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Thirteen people were killed and 31 others were injured Sunday morning when a tour bus returning from a casino crashed into a big-rig truck that was hauling food products on the 10 Freeway in Desert Hot Springs, California Highway Patrol officials said.

The tour bus, operated by USA Holiday, crashed into the freight truck at 5:17 a.m. near the Indian Canyon exit on the westbound side of the 10 Freeway. Forty-four passengers were on the bus, which was returning from the Red Earth Casino near the Salton Sea.

The driver of the bus was among those killed in the crash, while the driver of the semi-truck suffered minor injuries.

USA Holiday is based in Alhambra and employs one driver, the Los Angeles Times reported, citing federal records.

A CHP official said the 1996 MCI bus was traveling significantly faster than the semi-truck it plowed into.

"The speed of the bus was so significant that when it hit the back of the big-rig the trailer itself entered about 15 feet into the bus. So you can see it was substantial impact," CHP Chief Jim Abele during a news conference on Sunday afternoon.

Abele said there was no fog at the time of the crash. He said traffic, including the trailer, had slowed because a maintenance crew had been periodically stringing wires across the roadway.

The truck driver told authorities he "felt a thump in the back ... he didn't know what specifically happened," Abele said.

Some crash survivors said most of the passengers were asleep at the time of the crash.

"There were so many injuries that they had to establish an emergency triage right there and treat the people right there on the scene," he said.

Hospital officials said many of the victims suffered facial injuries because they were not restrained on the bus.

Fourteen patients were transported to Desert Regional Medical Center in Palm Springs, 11 were taken to Eisenhower Medical Center and five patients were sent to John F. Kennedy Memorial Hospital, officials reported. Four people remained in critical condition, one was in serious condition and one person was transported to a Los Angeles-area hospital, officials said Sunday night. Several people have been discharged.

Abele said all of the passengers on the bus were believed to be adults. The injuries ranged from mild to moderate.

The identities of the victims were not immediately released.

"In almost 35 years I've never been to a crash where there's been 13 confirmed fatal accidents. So, it's tough," Abele said. "It's tough for all of us."

CHP officials confirmed the tour bus had been inspected in 2014, 2015 and most recently in April of this year with no mechanical problems reported.

Early videos from the scene show dozens of firefighters with ladders inside the mangled remains of the tour bus, struggling to reach the victims. Rescuers also used a crane to sift through the wreckage.

The deadly bus crash was being investigated by the CHP and the National Transportation Safety Board, officials said.

It closed the westbound freeway lanes for hours before it was reopened about 4 p.m. Sunday, the Times reported.

Abele said it was unknown if alcohol, drugs, or fatigue were contributing factors in the deadly crash.

Henry Turcios, 58, was one of the passengers on the tour bus and was at a vigil in Koreatown, one of the bus company's stops, Sunday night.

He said told KTLA that, while he doesn’t know exactly what happened, he doesn’t think the bus driver fell asleep because he had been talking to Turcios' friend about 20 minutes before the crash.

“I flew off and I hit my chest, my face, messed up my neck, my jaw,” Turcios said about the crash.

He said many people were thrown from the back of the bus to the front.

“The first thing you hit is the next seat.”

KTLA's Cindy Von Quednow and Chris Wolfe contributed to this story.

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