A man sentenced to death for the 1995 torture slaying of an Orange County teenager died early Monday of unknown causes, prison officials said.

Noel Jesse Plata, 45, of Santa Ana, was found unresponsive in his cell at San Quentin State Prison before 4 a.m. and pronounced dead about a half-hour later despite employee’s attempt at life-saving measures and outside medical assistance, they said.

No foul play is suspected, but officials said his death is being investigated. The cause of death will be decided by a coroner.

He and an accomplice, Ronald Tri Tran, were sentenced to death for the first-degree murder of Linda Park, 18. She was a freshman at Irvine Valley College, according to the Los Angeles Times.

The two gang members tortured her into revealing where her family kept its valuables, then strangled her.

Plata was already in prison serving a sentence of life with the possibility of parole for another first-degree murder when he was convicted in Park’s slaying.

There are currently 710 people on California’s death row, the nation’s largest, though Gov. Gavin Newsom has imposed a moratorium on executions so long as he is governor.