PALM SPRINGS, Calif. — At least 13 people returning from a casino outing were killed early Sunday when a tour bus plowed into the back of a tractor-trailer truck on Interstate 10 near this resort city 100 miles east of Los Angeles.

California Highway Patrol (CHP) officials put the death toll at 13, with 31 injured.

"The intrusion into the bus compartment is significant," said CHP officer Stephanie Hamilton.

National Transportation Safety Board (NTSB) investigators on Sunday were expected to examine the crash scene, but Jim Abele, commander of CHP's Border Division, told reporters, "We may not determine how the accident occurred because the driver was killed."

Abele said the bus plowed 15 feet into the back of the truck in a "substantial impact.” The driver of the truck survived with minor injuries.

Abele said it was unclear why the bus was traveling at a much faster speed than the truck. It was also unclear whether drugs or alcohol were involved, or whether fatigue was a factor. The crash took place shortly after 5 a.m. local time.

“Right now we’re looking at everything,” Abele said.

He did not give the nationalities of the victims but said authorities were in contact with consular officials of Mexico, Japan and Australia.

Throughout the morning, firefighters struggled to reach victims. The bus crashed into the big rig's trailer, and a tow truck was used to lift the trailer and help officials gain easier access to the demolished vehicle. Another tow truck tried to pull the truck off the bus. Firefighters positioned ladders at windows of the bus to remove victims.

Patients were taken to Desert Regional Medical Center, Eisenhower Medical Center and JFK Memorial Hospital, Desert Regional director of marketing Rich Ramhoff said. His hospital was treating 14 patients, five in critical condition, he said. Three were in serious condition, and six had minor injuries, Ramhoff said.

Eisenhower Medical Center spokesperson Lee Rice said the 11 patients there all had minor injuries. JFK Medical Center received five patients, all of whom suffered minor injuries, chief development officer Linda Evans said.

The tour bus, which was carrying 44 passengers, belongs to USA Holiday, a Los Angeles-based tour company, and Hamilton said the driver was identified as one of its owners.

A passenger who was treated at Desert Regional for minor injuries said the tour bus had started in Los Angeles, traveled to Red Earth Casino in Thermal, and was making its return trip Sunday morning.

USA Holiday regularly carries passengers from the Los Angeles area to Southern California casinos. According to its Facebook page, USA Holiday has recently made trips to Tortoise Rock Casino, Red Earth Casino and Las Vegas.

All westbound lanes of I-10 were closed following the crash. Stuck in the traffic were Chris and Rachel Williams, who were about five vehicles east of the scene and waited more than an hour.

"We're doing good. We're just saying our prayers for the people involved in this situation," Chris Williams said. "This is a day that will be remembered."

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 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.

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

In a tweet posted Sunday afternoon, the NTSB said it was dispatching a team of investigators to the crash scene.

Contributing: John Bacon, USA TODAY; Ian James and Sherry Barkas, The Desert Sun. Follow Colin Atagi and Rosalie Murphy on Twitter: @TDSColinAtagi and @rozmurph