FLORISTON, Calif. -- Authorities say driver fatigue is under investigation as a possible cause of a tragic bus crash in California's Sierra Nevada that left one dead and 24 injured. Eleven remained hospitalized Sunday.

California Highway Patrol Officer Steve Skeen says investigators are also focusing on whether a medical condition contributed to the accident Saturday on Interstate 80.

An employee shuttle bus operated by the Resort at Squaw Creek veered off the road, "struck a guardrail and rolled several times" before stopping in a ditch along Interstate 80, about 20 miles west of Reno, said California Highway Patrol Officer Steve Skeen.

Some of the 30 or so people aboard were thrown from the bus. Twenty-five-year-old Pablo Olivas of Reno was pronounced dead at the scene, according to the Nevada County, Calif., sheriff's office.

The driver suffered "moderate to major" injuries and was being interviewed by officers.

Twenty-four people were taken to hospitals in Reno and Truckee, Calif., according to the California Highway Patrol.

The most severely injured were taken to Renown Regional Medical Center in Reno, where one was listed in critical condition, four in serious condition and five in fair condition. The condition of another passenger was unknown.

The bus was transporting employees from Reno to the upscale resort north of Lake Tahoe when the accident occurred, said company spokesman Les Pedersen.

"The driver had a very good record," he said. "The bus had no mechanical problems that I'm aware of."

The names of the driver and passengers were not immediately released.

The resort is located at the foot of the Squaw Valley USA ski resort, site of the 1960 Winter Games, and is separately owned.

Bella Fritz, a passer-by from Sparks, described seeing passengers at the scene before authorities arrived.

"One guy was stuck under the bus," she told the Reno Gazette-Journal. "I ran back and got a blanket and covered one lady who had flown out of it and hit her head on a rock. She was in bad shape, but breathing. I rubbed her back until someone got here."

The accident shut down both directions of travel on the interstate for about 90 minutes. The westbound lanes of I-80 were closed for several hours.

The cause of Saturday's incident remains under investigation.