WILLIAMS, Calif. -- Police have arrested the driver of a casino-bound charter bus that crashed in California on the weekend, killing eight people.

The California Highway Patrol say the driver was arrested on suspicion of driving under the influence of drugs.

Authorities were investigating whether prescription or non-prescription drugs were involved in Sunday's crash.

The bus, which had an invalid licence plate, drifted off a rural two-lane road before the driver "overcorrected" and the bus rolled over and toppled into a ditch.

Police say the driver, who has not been identified, had a valid commercial driver's licence.

But they say they are looking into whether he had the proper permits to drive a bus in California.

The California Highway Patrol also says the bus had a Texas licence plate that was "not valid" and that other registration serial numbers also came up invalid.

Authorities had not determined the name of the charter company by early Monday morning.

The bus had "Greyhound" marked on its side. But a Greyhound official said it was no longer operated by the company.

"It is not our bus. We sold it more than two years ago," said Kim Plaskett, a Greyhound spokeswoman. "It is an old bus."

The bus, which left from Sacramento, crashed about 15 kilometres short of its destination, the Colusa Casino Resort. Many of the passengers were Laotian.

"It looks like they were going up to the casino to have a good time," California Highway Patrol Commander Fran Clader said Monday. "Unfortunately, it resulted in tragedy."

CHP dispatcher Terry Troth said no other cars appeared to be involved in the crash, which happened in an area of rice fields and orchards a few kilometres east of Interstate 5 just north of Williams.

Troth said emergency responders were having trouble communicating with the passengers because many spoke Lao.

Helicopters took victims to several area hospitals.

Laura Hennum, a spokeswoman for Enloe Medical Centre, said 11 victims were at the hospital Monday morning. Four were in critical condition, one was in serious condition and six in fair condition.

She said one of the biggest challenges for the hospital was helping family members find one another, as passengers were taken to several hospitals.

"We were getting a huge volume of calls from distressed family members trying to find their loved ones," she said.