MAINE — The fire and subsequent explosion at a Maine lumber mill that killed a firefighter and injured a dozen other people was accidental and originated at the base of a silo, authorities said Tuesday. …Rapid ignition of particulate material resulted in an explosion in the silo that caused it to lift from its concrete base and release large amounts of sawdust and other materials, the Maine State Fire Marshal’s Office said. The silo then toppled and the surrounding area became engulfed in fire, the office said. Investigators will return to the facility in the coming months to conduct a more detailed examination, the fire marshal’s office said. “Investigators also determined the facility’s fire suppression system, which was located near the top of the silo, did not activate because temperatures at that elevation did not reach the activation threshold after the fire originated at the base of the silo,” the office’s statement said.