The bus was traveling west on Interstate 10 when the crash occurred near Palm Springs, a city 100 miles east of LA.
The bus was traveling west on Interstate 10 when the crash occurred near Palm Springs, a city about 100 miles (160 km) east of Los Angeles, California Highway Patrol (CHP) Chief Jim Abele told reporters.
Abele said because of the bus' speed, the truck's trailer pushed about 15 feet into the bus. The bus driver was among the 13 killed, he said. Abele said he believed the 31 injured passengers had minor to moderate injuries, but some could be worse.
"In almost 35 years I've never been to a crash where there's been 13 confirmed," deaths, Abele said. "So it's tough. It's tough for all of us."
Abele said the bus had been inspected as recently as April and there was no indication of mechanical problems. He said several survivors reported most of the passengers, all of whom were adults, were sleeping at the time of the crash.
Photographs from the scene showed the front of the tour bus wedged inside the back of the trailer, with emergency workers using metal ladders to reach the inside of the bus.
The injured were being treated at hospitals while all westbound lanes of the interstate were closed near the crash, officials said. Abele said the lanes would likely be reopened around 4 p.m.
Abele said the bus appeared to be ferrying casino-goers back to Los Angeles from the Red Earth Casino after a night of gambling.
The Los Angeles-area bus company, USA Holiday Bus, could not immediately be reached for comment. The local Desert Sun newspaper reported the driver was one of the company's owners.
Abele said USA Holiday appeared to be an "owner-operator" company and that the totaled bus was the only one owned.
The National Transportation Safety Board said it will investigate the crash and is sending an investigative team to the site.
Abele said it was not immediately clear if alcohol, drugs or fatigue was a factor in the crash. He said it was not clear whether the bus was speeding, but said it was traveling "significantly faster" than the tractor-trailer.