At least two of the 27 other people injured in the Sunday night crash remained in critical condition, the Los Angeles Times reported.
The charter bus, carrying a tour group of 38 people from Tijuana, Mexico, apparently experienced brake problems about 6:30 p.m. while on a narrow mountain road leading to the Big Bear resort area near Yucaipa.
The bus hit a car and then swerved into oncoming traffic and collided with a landscaper's truck. The impact sent the bus rolling, ejecting numerous passengers.
"It is a gruesome and horrible scene," Leon Lopez, a spokesman for the California Highway Patrol, told the Times. "It's one of the most horrific scenes I've ever seen in 10 years with the department."
Federal transportation records showed the bus and the company that owns it, Scapadas Magicas of National City, Calif., had been cited repeatedly in recent months for poor maintenance, the newspaper said.
Investigators questioned the bus driver, who the Times said was believed to be at an area hospital.
Highway Patrol spokesman Mario Lopez said Sunday night all of the fatalities appeared to have been people aboard the charter bus.
Firefighters spent more than 2 hours to get all the injured people out of the bus.