SAN BERNARDINO, California -- A deadly tour bus crash in the Southern California mountains left at least eight people dead and dozens injured Sunday, officials said.
The difficult terrain hampered rescue efforts in San Bernardino County, east of Los Angeles, as authorities tried to extricate some of the victims.
Though the early death toll stood at eight, that number is expected to rise, California Highway Patrol Officer Mario Lopez said.
Authorities believe the bus rear-ended a sedan, crashed with a pickup truck pulling a trailer and rolled over about 6:30 p.m. (9:30 p.m. ET).
The incident took place in a mountainous area of State Route 38, between Mentone and Forest Falls, San Bernardino Fire Department spokesman Eric Sherwin said.
At least 42 people suffered injuries ranging from minor to severe, he said.
A parade of ambulances snaked down a narrow, sloping highway Sunday night, waiting to transport the dozens injured.
The highway was reduced to one lane for both inbound and outbound ambulances, which had to contend with a downhill grade and winding roads.
By late Sunday night, all passengers had been removed from the bus, Sherwin said.
The cause of the crash was not immediately known.
Investigators pored over the debris with flashlights early Monday, searching for clues.
The top of the bus above the driver's seat was caved in, and part of the roof was pried open.
CNN's Holly Yan contributed to this report.