The crash occurred near Milepost 228 of Interstate 84 approximately 20 miles east of Pendleton,Oregon.
Following the crash, westbound lanes of I-84 were closed near LaGrande, Ore. through much of Sunday morning and afternoon. The westbound lanes re-opened shortly after sundown.
St. Anthony Hospital in Pendleton treated 26 people from the accident, including some who were treated and then transported to other medical facilities in La Grande and Hermiston, Ore., said hospital spokesman Larry Blanc.
About 40 people were on board the charter bus when it lost control on the ice and snow-covered interstate.
The bus skidded off the highway and plunged down an embankment of about 200 feet.
Initial reports put the death toll at five, but the AP reported Sunday evening that authorities said it had risen to nine.
Umatilla County Emergency Manager Jack Remillard said the bus was owned by Mi Joo travel in Vancouver, B.C., and state police said the bus was en route from Las Vegas to Vancouver.
A woman who answered the phone at a listing for the company confirmed with The Associated Press that it owned the bus and said it was on a tour of the Western U.S.