FOLSOM (CBS SDF) — A man convicted of first degree murder in Marin back in 1983 has been found dead in his cell at California State Prison-Sacramento, state prison officials announced Wednesday.

Prison officials and the Sacramento County District Attorney’s Office are investigating the death of 53-year-old Darryl Staples that occurred Monday as a homicide.

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Correctional officers found Staples unresponsive in his cell at 8:20 p.m. Custody and medical staff began lifesaving measures, but he was pronounced dead at 8:48 p.m.

Staples was serving a life-with-the-possibility-of-parole sentence from Marin County for first-degree murder and had been in prison since June 10, 1983.

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Staples’ cellmate, Rambo M. Martin, 26, has been identified as a suspect.

Martin is serving an eight-year sentence from San Bernardino County for assault with a firearm, his second strike. He is also serving a four-year sentence from Kern County for assault by a prisoner with a deadly weapon. Martin has been in prison since November 2013.

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CSP-SAC is a maximum-security prison that houses nearly 2,300 general population inmates and employs about 1,700 people. The institution houses inmates serving long sentences and those who have proven to be management problems at other institutions.