FORT INDIANTOWN GAP, Pa. (AP) – 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, cast-iron bomb fell on the trail Oct. 13 during a training mission for a pair of A-10 Thunderbolts. The bomb created a crater about 6 inches wide in the trail along an abandoned rail line in Schuylkill County.

At least one hiker was close enough to hear the thud. “It took a while for me to realize what had occurred,” the hiker said in an e-mail to the state Game Commission. “Couldn’t believe it! Retraced my steps. Still couldn’t believe it!”

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

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

The dropped bomb is “a significant concern on all fronts,” Cleaver said. “Whether it’s in peace time training environment or on the front lines of war, when you have a bomb that hangs up, that is a significant safety concern.”

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.

In 1997, officials closed a tank range after rounds ricocheted over the top of a mountain and fell on state game lands. A fisherman reported a near miss from one errant round from a tank gun.

AP-ES-10-23-04 2059EDT