STILLWATER (AP) -- An inmate accused of killing a guard at the Stillwater prison last month has been indicted on first-degree murder charges.

A grand jury in Washington County returned the indictment against Edward Muhammad Johnson on charges of first-degree premeditated murder and first-degree murder of a prison guard.

Department of Corrections Officer Joseph Gomm died after he was attacked with a hammer at the prison on July 18. The 42-year-old inmate was previously charged with second-degree murder and assault.

According to the charges, Johnson struck officer Gomm repeatedly in the head and face with a hammer that he checked out from an industrial workshop on the third floor of the correctional facility.

Johnson was serving a 29-year sentence for a fatal stabbing in 2002.