Phillips was 40. The death is being investigated as a suspected suicide, according to the report.
Phillips entered Kern Valley in 2008 after two separate incidents from 2005 in which he assaulted his then-girlfriend and ran his car into three teenagers at a youth football game. In September, Phillips was charged with first-degree murder after his cellmate, Damion Soward, was found strangled to death inside their shared cell.
Phillips claimed he was acting in self-defense. Tuesday, a judge said there was enough evidence to move forward with a trial, in which Phillips would face the death penalty if convicted.
Phillips was a standout at the University of Nebraska, where he averaged 143.5 yards per game as a sophomore in 1994. During his junior season, he was charged with assaulting his then-girlfriend, a Nebraska basketball player, dragging her down three flights of stairs.
Phillips was the sixth pick of the 1996 NFL draft by the Rams. He was released by the Rams midway through the 1997 season and signed with the Miami Dolphins. Miami released Phillips after he was charged with assaulting a woman at a Florida nightclub. He played for seven teams in five leagues and three countries.