No one was killed, but at least four people were airlifted to hospitals with serious injuries, said Robert Leyba, a spokesman for the Colorado State Patrol. A 17-year-old boy was in critical condition at Swedish Medical Center in Englewood, a spokeswoman said.

Everyone aboard was taken to one of eight Denver and Colorado Springs hospitals for treatment or examination. Most were released Saturday night.

The bus overturned at about 4 p.m. on 10,000-foot Kenosha Pass, about 45 miles southwest of Denver on busy U.S. 285. A three-mile stretch of the highway was closed in both directions for three hours. One lane of traffic was open by 7 p.m.

"I heard a bunch of screaming," said Tyler Haschig of Minneapolis. "We were going pretty fast down a hill and the bus just started fishtailing."

At least five helicopters were sent to the site, said Sgt. Martin Petrick of the State Patrol. Several motorists, including doctors, nurses and a psychologist, offered their assistance.

"There was a lot of crying, obviously, a lot of shock and people helping each other, which was very nice," Petrick said.

The driver, Greg David Wright, 54, of Minnesota City, Minn., suffered moderate injuries and was admitted in fair condition to University Hospital in Denver.

Police planned to arrest Wright after his release from the hospital on 46 counts of careless driving causing injury, Leyba said.

Depending on what the investigation shows, the charges could change or be dropped, he said.

The bus was registered to Minnesota City Bus Service of Minnesota City.

Leyba said the bus was carrying high school students from Burnsville, Minn., to the Frontier Ranch in Buena Vista, operated by a Young Life, a Christian youth organization in Colorado Springs.

Petrick said a preliminary investigation suggested debris in the roadway sliced open a tire, leading to the crash. He said investigators were also considering the possibility that the driver was traveling too fast for the wet road conditions.

Petrick said investigators do not believe alcohol or drugs were a factor.

The bus was lying on its left side on the inside shoulder of a curve, its nose resting in a stand of bent and broken aspen trees.

The State Patrol planned to inspect the 1996 bus Saturday at the scene and then have it towed to Denver for more inspection.

The youths had planned to stay in a hotel in Buena Vista Saturday night, raft on the Arkansas River Sunday and arrive at the ranch later in the day, Young Life spokesman Edward Jackson said.

The ranch, which hosts about 350 campers at a time, was inundated with calls from worried parents and acquaintances.

"We're asking people who are familiar with Young Life and this particular camp to pray with diligence, particularly for the most injured," Jackson said.

It was the second serious bus wreck in Colorado in two years. In December 1999, a bus carrying Texas youths on a church ski trip skidded off an icy road and crashed, killing three and injuring nearly 50.