Cellmate detained after inmate convicted of sexual assault found dead at California prison

California prison officials are investigating the death of an incarcerated man at California State Prison, Sacramento, as a homicide, authorities announced Wednesday.

Staff found 49-year-old John Cisneros unresponsive in his cell at about 9:15 p.m. Feb. 23, according to the California Department of Corrections and Rehabilitation (CDCR).

Medical personnel began life-saving measures, but he was pronounced dead just before 10 p.m.

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Cisneros' cellmate, 34-year-old Irvin Sanchez, was detained and removed from the cell.

He has since been placed in restricted housing pending the outcome of an investigation by the prison's Investigative Services Unit and the Sacramento County District Attorney's Office.

The Office of the Inspector General has also been notified.

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The Sacramento County Coroner's Office told Fox News Digital the cause and manner of death remain pending and that final reports could take at least six months from the date of death to complete.

Cisneros was originally sentenced in 2015 to five years for second-degree robbery. 

While incarcerated, he was later sentenced in 2017 to 28 years for "penetration, oral copulation and attempted rape with force/violence/fear of immediate bodily injury," according to CDCR.

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Sanchez is serving an 18-year sentence for attempted second-degree murder with gang and gun enhancements.

California State Prison, Sacramento, houses more than 2,200 medium-, maximum- and high-security incarcerated men.