PALM SPRINGS, Calif. >> A maintenance crew had slowed down traffic on a California highway through the night, and the work had gone on for hours without problems. Then a tour bus returning to Los Angeles from a casino trip slammed into the back of a semi-truck. Passengers who were asleep on the bus woke up to loud screams and the sound of crushing metal.

The gambling jaunt ended in one of the deadliest wrecks in California history with 13 people killed and 31 others injured.

Authorities said the bus was going much faster than the truck, causing it to plow about 15 feet into the truck on Interstate 10 just north of the desert resort town of Palm Springs.

“You can see it was a substantial impact,” California Highway Patrol Border Division Chief Jim Abele said.

It was not known if alcohol, drugs or fatigue played a role in the crash, but the bus was inspected in April and had no mechanical issues, Abele said. The bus driver was killed, and the truck driver received minor injuries.

The bus was coming from Red Earth Casino in the desert town of Thermal and was about 35 miles into its 135-mile trip back to Los Angeles.

CHP officers had been slowing traffic to allow utility workers to string wires across the freeway, Abele said.

Passengers told officials that most people were asleep when the crash occurred shortly after 5 a.m. Abele said it appeared the 1996 bus didn’t have seat belts and likely didn’t have data recorder that would reveal how fast the bus was traveling and whether the driver braked before impact.

“I was awakened by the sounds of people screaming for help,” passenger Ana Car, 61, told the Los Angeles Times. “I noticed a heavyset woman lying in the center aisle to my right yelling, ‘My legs! My legs!’”

Before April, the bus also was inspected in 2014 and 2015, the CHP said. Federal Motor Carrier Safety Administration records show it had no crashes in the past two years and had a satisfactory safety rating.

The front of the bus crumpled into the semi-truck’s trailer and debris was scattered across the key route through Southern California. Firefighters used ladders to climb into 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 Center in Palm Springs, the area’s only trauma center. Four remained in critical condition Sunday evening, two were transferred to other hospitals for continuing treatment and the rest were treated and released, hospital officials said.

Many suffered facial injuries, a telltale sign they were not wearing seat belts, said Dr. Ricard Townsend, a trauma surgeon.

“It seemed as though most of the victims were unrestrained and were therefore flown through the air and ended up sustaining facial trauma,” he said.

Two other hospitals received patients with minor injuries.

The tour bus company, USA Holiday, has one vehicle and one driver, according to federal regulators. A call to the company was not immediately returned.

The company says on social media that it has more than 25 years of experience traveling to casinos in Southern California. It posts about quick turnaround trips leaving the Los Angeles area to casinos in the Southern California desert area and Las Vegas.

The bus owner’s neighbor said she’d often see a tour bus with the sign “USA Holiday” parked on the street in front of his apartment in a working-class neighborhood in Alhambra, about 7 miles east of downtown 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 the bus with him.

Anderson described them as a kind, close-knit family. “All three of them are hard-working people. Good people. Beautiful family,” she said.

The father was “always working, coming in and out, providing for his family,” Anderson said.

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 median north of Sacramento and slammed into a bus full of high school students, killing 10 people.

In 1963, 32 Mexican farm workers who were in the Central Valley as part of a work visa program were killed when a freight train struck the flatbed truck they were riding on. In 1976, a bus carrying the Yuba City high school choir plunged off a freeway ramp in Martinez, killing 28 students and one teacher.