DESERT HOT SPRINGS, Calif. (AP) - Authorities say a maintenance crew was working on a California highway and slowing down traffic when a tour bus plowed into the back of a semi-truck, killing 13 people.

California Highway Patrol Border Division Chief Jim Abele says the cause of the accident isn't clear but that the bus carrying 44 passengers was going significantly faster than the truck when they crashed before dawn Sunday north of Palm Springs. An additional 31 people were injured.

CHP officers had been slowing traffic to allow Southern California Edison workers to string wires across Interstate 10. Abele says the work had gone on for hours without problems.

Abele says the bus was coming from Red Earth Casino in Thermal, California, and was about 35 miles into its 135-mile trip back to Los Angeles.

A trauma surgeon says many of those most severely wounded in a deadly tour bus crash in California suffered facial trauma but few broken bones, calling the injuries unusual for a high-speed vehicle wreck.

Dr. Ricard Townsend of Desert Regional Medical Center in Palm Springs told reporters Sunday that the bus struck the collapsible trailer of a semi-truck, sending many of the likely unrestrained passengers flying through the air.

He called the widespread facial injuries a hallmark of those not wearing seat belts. He says the injuries indicate the bus was slowing down at the point of impact.

Townsend says doctors treated several spine fractures but few other bone injuries. Five patients are in intensive care but stable and about seven people have been released.

Several others are being transferred in from other hospitals or still being assessed in the emergency room.

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