One of the four men serving life sentences for the grisly "Zebra" killings that terrorized San Francisco in the early seventies has died.

J.C.X. Simon, age 69, was pronounced dead in his San Quentin prison cell on Thursday night at 11:59 p.m. by California correctional officials. "The cause of death is pending an autopsy. All that can be released at this point is that he was unresponsive in his cell," Krissi Khokhobashvili, a spokeswoman for the California Department of Corrections and Rehabilitation told the Chronicle.

Simon, Larry Green, Manuel Moore and Jessie Lee Cooks were all convicted and sentenced to life in prison for the 'Zebra' murders in 1976. Named for the special radio band the police used in the investigation, the Zebra murders were a string of attacks and killings targeting white people in San Francisco from October of 1973 to the April of 1974. In total, 14 were killed and 8 were hurt, including future mayor Art Agnos.

Simon, himself, was convicted of two counts of first-degree murder, conspiracy to commit murder and assault with a deadly weapon and was given a life sentence with the possibility of parole, which was denied repeatedly.

All four men were incarcerated in separate prisons throughout the state, never being allowed to be housed in the same institution.

Read More: 5 Interesting Facts About The 'Zebra Killings' From The 1970s