The accident occurred before 10:30 a.m. PST when the driver lost control on Interstate 84 near Pendleton in an area known as Deadman Pass because it is treacherous in the winter, The (Portland) Oregonian reported. The bus, carrying about 40 people, slammed through a guardrail and tumbled nearly 100 feet down an embankment, police said.
The newspaper said police trained in rope rescues were deployed to help bring passengers up the embankment.
Initial reports had indicated five people had died but the toll grew through the day as the victims were removed from the bus, the newspaper said.
State police said the bus driver survived but hadn't been able to provide any information because of the severity of his injuries.
Umatilla County Emergency Manager Jack Remillard said the bus is owned by MiJoo Tour & Travel in Vancouver, British Columbia, and The Oregonian said an employee, Ryan Choi, said the company has rented out its two tour buses to travel companies for more than a decade.
Choi said the tour was run through another Canadian firm, Royal Tours Co. An unidentified representative for Royal Tours said the bus was headed back to Canada from Boise, Idaho, while the state police indicated the tour had started in Las Vegas.