A tour bus returning to Los Angeles from a casino trip smashed into the back of a lorry cab on a California highway, killing 13 people and injuring 31 others.
A maintenance crew had slowed down traffic on Interstate 10 before the vehicles crashed just north of the desert resort town of Palm Springs, California Highway Patrol Border Division Chief Jim Abele said.
He said the bus, carrying 44 passengers, was going much faster than the lorry, though a trauma surgeon said the injuries he saw indicated it was slowing down at the point of impact.
"The speed of bus was so significant that the trailer itself entered about 15 feet into the bus," Mr Abele said. "You can see it was a substantial impact."
It was not known if alcohol, drugs or fatigue played a role in the crash, about 100 miles east of Los Angeles, but the bus was inspected in April and had no mechanical issues, Mr Abele said. The bus driver was killed and the truck driver received minor injuries.
The bus was coming from Red Earth Casino in the unincorporated community of Thermal and was about 35 miles into its 135-mile trip back to Los Angeles.
CHP officers had been slowing traffic to allow Southern California Edison workers to string wires across the freeway, Mr Abele said.
Passengers said most people were asleep when the crash occurred at 5.17am. Mr Abele said it appeared the 1996 bus did not have seat belts and probably not a black box that newer vehicles feature.
The front of the bus crumpled into the cab's trailer and debris was scattered across the key route through Southern California. Firefighters used ladders to climb through the bus windows to remove bodies and tow trucks lifted the trailer to make it easier to reach the bus, whose front end was demolished.
Fourteen patients were sent to Desert Regional Medical Centre in Palm Springs, the area's only trauma centre. Five were admitted in critical condition but were stable and in intensive care by Sunday afternoon, said Dr Ricard Townsend, a trauma surgeon. Seven others had been released.
Many suffered facial injuries, a tell-tale sign they were not wearing seat belts, he said. He called the injuries unusual for this type of crash.
"When you usually see someone involved in a high-speed motor vehicle crash, the thing that you see are big-time broken bones. This was not one of the circumstances we were faced with," Dr Townsend said, referencing the collapsible trailer.
"It seemed as though most of the victims were unrestrained and were therefore flown through the air and ended up sustaining facial trauma."
Doctors treated several spinal fractures but few other bone injuries. The wounds indicated the bus was slowing down when it struck, Dr Townsend said.
Two other hospitals received patients with minor injuries.
Officer Stephanie Hamilton of the CHP told the Desert Sun newspaper in Palm Springs that the driver was one of the owners of tour bus company, USA Holiday, based near Los Angeles. The company has one vehicle and one driver, according to the Federal Motor Carrier Safety Administration.
A Facebook message from USA Holiday said it did not have much information about the crash.
The company says on social media that it has more than 25 years of experience traveling to casinos in Southern California. It posts about trips leaving the Los Angeles area to casinos in the Southern California desert area and Las Vegas.
The bus owner's neighbour said she would often see a tour bus with the sign "USA Holiday" parked on the street in front of his apartment in a working-class neighbourhood in Alhambra, about seven miles east of central Los Angeles.
Sonia Anderson said the family who lived there - a man, woman and their college student son - had lived in the apartment for about 17 years. She said the father generally drove the bus and his wife and son would sometimes travel on it with him.
She described them as a kind, close-knit family.
"All three of them are hard-working people. Good people. Beautiful family," she said, adding that t he father was "always working, coming in and out, providing for his family".
The National Transportation Safety Board is sending a team to California to investigate.
The crash comes two years after a FedEx truck veered across an interstate central reservation north of Sacramento and hit a bus full of high school students, killing 10 people. In August, a bus in central California hit a highway sign post that tore through the vehicle and left four people dead.