Military officials are investigating why a jet fighter accidentally dropped a 25-pound practice bomb on a hiking trail a mile from its intended target in southeastern Pennsylvania.

No one was injured when the grapefruit-sized bomb fell on the trail Oct. 13 during a training mission for a pair of A-10 Thunderbolts (search). The bomb created a crater about 6 inches wide in the trail.

The plane that dropped the bomb was assigned to the 111th Fighter Wing (search) at Willow Grove Naval Air Station, said Col. Chris Cleaver, spokesman for Fort Indiantown Gap, an 18,000-acre military training site managed by the Pennsylvania National Guard (search).

The plane has been grounded while officials investigate what caused the bomb to drop incorrectly. Investigators have already ruled out pilot error as the cause, Cleaver said.

The National Guard has been seeking permission to acquire 2,100 acres of Pennsylvania Game Commission land to act as a buffer zone for a new tank range. The base is located about 25 miles northwest of Harrisburg.