PALM SPRINGS, Calif. — Investigators continue to work Monday to determine what caused one of the deadliest crashes in California in recent years. Thirteen people were killed Sunday after a tour bus slammed into the back of a big rig on Interstate 10 near Palm Springs while traveling to Los Angeles from a casino near the Salton Sea.

Thirty one others were injured in the predawn crash, which shut down the westbound side of the freeway, according to the California Highway Patrol (CHP). The passengers were believed to be sleeping when the bus collided with the tractor-trailer truck shortly after 5 a.m., CHP Chief Jim Abele said at a news conference Sunday.

“It was a substantial impact,” Abele said, explaining that the bus plowed 15 feet into the back of the truck.

Sunday's pre-dawn crash took place about 100 miles east of Los Angeles as California Highway Patrol officers slowed traffic to allow utility workers to string wires across Interstate 10.

The National Transportation Safety Board, which is investigating the crash, scheduled a news conference for Monday at 1 p.m. PT (4 p.m. ET).

The bus, which had 43 passengers, was on its way back from Red Earth Casino in the Salton Sea Beach and its driver was among those killed, Abele said. The bus was about 35 miles into its 135-mile trip back to Los Angeles.

All victims were adults, the chief described the survivors' injuries as minor to moderate and said the big rig's driver also suffered minor injuries.

He did not give the nationalities of the victims, but said authorities were in contact with consular officials of Mexico, Japan and Australia and that most of the passengers were Latino. He said authorities have struggled to identify some of the passengers and contact their families, partly because some of them were not carrying "valid" identification.

The tour bus belonged to USA Holiday, a Los Angeles-based tour company, which only owns one bus and has one driver licensed to transport passengers, according to records from the Federal Motor Carrier Safety Administration. In 2013, the bus logged 68,780 miles.

Abele said the agency inspected the bus in 2014, 2015 and 2016 and found no mechanical violations. The bus is a 1996 model, however, and probably did not have a black box device that could provide investigators with information about what was happening on the bus in the minutes before the crash.

USA Holiday regularly carries passengers from the Los Angeles area to Southern California casinos, and the company's Instagram page says it has been doing that for 25 years. According to posts on social media, USA Holiday recently made trips to Tortoise Rock Casino in Twentynine Palms, Red Earth Casino and Las Vegas.

USA Holiday did not return requests for comment.

Red Earth Casino is a small casino in remote Imperial County operated by the Torres-Martinez Desert Cahuilla Indians. It sits beside Highway 86, a two-lane road that traces the Salton Sea to the U.S.-Mexico border, and often functions as a truck stop and convenience store as well.

Patients were taken to all three Coachella Valley hospitals. Desert Regional Medical Center, which has the valley's only trauma center, was treating 14 patients.

By Sunday evening, four patients remained in critical condition; one patient, in serious condition, was received as a transfer from another local hospital; one patient was transferred to a hospital in Los Angeles for continuing treatment and nine patients had been treated and released from the hospital, director of marketing Rich Ramhoff said.

Trauma surgeon Dr. Ricard Townsend said the critical patients were in stable condition and seven had already been discharged. He said all the patients he came into contact with spoke only Spanish.

Townsend wouldn't talk about specific injuries, but he said many patients came in with facial trauma requiring eye surgery. He said this is often indicative of patients who were not wearing seat belts, but did not know whether seat belts were available on the vehicle.

No one was expected to be hospitalized long and Townsend said everyone would probably recover.

Eisenhower Medical Center spokeswoman Lee Rice said the Rancho Mirage hospital received 12 patients who had minor injuries. Six of them were released, but the others were kept at the hospital in "good condition," she said.

JFK Medical Center in Indio, Calif., received five patients who had minor injuries, Chief Development Officer Linda Evans said.

The crash was among the deadliest in California in years. In April 2014, 10 people were killed in a fiery crash near Orland, Calif., when a FedEx tractor-trailer crossed a narrow median on Interstate 5 and slammed into a bus carrying high school students from Los Angeles who were on their way to visit a college campus. The National Transportation Safety Board investigated that collision and said more than a year later that it was a mystery why the truck driver had crossed the median.

After that crash, the NTSB also urged the National Highway Traffic Safety Administration to require secondary doors as emergency exits on new bus designs.

Contributing: Corinne Kennedy, Ian James and Sherry Barkas, The (Palm Springs, Calif.) Desert Sun; John Bacon, USA TODAY; The Associated Press. Follow Colin Atagi and Rosalie Murphy on Twitter: @TDSColinAtagi and@rozmurph