SAN DIEGO -- The stepson and key witness in the trial of a Ramona man accused of killing his wife and burying her body in their backyard died Sunday, the Medical Examiner's Office confirmed Monday.

Sean Christopher Turner, 25, was found dead in his bed at the home he shared with his grandmother in National City, officials said.

Turner died at 5:55 p.m. Sunday, according to officials at the Medical Examiner's Office who continue to investigate the cause of his death.

Turner's stepfather, Keith Harold Turner, was convicted in December of voluntary manslaughter for the death of Toby Turner in 2005.

Sean Turner testified that he heard the sound of breaking glass, then the sound of something being dragged the night his mother died.

Turner said he saw his stepfather standing over his mother's body putting pressure on her neck with his foot.

Investigators uncovered Toby Turner's remains in 2007 in two shallow graves after Sean Turner told them what he saw.

Sean Turner also said his stepfather told him that he had to choke Toby Turner three times before she died.

His stepfather also asked him to help bury her body, but Sean Turner refused.

Sean Turner said he moved to San Francisco after the killing because he was scared and his stepfather threatened him.

However, he said he returned to San Diego County in 2007 to reunite with his biological father, who persuaded him to go to the police about his mother's death.