INDIANA, Pa. — Prison officials say a teenager who pleaded guilty to killing his grandparents has committed suicide inside his cell.

Judy White, spokesman for the Pine Grove state prison in Indiana, said the body of 15-year-old Zachary Proper was found Monday. White says Proper used a sheet to hang himself.

The teenager from Oil City was sentenced to 35 to 80 years in prison for shooting his grandparents, George and Dorothy Fross, in October 2012. He pleaded guilty in January.

Proper was alone in his cell.

State police and prison authorities are conducting investigations into Proper’s death.