TEMPE, Ariz. — The Tempe Fire Department is investigating what caused a city bus to erupt in flames Monday night. The exact cause is not known but officials believe the fire's origin was mechanical or electrical in nature.

The bus was one of Tempe's Orbit buses, which offers a free route around the city. It runs on an alternative fuel of compressed natural gas. The bus was not in service and had no passengers. It was returning to the East Valley Bus Operations and Maintenance facility when the operator noticed a fire in the back of the bus.

Upon responding to the dramatic scene Monday night near the Loop 202 and Priest Drive, fire crews first had to take a defensive approach because of the involvement of compressed natural gas (CNG) tanks. Flames were shooting from the roof of the bus wh

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