STILLWATER — Prosecutors said they will seek first-degree murder charges in the July 18 death of a Minnesota prison guard.

Department of Corrections Officer Joseph Gomm died after he was attacked with a hammer at the Stillwater prison.

Inmate Edward Johnson was charged with second-degree murder and assault, but prosecutors said Friday they’ll convene a grand jury and seek more serious murder charges.

The 42-year-old Johnson said little at his first court appearance Friday, but answered “Yes” and “No” to questions from a judge.

Washington County Attorney Pete Orput called Gomm’s killing “completely senseless” and said authorities might never know what triggered the attack, which happened in an industrial building.

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