It took 51 years to crack, but one of the taunting messages written in code and attributed to the Zodiac Killer has been solved, according to the F.B.I.

The mysterious 340-character cipher, which was mailed to The San Francisco Chronicle in November 1969, does not reveal the killer’s identity. But it does build on his image as an attention-seeking killer who reveled in terrorizing the Bay Area in the late 1960s.

“I hope you are having lots of fun in trying to catch me” and “I am not afraid of the gas chamber” are two of the dark boasts in the message, according to David Oranchak, a software developer in Virginia who said he had decrypted the cipher with the help of Sam Blake, an applied mathematician in Melbourne, Australia, and Jarl Van Eycke, a warehouse operator and computer programmer in Belgium.

Mr. Oranchak, who runs a website and YouTube series about the Zodiac Killer’s ciphers, said he was excited to have solved the code after 14 years of trying to break it. But he said he was also worried about the effect it might have on victims’ families.

“The message in that cipher — I don’t see it as being helpful to them,” he said. “It’s more of the same junk that the killer liked to write about. It’s just intended to hurt people and make them afraid.”