Authorities are investigating what caused an early morning bus fire in Manhattan.

No injuries were reported after the Metropolitan Transportation Authority bus caught fire around 6:45 a.m. Thursday at the corner of Lexington Avenue and East 46th Street.

Transit officials say there were no passengers when the bus burst into flames, spewing heavy gray smoke and flames and blackening part of the vehicle.

MTA spokesman Christopher McKniff says the fire started in the back of the bus.