An explosion tore through an earth-covered bunker at an Army ammunition plant Wednesday, killing one worker, leaving another missing and creating a thunderous blast that could be heard for miles.

Mayor George Killebrew (search) said a third worker was airlifted to a hospital after the explosion at the Milan Army Ammunition plant (search). The factory produces medium-caliber ammo for the Army and employs about 600 people.

The cause was not immediately determined, but FBI agent George Bolds had been told by authorities that the blast appeared to be an accident. It occurred in an area where ammunition is stored.

"As far as we know, they were moving some barrels of some sort of volatile material and one of the barrels flashed," Bolds said.

Darrell Rinks, shop foreman at the Gibson County Highway Department, said he heard the explosion at his workplace some 15 miles away.

Killebrew said the explosion would be difficult for family members of workers at the plant, but added that "when we have the arsenal here, it's something we're always cognizant that could happen."

All three individuals are employees of American Ordnance (search), the company that operates the plant for the Army, said Michelle McCaskill, spokeswoman with the U.S. Army Materiel Command in Fort Belvoir, Va.

The plant, established during World War II, got an $11 million contract in 2003 to load, assemble and pack cartridges used in grenade machine guns.

Milan, a town of about 7,600 people, is about 90 miles northeast of Memphis.