Station in the Upper Haight neighborhood. The uncle of Brian McDonnell was Sergeant Joseph Emmett Lacey (40), murdered on December 30th 1956 while socializing with a friend at Eddies' Cocktail Lounge at 502 Ellis Street, San Francisco. The two were enjoying a drink when two armed men burst into the establishment with the intention of robbing it. Sergeant Lacey, despite being off-duty at the time, drew his gun and attempted to put a stop to the robbery, but was beaten to the draw and sadly killed. One of the robbers was apprehended shortly after the crime and plead guilty to first-degree murder. The actual shooter of Sergeant Lacey remained a free man for nearly three years, before being captured and found guilty of second-degree murder. Fourteen years after the murder of Joseph Lacey, the end of watch would unfortunately befall his nephew, Brian McDonnell. Whether this historical case was mentioned in the newspapers to some degree in relation to the Brian McDonnell murder is unknown - but if it was featured and noted by the Zodiac Killer, the sentence of "there is more glory in killing a cop than a cid because a cop can shoot back" would certainly have more meaning with respect to the April 20th 1970 communication (see foot of page).
Fence staples used in the bomb at the Park Police Station
The code has a degree of symmetry in its design in the form of 8 alphabetical characters, three identical symbols at the center which appear to be circled 8's, and two characters arranged four in from either side (crosshairs and an Anchor). Some people have considered that the circled 8's are actually circled Taurus symbols. This carries a measure of credibility because the Zodiac Killer may have chosen his moniker based on the 12 signs of the Zodiac horoscope and therefore including them in his ciphers may not be too surprising.
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). This news was likely what prompted the Zodiac Killer to open this communication with the words "I am mildly cerous as to how much money you have on my head now". 
 Click to enlarge
Harvey Hines, a retired law enforcement officer from Groveland, pitched in with his theory on the cipher with respect to Lawrence Kane, one of the high profile suspects in the Zodiac case. Harvey Hines, who retired from the California Police Department in 1992, had an avid interest in the Donna Lass case and remained resolute in his determination to solve her disappearance by forming a close association with the Lass family members. Along with Mary and Don Pilker, the sister and nephew of Donna Lass, they became convinced that the Zodiac Killer was responsible for the abduction and probable murder of the young nurse in South Lake Tahoe. Harvey Hines believed Lawrence Kane to be responsible, and highlighted the name Kane in the 13-Symbol cipher to bolster his arguments, which was clearly evident on the left hand portion of the code. He claimed that the three 8's when totalled up, pointed to the killer's birth year of 1924, which unsurprisingly corresponded with the birth year of Lawrence Kane of April 26th 1924. To watch a video on this subject click here.
One researcher who has dedicated his time and energy to the Zodiac ciphers is Dave Oranchak of the hugely successful website Zodiac Killer Ciphers. This should be your first port of call when analyzing the various codes and ciphers, explaining why we are the 'Most Pattern Seeking Animal of All'. In many pages on this site we have discussed how the Zodiac Killer had a penchant for responding to the newspapers - sometimes within days - to either counter claims made about him, or to supply extra details about his crimes in the Bay Area and beyond, with this correspondence no exception. Michael Cole, an avid Zodiac researcher and host of the Zodiac Killer website The Zodiac Revisited, produced an interesting article entitled My Name is Cipher Motivation, showing where the inspiration for this correspondence may have originated. Here is a short excerpt: "The article of interest, however, wasn't one of these front page stories; rather, it was an article relegated to page nine, entitled Cipher Expert Dares Zodiac To Tell Name, the story described "a challenge perhaps unique in the annals of American crime...." Read more. 
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'13 Symbol' cipher. Click to enlarge letter.
Bus bomb diagram. Click to enlarge.
Chief Al Nelder speaks to Phyllis McDonnell
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PHONE CALLS RECEIVED MARCH 18TH 1970
The San Francisco Examiner,
Tuesday, 17th February 1970
SAN FRANCISCO CHRONICLE, DECEMBER 31ST 1956
