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DESERT HOT SPRINGS, Calif. — A tour bus returning from a casino slammed into the rear of a tractor-trailer on Sunday near Palm Springs, Calif., killing 13 people and injuring 31 others, according to state police.

Eleven of the 13 victims were identified Monday.

The bus driver was among those killed and the truck driver was injured, California Highway Patrol Division Chief Jim Abele said. Injuries range from minor to moderate, he said.

“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,” Abele said.

The 1996 MCI bus, operated by USA Holiday, was returning to Los Angeles from the Red Earth Casino near the Salton Sea, according to Abele.

The accident happened at 5:17 a.m. on westbound Interstate 10, officials said. The cause was not immediately known.

The CHP and the National Transportation Safety Board are investigating.

It’s unknown whether drugs, alcohol or fatigue were factors in the accident.

Eleven of the 13 crash victims have been identified by the Riverside County Coroner’s Office: Isabel Jimenez Hernandez, 66, of Los Angeles; Rosa Ruiz, 53, of Los Angeles; Gustavo Green, 62, of Los Angeles; Zoila Aguilera, 72, of Los Angeles; Teodulo Vides, 59, of Los Angeles; Milagros Gonzales, 72, of Los Angeles; Conception Corvera, 57, of Palmdale, Calif.; Aracely Tije, 63, of Los Angeles; Dora Galvez de Rodriguez, 69, of Los Angeles; Elvia Sanchez, 52, of Los Angeles; and Ana Gomes de Magallon, 71, of Los Angeles.

One male and one female victim remain unidentified.

Bus was inspected in April

Survivors said most passengers were asleep, Abele said. The victims were found in their seats and most of those killed were sitting in the front of the bus, Abele said.

The bus was the only one the company operated. The chief didn’t know whether the vehicle had seat belts.

“In almost 35 years I’ve never been to a crash where there’s been 13 confirmed fatalities,” Abele said. “So, it’s tough, it’s tough for all of us.”

He said the 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.

KESQ reported that rescuers used a crane to search the wreckage.

Bus going faster than tractor trailer

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.

Abele said “the bus was traveling significantly faster than the tractor trailer that it struck from behind.”

The truck driver told authorities he “felt a thump in the back … he didn’t know what specifically happened,” Abele said.

The CHP shut all lanes of the highway for first responders to care for the injured.

“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.

Police said the injured were taken to three hospitals, including Desert Regional Medical Center in Palm Springs. The hospital treated 14 patients, officials said. Five of those patients were in intensive care, but stable. Several were discharged.

Shortly after 4 p.m., police reopened Interstate Highway 10. Sunday’s crash is among the deadliest in California in recent years.

Ten people were killed in April 2014 when a FedEx tractor-trailer crossed a median on Interstate 5 north of Sacramento and slammed into a bus carrying high school students from Los Angeles on their way to visit a college campus.