A phone message left at Nonlethal Technologies in Homer City was not immediately returned Tuesday.
Indiana County 911 dispatchers say the fire was reported about 10:30 a.m. after at least one explosion was also reported.
It was not immediately clear if anyone has been injured at the plant about 40 miles northeast of Pittsburgh.
The plant is on Route 286, which has been closed in both directions.
A dispatcher says crews were still on the scene about 90 minutes later, but were primarily dealing with smoke as the fire has reportedly been extinguished.