10:15 p.m.WILLIAMS, Calif. — The driver of a casino-bound charter bus that crashed and killed eight people was arrested on suspicion of driving under the influence, the California Highway Patrol said Monday.

Authorities were investigating whether prescription or nonprescription drugs were involved in Sunday’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 Monday.

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 some passengers as it rolled and crushing others, 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, which left from Sacramento, crashed 10 miles 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,” CHP Commander Fran Clader said Monday. “Unfortunately, it resulted in tragedy.”

The accident was at least the fifth this year involving buses carrying people to and from casinos that caused deaths or many injuries.

Don Kennedy, marketing director at Colusa Casino Resort, said the bus was not scheduled to bring guests to the facility, but he added that the casino accepts unscheduled visits from charter buses.

The driver, who has not been identified, had a valid commercial driver’s license but the patrol is looking into whether he had the proper permits to drive a bus in California.

Authorities had not determined the name of the charter company 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, the Greyhound spokeswoman. “It is an old bus.”

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

“There are still several pieces of this puzzle that’s missing,” Kays said. “We will find out who owns the bus.”

Firefighters used flashlights and infrared sensors to search the tall grass near the overturned bus Sunday night for victims. The mud-covered bus was pulled from the ditch and was on a large tow truck.

CHP dispatcher Terry Troth said no other cars appeared to be involved in the crash.

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

Helicopters took victims to several area hospitals.

Yvonne Haynes, 35, of Merced got a phone call Sunday night from her brother Tou Xiong, 29, saying there had been a bus accident. They called hospitals looking for their mother Mai Cha, 74, of Sacramento who had gone on a day trip to the Colusa Casino Resort and was expected to return, on a bus, that evening.

“We couldn’t find her,” said Xiong. “She came in as a Jane Doe. Her purse was lost in the accident and she did not have any ID.”

Finally, at 3 a.m. the siblings found their mother at UC Davis Medical Center in Sacramento. Cha has a fractured rib, face and right wrist and with the tubes in her mouth, she cannot speak, said Haynes.

“She squeezed my hand so I know she hears me,” said Haynes.

The two also had an aunt on the bus, 67-year-old Ge Vue of Sacramento who Haynes said is in a hospital in Calusa and is expected to be released Monday.

The crash took place in an area of rice fields and orchards a few miles east of Interstate 5 just north of Williams.

Laura Hennum, a spokeswoman for Enloe Medical Center, 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.

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Associated Press writer Sofia Mannos contributed to this report from Washington, D.C.