NORTH BETHESDA, Md. -- Kids and their parents who spent Wednesday sightseeing in the nation's capital were headed home to Pennsylvania when their bus plunged off a highway and rolled, killing the driver and injuring at least a dozen, authorities said.

The Pennsylvania-based Wolf's Bus Lines Inc. bus careened some 45 feet off a skyramp of the Capital Beltway and stopped below along Interstate 270 in Bethesda, a suburb of D.C., said Maryland State Police Sgt. Michael Brennan.

Some on the bus had been to the National Mall and National Zoo, according to state police spokesman Greg Shipley, and a pastor said some were students at St. Patrick School in Carlisle, Pa.

Troopers believe the bus crashed through a guard rail, hit a concrete barrier on the ramp and plunged down a hill, Shipley said. It rolled once and came to rest with its wheels on the barrier. What caused the crash may not be known for days or weeks. The National Transportation Safety Board is investigating.

Dr. Barton Leonard, an emergency physician at Suburban Hospital where eight patients were taken, said considering how far the bus fell, "I'm surprised there weren't more severe injuries."

Earlier, television footage showed crews in yellow jackets and helmets walking up ladders and crawling into the holed-out windows of the heavily damaged white bus. Rescuers spread out tarp on the roadway where a woman lay with a brace around her neck and held a compress to her head.

Some children sat in the triage areas surrounded by backpacks, including a girl with a white bandage wrapped around her head. A firefighter could be seen carrying a child in his arms, while others were taken away on stretchers.

Crews had to extricate several people from the "limousine-style tour bus," said Montgomery County fire department Assistant Chief Scott Graham. Police dogs for a time searched to make certain no one was ejected.

The crash occurred about 4 p.m. as the afternoon rush hour started to build, creating a massive traffic jam northwest of Washington. Shipley identified the driver as 66-year-old Joseph A. Clabaugh Jr. of Hanover, Pa. His body was taken to the medical examiner's office in Baltimore.

Twelve people were taken to area hospitals, two with life-threatening injuries, Graham said. A 13th person refused treatment.

Three of the four children treated at Suburban were siblings, ages 9, 11 and 12, said spokeswoman Ronna Borenstein. The fourth was a 6-year-old girl whose mother also was a patient and had chaperoned the children on a trip to the zoo. She said other passengers were visiting museums in Washington.

The Rev. William C. Forrey, pastor at St. Patrick School in Carlisle, Pa., confirmed that some of his students were on the bus and that he they had "some bumps and bruises" and some got stitches, he told The Sentinel newspaper of Carlisle.

"Our thoughts and prayers are with not only the students who were on the bus from St. Patrick's, but all of the people who were on the bus, especially to the man who passed away," he said.

Leonard did not outline the injuries, but said some patients might be released later. Among the patients were a mother and daughter. Two adults were in fair condition and two other adults and four children were in good condition.

"They are in good spirits," said Leonard, who personally cared for some of the patients.

A person who answered the telephone at Wolf's Bus Lines Inc. said she had no information about the crash. Federal Motor Carrier Safety Administration spokeswoman Candice Tolliver wrote in an e-mail that the bus line got a satisfactory rating in August.

Montgomery County Council President Nancy Floreen said she was shocked to learn of what she called probably the worst accident in her 10 years in local public office.

"Our hearts and prayers go out to the families of those people, those children," Floreen said.

I-270 reopened to traffic late.