SACRAMENTO, Calif. (AP/CBS13) — 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.

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He was found during a headcount 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.

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The Marin County coroner did not immediately provide a cause of death, and corrections officials wouldn’t give more details, citing the investigation.

The Associated Press found in a 2015 analysis that male sex offenders were being killed at a rate double their percentage in the prison population.

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San Quentin, north of San Fransico, is California’s oldest prison. It houses the state’s death row, but also general population inmates.