(CNN) — A parade of ambulances snaked down a narrow, mountainous highway Sunday night, waiting to transport the dozens injured from a fatal tour bus crash in Southern California.
The difficult terrain hampered rescue efforts in San Bernardino County, east of Los Angeles, as authorities tried to extricate some of the victims.
The San Bernardino County Fire Department said multiple people were killed, but did not have an exact number.
The bus rolled over and crashed about 6:30 p.m. (9:30 p.m. ET) in a mountainous area of State Route 38, between Mentone and Forest Falls, Fire Department spokesman Eric Sherwin said.
At least 27 people suffered injuries ranging from minor to severe, he said.
The highway was reduced to one lane for both inbound and outbound ambulances, which had to contend with a downhill grade and winding roads.
The cause of the crash was not immediately known.
CNN's Holly Yan contributed to this report.