STILLWATER — A man in prison for murder has been indicted in the death of a Minnesota prison guard who was killed with a hammer.

A grand jury returned the indictment on first-degree murder charges Tuesday against Edward Muhammad Johnson, 42. He’s accused of killing Department of Corrections Officer Joseph Gomm at the Stillwater state prison on July 18.

According to the new charges, Johnson repeatedly struck Gomm 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 initially charged with second-degree murder and assault. He’s now charged with first-degree premeditated murder and first-degree murder of a prison guard.

Minnesota Public Radio reports that state law requires a grand jury to approve the more serious charges. In Minnesota, a first-degree murder conviction carries a life sentence without possibility of parole.

Johnson’s public defender, Laurel Ann O’Rourke, declined comment Thursday except to say Johnson is presumed innocent. Johnson is due in court Friday on the first-degree murder indictment.

Johnson is serving a 29-year sentence for fatally stabbing his roommate in 2002. Gomm, who had just marked 16 years as a correctional officer, is believed to be the first Minnesota correctional officer killed while on duty. His funeral drew thousands of law enforcement officers.

After Gomm’s death, union leaders representing Minnesota corrections workers called for more prison officers.

More than 1,600 inmates are housed at the century-old Stillwater prison. About a third of the prisoners are serving time for homicide.