BUFFALO, N.Y. — The Niagara Frontier Transportation Authority and Buffalo Police are investigating what caused a rollover that sent at least four to the hospital late Monday morning.

Officials aren't saying at this point who was at fault for the rollover, or whether speed was a factor, though witnesses on the scene say the bus collided with an SUV at the corner of Clinton and Smith Street before ending up on its side.

According to sources at the scene, the bus was carrying one passenger at the time.

Rural Metro tells YNN four people were transported from the scene to area hospitals, two of them with serious injuries.

Officials say this is still an active investigation. We'll have more details when they become available.