Thirteen people were killed when a tour bus slammed into the back of a big rig on Interstate 10 near Palm Springs on Sunday morning while the bus was carrying a group of passengers to Los Angeles after a trip to a casino near the Salton Sea.

Thirty-one people were injured in the the predawn crash, which shut down the westbound side of the freeway for much of the day.

Passengers were believed to be sleeping when the bus collided with the tractor-trailer truck shortly after 5 a.m., California Highway Patrol Chief Jim Abele said at a news conference.

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

A total of 44 passengers were aboard the bus. The bus driver was among those killed, Abele said. He described the survivors' injuries as minor to moderate.

He said it was unclear why the bus was traveling at a much faster speed than the truck. It's also unclear whether drugs or alcohol were involved, or whether fatigue was a factor, Abele said.

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

He did not give the nationalities of the victims but said authorities were in contact with consular officials of Mexico, Japan and Australia. He said the driver of the truck survived and had minor injuries.

Officials with the National Transportation Safety Board were on their way to participate in the investigation.

The freeway was littered with mangled wreckage and the front of the bus was enveloped by the big rig's trailer.

Dozens of firefighters swarmed the scene, struggling to reach victims. They used chainsaws to break open the vehicle and make their way into its front, even though it was clear there was barely any room to fit inside.

A tow truck was used to lift the trailer, and another pulled the big rig forward, off the bus. Firefighters positioned ladders in the bus' windows to pull victims out.

Surviving victims were placed in ambulances and rushed to hospitals while a few remained, laying on stretchers with their heads raised.

Abele said in his 35-year career, he has almost never witnessed such a scene.

"It’s tough. It’s tough for all of us," he said, his voice quavering. "Our sympathies to the families."

All westbound lanes of I-10 were closed at Indian Canyon following the crash. Traffic was being diverted from I-10 at Indian Canyon. CHP officials said the freeway's westbound lanes would be closed until about 4 p.m.

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 Sunday morning. Five were in critical condition, three in serious condition, and six had minor injuries, director of marketing Rich Ramhoff said. All the patients were adults.

Ramhoff said that in his six years at the hospital, this was the largest number of victims he'd seen from a single incident.

Eisenhower Medical Center spokesperson Lee Rice said the Rancho Mirage hospital received 11 adult patients, all with minor injuries.

JFK Medical Center received five patients, all of whom suffered minor injuries, according to chief development officer Linda Evans.

The tour bus belonged to USA Holiday, a Los Angeles-based tour company, and Hamilton said the driver was one of the company's 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, and the company's Instagram page says it has been doing so for 25 years. According to posts on social media, USA Holiday has recently made trips to Tortoise Rock Casino, Red Earth Casino and Las Vegas, but the company hasn't promoted any trips on Facebook or Instagram since mid-September.

USA Holiday's trips usually depart from Koreatown, southeast Los Angeles and the San Fernando Valley, according to its social media posts. In September, the company took passengers on a 24-hour jaunt to Las Vegas, which cost $60 per person including transportation, snacks and drinks. Two weeks earlier, they led a trip to Tortoise Rock Casino that departed at 5 p.m. and returning at 5 a.m., for which passengers paid $25. The company's last post from Red Earth Casino was in July.

According to records from the Federal Motor Carrier Safety Administration, USA Holiday only owns one bus and has one driver licensed to transport passengers. In 2013, the bus logged 68,780 miles.

The bus underwent a complete inspection sometime in the last two years and had no violations. The company didn’t report any crashes in the last two years, either.

The company has not yet returned requests for comment.

As the hours passed Sunday, firefighters continued to work outside the mangled bus as passersby pulled over in the eastbound lanes to catch a glimpse of the grisly scene before being ordered to leave by officials. Line after line of vehicles backed up along the freeway, many motorists getting out of their vehicles and watching from afar. Those closest to the scene said it was too dark to see the actual crash, but they were shocked to see the remnants as the sun came up.

Poet Aleman, who lives in the Coachella Valley, was driving west on I-10 around 5 a.m. when traffic came to a sudden halt. From her vantage point, she said it looked like the big rig had tried to merge into the lane the passenger bus was using.

"The whole front of the bus up to about the second or third row of seats was completely crushed into the left rear side of the semi truck," Aleman told The Desert Sun. "By the time I got there there were at least a dozen firefighters climbing into the passenger bus pulling people out."

She finally got off the freeway around 7:30 a.m.

Chris and Rachel Williams were delivering frozen chicken to Los Angeles when they got stuck in traffic. They 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 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.

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