Thirteen people were killed and 31 injured when their coach collided with a truck on a major motorway in southern California.
"They believe everyone was asleep on the bus," an emotional California Highway Police chief Jim Abele said, giving condolences to families of the victims, who were believed to have been visiting casinos.
The front of the bus was a mangled mass of metal after apparently plowing into the back of the large truck.
"Because of the impact where the bus entered the back of the big rig, the majority of the people who were killed were in the front section of the bus," Chief Abele said.
Although it was not known what caused the deadly crash, the bus was traveling fast - faster than the truck it crashed into, he added.
"The investigation will be ongoing," he said.
The bus driver was killed, the truck driver survived. None of the victims were immediately identified.
The bus belongs to USA Holiday, a Los Angeles area-based tour company that ferries customers to casinos, The Desert Sun newspaper reported.
The crash happened about 180kms east of Los Angeles.
