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 motorcoach 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.