Rescuers faced the challenge of bringing survivors 200 feet up a steep cliff, Pendleton Fire Chief Gary Woodson said. They descended the hill and used ropes to help retrieve people from the wreckage in freezing weather.
Some survivors were carried on backboards by six or eight rescuers. Others were hoisted in baskets, and an all-terrain vehicle arrived toward the end of the operation, Woodson said.
Officials said 39 people were taken to hospitals, and 10 of them had been treated and released.
The National Transportation Safety Board said two investigators were expected to arrive at the crash site Monday. They will look into why the bus left the road, the condition of the road at the time, the condition of the guardrail, the actions of the driver, and the operations of the company that owns the bus, the agency said.
The Oregonian newspaper quoted one survivor, 25-year-old Yoo Byung Woo, as saying he and some other passengers thought the driver was "going too fast."
"I worried about the bus," Yoo said, adding it was snowing and foggy at the time. He said one rider was frightened and asked if they could take another route. Some passengers were dozing when the driver slammed on the brakes.
Yoo said rocks smashed through windows after the bus crashed through the guard rail and rolled down. The NTSB said the bus rolled at least once.
Umatilla County Emergency Manager Jack Remillard said the bus was owned by Mi Joo Tour & Travel in Vancouver, B.C.
A bus safety website run by the U.S. Department of Transportation said Mi Joo has six buses, none of which have been involved in any accidents in at least the past two years.
A spokesman for the NTSB, Peter Knudson, said seatbelts aren't required on such buses. "We have been concerned about this for some time," Knudson said.
The local Red Cross shelter has been offering food, clothing and hotel arrangements for survivors as they are released from the hospital. Passengers' relatives also have gone to the shelter, seeking information about their loved ones.
Jake Contor, a Pendleton resident who speaks Korean and helped translate for the Red Cross, said he had spoken with several of the survivors.
"The stories have been fairly consistent: braking, swerving, sliding on the ice, hitting the guardrail, then sliding down the embankment," Contor said.
He said the victims told him the bus left Boise, Idaho, on Sunday morning and was supposed to arrive in Vancouver that night. The survivors who spoke to Contor were seated at the back of the bus and said it appeared that the front and center of the coach sustained the most damage.
The interstate links Boise and Portland through the Blue Mountains and the Columbia Gorge.
AP correspondent Jeff Barnard in Grants Pass, Ore., contributed to this report.