CHP Sgt. Patrick Landreth said the accident was reported at 6:18 p.m. about six miles east of Interstate 5. The bus was traveling north from Sacramento on Lone Star Road in Colusa County, headed to Colusa Casino Resort.
About 60 miles into the drive, the bus "veered off the road, the driver lost control . . . spun out, rolled completely over and ended up on its wheels in a ditch," Landreth 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," he said. "It was facing the opposite direction and it was on its wheels."
Several passengers were thrown from or tossed around inside the bus while it rolled, Landreth said.
At least 37 passengers, as well as the driver, were taken to hospitals throughout the region, including Enloe Medical Center, Colusa Regional Medical Center, Mercy San Juan Medical Center, Sutter Roseville Medical Center, UC Davis Medical Center and Woodland Memorial Hospital, by medevac helicopters and other means.
Firefighters used flashlights and infrared sensors to search the tall grass near the overturned bus Sunday night for more possible bodies. The bus, covered in mud, had a crumpled roof and broken windows.
The crash took place in an area of rice fields and orchards a few miles east of Interstate 5 just north of Williams
Landreth did not have any more information on the driver. He said at least 46 people were believed to be on the bus.
Enloe Medical Center received six patients, four in critical condition and two in serious condition, and was expecting four more.
No other vehicles were involved, Landreth said, and the cause of the accident is under investigation.
Although the bus had "Greyhound" marked on its side, a Greyhound official said it was no longer operated by the company and had been sold more than two years ago.
CHP spokesman Robert Kays said the bus had a Texas license plate that was "not valid." He said other registration serial numbers also came up invalid.
The Associated Press contributed to this report.