Former NFL running back Lawrence Phillips is scheduled to be officially charged with first-degree murder Thursday afternoon in Kern County Superior Court.

Phillips, who is serving time in Kern Valley State Prison, is accused of killing his cellmate back in April. 

Damion Soward was found unresponsive in his cell after Phillips apparently called a "man down". Soward wasn't breathing and had no pulse.

According to USAToday, Phillips wrote a letter to his mom the month before Soward's death. In it, Phillips, who also starred at the University of Nebraska, said, "I feel myself very close to snapping. 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 is scheduled to be in court at 3 p.m.