A former NFL running back is suspected of killing his cellmate in the California prison where he is serving decades for a number of criminal charges.

Lawrence Phillips, 39, is accused of killing his cellmate at Kern Valley State Prison, after Damion Soward, 37, was found unresponsive in his cell just before 1 a.m. on Saturday, officials said.

Phillips is serving a 31-year sentence for multiple charges, including spousal abuse and auto theft, officials said. He is now in a segregation unit of the prison, according to the California Department of Corrections.

Phillips was once one of the nation's top college football players at Nebraska. He was drafted 6th overall by the St. Louis Rams in the 1996 NFL Draft. St. Louis even traded Jerome Bettis, because they viewed Phillips as the superior running back.

He only played in 25 games and scored 12 touchdowns for the Rams while Bettis had six consecutive 1,000-yard seasons with the Pittsburgh Steelers and was elected to the Pro Football Hall of Fame this year. Phillips also had short stints with the Miami Dolphins and San Francisco 49ers.

In 2005, jurors convicted Phillips of choking a former girlfriend twice in San Diego, once into unconsciousness. He was also convicted of hitting three teenagers with his car in Los Angeles.

Soward had been serving an 82-year sentence on a 2009 conviction on first-degree murder charges in San Bernardino County.

It wasn't clear if Phillips had an attorney.

Meanwhile, prison officials said they suspect a second inmate at the prison of killing his cellmate. It's unclear if the two deaths are related.

John Munoz, a 24-year-old sentenced to 42 years on for sexual battery and other charges in Los Angeles County, is suspected of killing cellmate, 41-year-old Rattanak Kim, who was serving a sentence of 35-years to life for conspiracy to commit murder in San Diego County.

Prison officials are investigating both deaths along with the Kern County Coroner and Kern County District Attorney, prison officials said.