A former NFL running back, who was in jail for assault and was recently charged with murdering his cellmate, was found dead on Wednesday from an apparent suicide.

Staff conducting a security check at Kern Valley State Prison in California found Lawrence Phillips unresponsive just after midnight. He was pronounced dead less than two hours later.

Phillips, 40, was originally imprisoned in 2008, sentenced to 31 years for assault and driving his car into three teens. He was accused of strangling his cellmate to death in April 2015, and was in the early stages of a trial for that alleged crime when he died. Phillips, charged with first-degree murder, could have faced the death penalty.

He wrote a letter to his mom in March 2015, just before the alleged murder, expressing anger and even predicting his demise.

“I feel myself very close to snapping,” Phillips wrote in the letter, obtained by USA Today. “My anger grows daily as I have become fed up with prison. I feel my anger is near bursting and that will result in my death or the death of someone else.”

Phillips was a star running back for the University of Nebraska before he was selected sixth-overall by the St. Louis Rams in the 1996 draft. He played three seasons, with the Rams, Miami Dolphins and San Francisco 49ers, scoring 14 touchdowns in 35 games.