Imprisoned former NFL running back Lawrence Phillips committed suicide while he was awaiting a trial that could have brought him the death penalty, a California coroner said Friday.

The Kern County coroner's office released no other details.

Phillips, 40, was found unresponsive at Kern Valley State Prison and died at a hospital early Wednesday.

He had been housed alone since April after he was suspected of killing his cellmate, Damion Soward, 37, the cousin of former University of Southern California and NFL wide receiver R. Jay Soward.

His attorney, Jesse Whitten, said his client seemed upbeat a day before his death even though a judge ordered him to face trial on a charge of first-degree murder. A conviction could have led to the death penalty, but prosecutors said no decision on that had been made.

The death will be reviewed by the federal official who controls the prison medical system, along with federal officials and lawyers involved in a long-running lawsuit over the care of mentally ill inmates.

Phillips was sentenced to more than 31 years in prison in 2008 after he was convicted of twice choking his girlfriend in 2005 in San Diego and of driving his car into three teens later that year after a pickup football game in Los Angeles.

Phillips went from a West Covina, California, group home to become a star running back at the University of Nebraska. But he pleaded no contest to beating a former girlfriend hours after a spectacular performance in a September 1995 win at Michigan State University.

He was reinstated after a six-game suspension and inpatient therapy for anger management.

He was drafted No. 6 overall by the St. Louis Rams in 1996, but released the next year for insubordination. He also played for the Miami Dolphins and San Francisco 49ers but was out of the NFL by 2000.