PORTLAND, Oregon (Reuters) - Federal safety investigators joined the Oregon state police on Monday in seeking to determine what caused a tour bus to careen off a mountain highway and plunge down a snow-covered slope, killing nine passengers and injuring at least 27 others.
The charter bus was carrying about 40 people through the Blue Mountains of northern Oregon en route from Las Vegas to Vancouver, British Columbia, when it crashed through a guard rail on Interstate 84 on Sunday morning, authorities said.
The motor coach rolled over at least once on the way down the embankment before coming to rest upright in the snow at the bottom of a hill, according to the National Transportation Safety Board.
Photos of the crash scene posted by the state police showed a portion of the vehicle's roof appeared crushed and one side of the bus torn open.
The driver survived the wreck but had yet to be interviewed by investigators as of Sunday night due to the severity of his injuries, police said.
Authorities said they could not immediately explain what triggered the accident, which occurred near the town of Pendleton, about 200 miles east of Portland.
Attention turned early on to road conditions. The Oregon State Police said on Sunday that a preliminary investigation showed the bus "lost control on the snow/ice covered westbound lanes of Interstate 84."
A spokesman for the state Transportation Department, Dave Thompson, told Reuters on Monday that the area where the crash occurred "is well known for treacherous conditions in winter."
Road surfaces there at the time were generally were "icy in spots, with some areas of packed snow, and that's absolutely typical for the area at this of the year."
Thompson said road crews had been out spreading sand in the area and that one sand truck "was just turning around after having passed the bus" on the highway shortly before the crash, but he said he did not know the condition of the highway surface where the bus veered off the interstate.
The NTSB, which has made improved motor coach transportation safety a top priority, said Monday it was sending a team to investigate the accident. A spokesman for the agency in Washington, Terry Williams said, "We'll be looking at the man, the environment and the machine."
In addition to nine passengers confirmed dead from the crash, more than two dozen people were injured. The NTSB put the number of injured at 27, including the driver, but reports from area hospital suggested the number of people hurt was higher.
St. Anthony Hospital in Pendleton initially received 26 surviving patients from the crash scene, and six of them were transported to larger hospitals in the region because they required a "higher level of care," spokesman Larry Blanc said.
Blanc said 11 other people from the wreck were sent from the scene to various other hospitals, but he had no information about their conditions.
Of the 20 patients treated at St. Anthony, seven were discharged by Monday morning, one remained in serious condition in the hospital's intensive care unit, and the remaining 12 patients were listed in fair condition, Blanc said.
According to the NTSB, more than 250 people were killed and 20,000 injured in bus-related crashes in 2009.
(Reporting by Teresa Carson; Additional reporting and writing by Steve Gorman; Editing by M.D. Golan)