Story highlights Lawrence Phillips had great potential but demons, too, former college coach tells newspaper

He was in prison for domestic violence and other charges

Phillips, 40, was to stand trial again, accused of killing a cellmate

(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 said, he killed his cellmate in April of last year. A trial for that killing was upcoming in Kern County, California.

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