A bus carrying elementary school children on a field trip has overturned on a freeway near Birmingham, Ala. State troopers say at least four people have been injured.

The accident on Interstate 59 occurred Friday during the morning rush hour, and it isn't immediately clear how it happened. State patrol spokesman David Richards says none of the injuries are believed to be critical.

Authorities have accounted for everyone on the bus. It overturned about 30 miles northeast of Birmingham on its way to Chattanooga, Tenn.