SACRAMENTO, Calif. (AP) — Former NFL running back Lawrence Phillips was found dead in his California prison cell early Wednesday, and officials said they suspect suicide.

Guards at Kern Valley State Prison found Phillips, 40, unresponsive in his cell shortly after midnight. He was taken to an outside hospital and pronounced dead about 1:30 a.m., the Department of Corrections and Rehabilitation said.

He had been housed alone in a segregation cell since April 2013 after he was suspected of killing his cellmate. A Kern County judge had ruled Tuesday that there was enough evidence to try Phillips in the death of Damion Soward, 37.

Phillips played for the St. Louis Rams before being released in 1997 for insubordination. He also played for the Miami Dolphins and San Francisco 49ers.

He was once one of the top U.S. college football players at Nebraska.

Phillips was sent to prison in 2008 to serve a sentence of more than 31 years 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.

He was sentenced for inflicting great bodily injury involving domestic violence, corporal injury to a spouse, false imprisonment and vehicle theft.

Phillips was housed with about 3,900 other inmates in the prison about 80 miles south of Fresno in California’s Central Valley.