The Pennsylvania-based Wolf's Bus Lines Inc. bus careened some 45 feet off a high-occupancy skyramp of Interstate 270 on Wednesday afternoon and stopped below along I-270 in Bethesda, a suburb of D.C., said Maryland State Police Sgt. Michael Brennan.
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."
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.
Federal investigators on Thursday will begin looking into the records of the company and driver, the work schedule and condition of the vehicle and roads, said National Transportation Safety Board member Robert Sumwalt. The cause of the crash would not be determined for a year or more, he said.
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. Graham said early Thursday that as of midnight Wednesday, the victims remained at area hospitals but he did not know whether they were still admitted or if they were waiting for family members.
Three of the four children treated at Suburban were siblings, ages 9, 11 and 12, spokeswoman Ronna Borenstein said. 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.
Federal Motor Carrier Safety Administration spokeswoman Candice Tolliver wrote in an e-mail that Wolf's Bus Lines Inc. got a satisfactory rating in August.
Associated Press writers Natasha Metzler in Bethesda, Md., and Jessica Gresko, Kathleen Miller and Nafeesa Syeed in Washington contributed to this report.