(CNN) -- Lawrence Phillips, an imprisoned former NFL running back who was awaiting trial on charges of killing his cellmate last year, died Wednesday morning after being found unresponsive in his prison cell, the California Department of Corrections and Rehabilitation said.
The death of Phillips, 40, is being investigated as a suicide, the department said.
Phillips was a star running back at Nebraska and a first-round pick for the St. Louis Rams in 1996, but lasted in the NFL only three seasons.
He was sent to a California prison in 2008 after being convicted of domestic violence, false imprisonment and vehicle theft charges.
While serving a 31-year sentence, authorities say, he killed his cellmate in April of last year. A trial for that killing was anticipated in Kern County, California.
Phillips was found unresponsive at 12:05 a.m. Wednesday in his cell at the Kern Valley State Prison, authorities said. He was taken to a hospital, where he was pronounced dead at 1:27 a.m.
The Corrections Department didn't say what led investigators to believe he committed suicide.
Phillips rushed for 1,453 yards and 14 touchdowns over three seasons for the Rams, the Miami Dolphins and the San Francisco 49ers in 1996, 1997 and 1999. He added 219 receiving yards and one receiving touchdown.
He also played for the Canadian Football League's Montreal Alouettes and the Calgary Stampeders early last decade, winning the league's championship with the Alouettes in 2002.