FRESNO, Calif. - Former NFL running back Lawrence Phillips is suspected of killing his cellmate in a Central California prison, officials said Monday.

Phillips' cellmate at Kern Valley State Prison was found lifeless early Saturday and later pronounced dead, prison spokesman Lt. Marshall Denning said in a statement.

Phillips, 39, was once one of the nation's top college football players at Nebraska. He played for the St. Louis Rams, until being released in 1997 for insubordination. He also played for the Miami Dolphins and San Francisco 49ers.

Phillips is serving a sentence of more than 31 years. He was convicted of twice choking his girlfriend in 2005 in San Diego and later that year of driving his car into three teens after a pickup football game in Los Angeles. It wasn't clear if Phillips has an attorney.

He is suspected of killing Damion Soward, a 37-year-old inmate from San Bernardino County serving 82 years to life for a first-degree murder conviction.

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

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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.