A 66-year-old inmate was found unresponsive in his cell at San Quentin State Prison early Wednesday, and state corrections officials said they are treating his death as a homicide.
John Sullivan had served half of his 10-year sentence from Placer County for failing to register as a sex offender, a second-strike.
He was found during a head count shortly after midnight and pronounced dead less than 40 minutes later.
Officials said they suspect his 28-year-old cellmate in the death. He has not been charged, but was serving a seven-year sentence from Los Angeles County for first-degree burglary and injury to a dependent adult causing death or great bodily injury.