J.C.X. Simon, one of the four infamous so-called Zebra killers responsible for a series of racially motivated murders in San Francisco during the 1970s, was pronounced dead after officials found him unresponsive in his cell at San Quentin State Prison. Simon, 69, was in the midst of serving a life sentence when he died alone of unknown causes late Thursday.

Simon was one of four African-American men convicted of killing 14 people and injuring seven others in San Francisco. The four men were convicted of targeting white victims at random in an attempt to “run all the white out of San Francisco,” the Los Angeles Times reported.

“The cause of death is pending an autopsy,” Krissi Khokhobashvili, a representative of the California Department of Corrections and Rehabilitation, told the San Francisco Chronicle. “All that can be released at this point is that he was unresponsive in his cell.”

The police search for Simon and the other Zebra killers -- Jessie Lee Cooks, Larry Green and Manuel Moore -- became so intense that officers stopped black men at random, until a federal judge ruled the practice unconstitutional. The group’s members were dubbed the Zebra killers after the police-radio band (Z as in Zebra) designated exclusively for the investigation of the murders committed between October 1973 and April 1974.

Simon was convicted of two counts of first-degree murder, conspiracy to commit first-degree murder and assault with a deadly weapon, according to the L.A. Times. Each of Simon’s partners in crime also was convicted and handed a life sentence.