ABC reported that Blake worked on decoding the message using a University of Melbourne supercomputer called Spartan.

“During the year we tested, by trial and error, around 650,000 different reading directions through the cipher. This search turned up — more or less — nothing,” Blake said. “However, one of these searches uncovered a surprising combination of words: GAS CHAMBER. That such a macabre phase should pop up in a sea of noise warranted further attention.

“From this fragment, David, Jarl van Eycke and I reworked the key and corrected an error Zodiac made in his diagonal enumeration of the second vertical segment of the cipher.

“Jarl’s fantastic program, azdecrypt, was essential in this process,” Black said.

The San Francisco office of the FBI on Friday confirmed the group had cracked the coded message, and said the investigation into the half-century-old case was ongoing.

“The FBI is aware that a cipher attributed to the Zodiac Killer was recently solved by private citizens,” the FBI said in a statement posted on Twitter. “The Zodiac Killer terrorised multiple communities across Northern California, and even though decades have gone by, we continue to seek justice for the victims of these brutal crimes.”

No one was ever charged in the Zodiac case, and theories abound as to the killer’s real identity.