Investigators confer at the scene of a mass casualty bus crash on the westbound Interstate 10 freeway near Palm Springs, California, October 23, 2016. [Photo/Agencies]

PALM SPRINGS, Calif. - A tour bus crashed into the back of a tractor-trailer on a Southern California highway before dawn on Sunday, killing 13 people and injuring 31, authorities said.

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.