BEAVER FALLS, Pa. (AP) - Police don't plan to cite the drivers of a truck and school bus which crashed in western Pennsylvania.

But only because neither vehicle had a driver when they wrecked.

Police in Patterson Township tell the Beaver County Times (http://bit.ly/UBPOwQ ) the incident happened just before 10 a.m. Tuesday when the parked bus began to roll down a hill. Police aren't sure why that happened, because the driver had engaged the parking brake.

The bus rear-ended a parked truck, which also began rolling down the hill alongside the bus, until the truck flipped onto its side. The bus continued on, shearing off one utility pole and hitting another before rolling to a stop a few feet from the porch of a home.

Nobody was hurt.

Patterson Township is about 30 miles northwest of Pittsburgh.

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Information from: Beaver County Times, http://www.timesonline.com/