Thirteen people were killed and 31 injured on Sunday when their tour bus collided with a truck on a major highway in southern California, the authorities said.
"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, news photographs showed.
"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," Abele said.
Although it was not known what caused the deadly crash, the bus was traveling fast -- faster than the truck it rammed, he added.
"The investigation will be ongoing," he said. The bus driver was killed while the truck driver survived. None of the victims was immediately identified.
"Unfortunately, several passengers have not been identified," Abele said. "Some IDs were not valid."
The bus belongs to USA Holiday, a Los Angeles area-based tour company that ferries customers to casinos.
The crash happened around 5:00 am (1200 GMT) in Palm Springs, some 110 miles (180 kilometers) east of Los Angeles.
Most of the passengers on the bus that initially left Los Angeles were Hispanic, the CHP chief said.