At least eight people were killed and another 38 were injured after a tour bus collided with two other buses and flipped over in the mountains of Southern California on Sunday evening, authorities said.

The accident that occurred around 6:30 p.m. involved three vehicles -- a tour bus carrying a group of people from Tijuana, Mexico, a sedan and a pickup truck pulling a trailer -- on Highway 38 near Mentone about 70 miles (113 km) east of Los Angeles.

According to an Associated Press report quoting California Highway Patrol spokesman Mario Lopez, the surviving bus driver said the brakes failed as the bus was descending a mountain; it first hit the sedan and then flipping over and crashed into the truck.

"Parties in the tour bus were ejected from the bus,” fire department spokesman Eric Sherwin told Reuters. “We have multiple fatalities on the scene."

However, he said the precise cause of the accident had yet to be ascertained.

“Investigation into the cause hasn’t even begun, because patient treatment and transport takes priority,” Sherwin said, according to the Los Angeles Times.

Twenty-seven of the injured passengers were taken to local hospitals for treatment; the rest were treated at the scene. Nine ambulances were called to the scene, Sherwin said. All fatalities were reported from the bus.

Rescue workers were unable to confirm the total number of fatalities as operations to pull out victims from the wreckage continued.

The highway is one of the three roads leading to Big Bear Lake, a recreational area popular with skiers, Sherwin said.

Mexico