A rural transit bus carrying passengers to a program that offers services for people with developmental disabilities and a freight train crashed at an unmarked railroad crossing Friday morning, injuring at least 10 people.

It was not immediately clear if the bus drove into the train, or vice versa, and police were investigating whether dense morning fog contributed to the crash.

The crash occurred in Evans City, about 25 miles northwest of Pittsburgh, at about 8:10 a.m. Friday. Officials say at least two people suffered serious injuries and three others were treated and released from area hospitals. The passengers ranged in age from 26 to 92.

Evans City police Chief Joseph McCombs says it appears the bus entered the crossing and stopped on the tracks. The engineer sounded a horn and attempted to stop, but couldn’t avoid the small bus, which came to rest about 20 feet away.

Police were investigating whether dense morning fog contributed to the crash.