It took 51 years and a machine to solve the Zodiac Killer's second cipher after the first was cracked within days by humans.

The code, named Z340, was cracked by David Oranchak, Sam Blake, an applied mathematician, and Jarl Van Eyke, a computer programmer along with their online community and software that Van Eyke created. Oh, and some luck, too.

“By luck,” Blake said on Iflscience.com, “we discovered that (Zodiac) split it into three pieces and rearranged the message in a predictable diagonal pattern in the first two pieces.” The message was contained in the first two parts, but did not follow a left-to-right pattern like the first cipher.

Probably indignant that the first cipher was solved so quickly, Zodiac made the second one much harder.

Cipher solved in 2020

The second cipher was cracked back in 2020, but has not been vetted and confirmed until now.

“On December 5, 2020, the FBI received the solution to a cipher popularly known as Z340 from a cryptologic researcher and independently verified the decryption,” an FBI statement said.

“This cipher was first submitted to the FBI Laboratory on November 13, 1969, but not successfully decrypted,” the statement continued. “Over the past 51 years CRRU [the FBI Cryptanalysis and Racketeering Records Unit] has reviewed numerous proposed solutions from the public – none of which had merit.”

A whitepaper was published explaining the cipher's solution in late March.

The infamous killer

The Zodiac Killer was known to have killed at least five victims in Northern California in 1968 and 1969, but is rumored to have killed as many as 49.

He claims to have killed 37, and left clues at the crime scenes as well as taunting police with the ciphers and mailing clothing to the San Francisco Chronicle.

He threatened to shoot school buses full of children and threatened to kill more people if the newspapers didn't publish the ciphers, which they did.

He was never caught or even identified, at least not definitively.

In 2021, a group of private crime-solvers claimed they identified the Zodiac Killer as Gary Francis Poste, who died in prison in 2018.

Police have not confirmed that Poste was the Zodiac killer, however.