(CNN) - 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.
Lopez identified the bus company as Scapadas Magicas of National City, California. The company did not immediately return a request for comment early Monday.
Federal safety records indicate Scapadas Magicas had no reported crashes in the two-year reporting period leading up to Sunday. The company received a "satisfactory" rating in a safety review last month.
Investigators pored over the debris with flashlights overnight and Monday morning, searching for clues.
But evidence of a rollover seemed abundantly clear: The top of the bus over the driver's seat was caved in, and part of the roof was ripped open.