SUMMIT TWP — A school bus driver was flown by medical helicopter to a Pittsburgh hospital Wednesday after his bus crashed into the rear of a tanker truck on Route 422, state police said.

The wreck occurred about 12:45 p.m. at the four-lane highway's intersection with Bonniebrook Road.

It took rescue crews about an hour to extricate bus driver Daniel C. Funk, 67, of Sharon, Mercer County, before he was taken to Allegheny General Hospital.

Police said Funk was in critical condition Wednesday night. His condition this morning was not available.

At least four high school-aged students from the Indiana School District in Indiana County were on the bus, authorities said. Those students were taken to Butler Memorial Hospital by private vehicle for precautionary reasons.

Trooper Dan Kesten said a teacher for the school district was following the bus at the time of the accident and took the students to the hospital.

The truck driver also was injured and taken to Butler Memorial, although the extent of his injuries were not known at the scene.

Police said the tanker, which was empty, was stopped at a red light at the intersection in the westbound lanes when the bus rear-ended it.

The force of the collision pushed the truck about 77 feet forward, investigators said.

Police are investigating what caused the flat-front bus to hit the back of the rig.

Troopers were unable to speak to Funk, who was conscious but disoriented after the wreck.

Kesten said officers would speak with the students for additional information on the crash.

“We don't know a whole lot about what happened right now,” he said at the scene.

Kesten said Funk was wearing a seat belt.

“Seat belts can do a terrific job,” he said. “This is a good depiction on how a seat belt can be effective.”

The bus was from the Hermitage School District and was taking the Indiana students to a music event at a Hermitage district school.

The rig was from the Safely-Kleen company of Breslau, Ontario.

Police at the scene were not sure if charges would be filed.

Traffic in both directions on Route 422 was backed up for miles as emergency crews worked to extricate the driver. The intersection of the road was shut down for more than an hour to allow the medical helicopter to land.

The helicopter left the scene about 2:20 p.m.

The responding emergency units were Herman, Chicora, East Butler, South Butler, Lick Hill and Butler city fire department.

Butler and East Butler ambulances also responded.

The last unit left the scene at 3:38 p.m.