A coded message sent by a brutal serial killer who has never been caught has been cracked more than 51 years after it was sent.

The male suspect, known as the Zodiac Killer, killed at least five people and attempted to kill at least two more in Northern California in 1968 and 1969. In the first three attacks, he targeted couples. The first two murder victims were high school students who were parked in a car on their first date. In attacks on the other two couples, he managed to kill the women, but the men survived. A male San Francisco cab driver was the last known victim.

During the murder spree, the Zodiac Killer sent media outlets a series of letters taking credit for the slayings. To prove the authenticity of the claims, the letters included unreleased details and evidence from the crime scenes.

In August 1969, following the murders of three of the five known victims, the Zodiac Killer sent three almost identical letters to three Bay Area newspapers. Each letter also included one-third of a 408-symbol cryptogram that the suspect said would reveal his identity. The killer demanded the papers publish the letters in full or he would kill again.

A week after the letters were sent, a couple in Salinas, California, cracked the cipher. The Zodiac Killer, the plaintext revealed, said he was collecting slaves for the afterlife and that he wouldn’t disclose his identity because doing so would interfere with those plans.

In November 1969, after killing the remaining two known victims, the Zodiac Killer sent a letter to The San Francisco Chronicle that included a new puzzle. The cryptogram was known as the Z-340, or simply the 340, because it contained 340 characters in it.

Photograph: Getty Images

Ever since then, both amateur and professional cryptographers, including those working for the FBI, have worked to crack the cipher. It wasn’t until this week that an international team solved it.

“The cipher had been unsolved for so long, it had a huge target on its back, and I felt like it was a challenge that had a chance of being solved,” Dave Oranchak, one of the three men who cracked the encoded message, said by email. “It was an exciting project to work on, and it was on many people’s ‘top unsolved ciphers of all time’ lists.”