A 23-year-old inmate beaten to death at the Oklahoma State Penitentiary had been put in a cell with convicted killer he had testified against.
Prison spokesman Jerry Massie says Paul Duran Jr. fought with one cellmate and then was put in a cell with Jessie James Dalton.
Duran was found beaten to death about 15 minutes later.
Massie says the two were not supposed to be put in the same cell and prison officials are trying to determine how it happened.
Duran and Dalton were co-defendants in the January 2002 shooting death of Billy Wayne Ray in Oklahoma City.
Duran pleaded guilty to a robbery charge and testified against Dalton who was convicted of murder and sentenced to life without parole.
Massie says investigators will present their findings to a district attorney who will decide if criminal charges will be filed.