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STERLING, Mass. (AP) — Massachusetts officials say an unoccupied school bus was destroyed when it caught fire before its morning commute.

Fire officials responded to the fire in a private lot in Sterling around 8:30 a.m. Thursday, but by the time they arrived the bus was fully engulfed in flames.

Fire Chief David Hurlbut says the bus driver had started the Wachusett Regional School District bus so it could warm up for the morning run. No injuries were reported.

Officials say the driver believes the cause was a heating unit in the back of the bus. An investigation is ongoing.