BIDDEFORD — Firefighters responded to an explosion at the Maine Energy Recovery Co.’s trash-to-energy incinerator on Lincoln Street in Biddeford at 9:30 a.m. today..

Fire officials believe the explosion was caused by a propane tank that was mixed in the waste stream, but because the incinerator grinds material before burning it and because of the explosion itself, there was little left for investigators to determine what actually exploded.

Historically, explosions that have occurred at the incinerator have been the result of discarded propane tanks being mingled with the waste stream. The explosions have sometimes resulted in fires at the plant but in this case a fire was not ignited, fire officials said.