Emergency responders search through high weeds in a water-filled ditch after a tour bus crashed on a rural two-lane road near Williams, Calif., on Sunday. (Steve Yeater/Associated Press)

The driver of a casino-bound charter bus that crashed and killed eight people in northern California has been arrested on suspicion of driving under the influence of drugs, police said Monday.

The California Highway Patrol said it is investigating whether prescription or non-prescription drugs were involved in Sunday evening's crash.

"We believe he was driving under the influence of something. That's why we placed him under arrest," patrol spokesman Robert Kays said.

The bus, which had an invalid license plate, drifted off a rural two-lane road before the driver "overcorrected" and swerved back, patrol spokesman Patrick Landreth said.

The bus tumbled into a ditch, ejecting passengers as it rolled over and crushing some of them before coming to rest on its wheels, police said.

"The roof was collapsed down, the windows were broken out, and the bus was not only rolled over onto its side, it rolled completely over," Landreth said. "It was facing the opposite direction and it was on its wheels."

The bus driver has not been identified and police had not yet determined the name of the charter company Monday.

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, the Greyhound spokeswoman. "It is an old bus."

Licence plate 'not valid'

Kays said the bus had a Texas licence plate that was "not valid." He said other registration serial numbers also came up invalid.

The bus, which left from Sacramento, crashed 16 kilometres short of its destination, the Colusa Casino Resort. Many of the passengers were Laotian.

Firefighters used flashlights and infrared sensors to search the tall grass at the accident site for more victims. Helicopters took the injured to several hospitals.

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

It wasn't immediately known how many people were being treated in other hospitals. Police estimated earlier that up to 35 were injured.

With files from the Associated Press