LAWTON, Okla. (AP) _ An Air Force fighter jet on a training exercise dropped a 500-pound bomb too soon, killing an Army officer and injuring eight servicemen on the ground at the Fort Sill Army base.

Army safety inspectors today began trying to determine why the 500-pound bomb was dropped in error Tuesday. It came from an OA-10 Thunderbolt II attack plane assigned to the 917th Wing Air Force Reserve at Barksdale Air Force Base in Bossier City, La.

The bomb struck about five yards behind five military vehicles parked at an observation post atop a hill, Army Capt. Ralph Bivona said. The blast destroyed two Humvees.

The inspectors from the Army's Safety Center at Fort Rucker, Ala., will look into whether the accident was caused by faulty training procedures or human error, Fort Sill spokesman Daran Neal said.

The explosion killed Capt. Christopher Williams, 29, of Houston, commander of Battery A, 2nd Battalion, 17th Field Artillery.

Four reservists from the 1st Battalion, 14th Marine Regiment and four active duty soldiers at Fort Sill were injured. Five were hospitalized in good condition; three of the Marines were treated and released.

The injuries included burns, scrapes, bruises, cuts and concussions, Neal said.

The OA-10s are flown in pairs and used daily in training exercises, Gramlick said. One pilot was aboard each fighter plane.

During the training, ground troops practice calling in aircraft to bomb targets, Army officials said.