The Oklahoma Department of Corrections on Saturday announced the death was considered a homicide and an autopsy has been ordered.
An officer doing a routine security check around 7:30 p.m. Friday, at the prison in McAlester, found the victim unresponsive in his maximum-security cell on C-Unit. Medical staffers were unable to revive the victim.
Investigators believe the cellmate, also serving life behind bars for first-degree murder, was responsible for the death. No charges were immediately filed.
Officials didn't immediately release names of either inmate.
McAlester is 90 miles (144.83 kilometers) south of Tulsa.
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