One adult remains in critical condition following a collision involving a loaded school bus and a tractor-trailer that killed the truck's driver in western Pennsylvania.

Authorities are still trying to determine why 31-year-old Robert Dodson Jr. left his lane Wednesday afternoon, colliding nearly head-on with a Turkeyfoot Valley Area School District bus on a rural Somerset County road.

State police say 21 students and two adults were on the bus. One adult was listed in critical condition Thursday morning, with another adult and two children in fair condition.

One other student was flown to Children's Hospital of Pittsburgh and others were treated at a nearby hospital.

Police say the bus was returning from a technical school in Somerset when the crash occurred.

Coroner Wallace Miller says an autopsy is planned for Dodson.