Cracking the Zodiac Killer’s Cipher – How Mathematicians Finally Solved the Z340 Mystery

Discover how mathematicians cracked the Zodiac Killer Z340 cipher, about half a century of relentless pursuit, and why this case remains one of the most famous unsolved mysteries.

By Stephen Ornes
Dec 18, 2021 4:15 PMJun 19, 2025 8:08 PM
Zodiac killer cipher sent to the San Francisco Chronicle in 1969
Zodiac killer cipher sent to the San Francisco Chronicle in 1969 (Credit: Bettmann Archive/Getty Images)

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Key Takeaways on the Zodiac Killer’s Cipher

  • In the late 1960s, a serial killer self-identifying as “the Zodiac” killed at least five people in Northern California and claimed to have murdered more. The identity of the Zodiac Killer remains unknown. 

  • The Zodiac Killer sent four coded messages in total to the San Francisco Chronicle in 1969 and 1970. The first had 408 characters and was cracked in a week. The recently solved Z340 cipher was the second. After those, the killer sent two very short ciphers, one with 13 characters and the other with only 32.

  • This case has remained one of the most famous unsolved mysteries in the history of American criminal investigations.


Few cases have captured the public's imagination and baffled investigators quite like the Zodiac Killer. For decades, this unidentified serial murderer taunted law enforcement and the public alike with cryptic messages and ciphers, leaving a trail of terror in his wake.

One of the most infamous aspects of the Zodiac's sinister saga was the series of letters he sent to local newspapers, in which he claimed responsibility for his gruesome crimes and taunted authorities with cryptograms. Among these letters, one stood out as a tantalizing puzzle that would endure for years – the Zodiac Killer letter that finally got decoded.

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