How Three Amateurs Solved the Zodiac Killer's '340' Cipher

The mysterious code stumped the FBI and NSA for 53 years. What does the key, and the resulting solution, tell us about the infamous murderer?

- KATHRYN MILES

THE ENVELOPE ARRIVED at the San Francisco Chronicle in November 1969 without a return address, its directive to the recipient, in handwriting distinctively slanted and words unevenly spaced, to “please rush to editor.” The Chronicle newsroom had seen the scrawl before, on previous letters sent from the Zodiac, a self-monikered serial killer who threatened to go on a “kill rampage” if the paper didn’t publish his writing on its front page. By the time of the November letter, the Zodiac had already attacked seven people, murdering five. His most recent murder—of a San Francisco cab driver, by gunshot—had occurred just four weeks before this new envelope arrived. The Zodiac had mailed the Chronicle a piece of the victim’s bloodied shirt as evidence of the crime.

The Zodiac’s letters were replete with grisly imagery. He signed his “name” with a crosshairs symbol. He shared haunting details of his attacks. He promised to blow up buses of schoolchildren and unleash a “death machine” on San Francisco. But in addition to these overt threats, he included baffling ciphers for investigators to crack, troubling grids of symbols and letters that presumably masked a secret about his identity, intentions, or victims (to this day, the killer has never been found). The Zodiac’s first cipher, included in the July 31 letter, had been solved within a week by an amateur husband-and-wife team—but it had only revealed more of the killer’s raving. The second, now known as “the 340” due to the number of characters in it, would prove a much more difficult challenge. It came with a letter for the Chronicle, reading in part: