The driver, 52-year-old Quintin Watts of Stockton, has been charged with driving under the influence. Authorities are investigating whether drugs, alcohol or a combination of both were involved.
"His driving was observed by an off-duty Colusa County Deputy, and there was very erratic, and just the nature of the collision," California Highway Patrol Assistant Chief Stephen Bell said.
Watts had a valid Class A commercial license, but none of the additional endorsements required by law to drive passengers. In 2005 and 2006, Department of Motor Vehicles records show the former truck driver received six traffic citations, from speeding to failure to wear a seat belt. In the early 90's, Watts had nearly a dozen arrests ranging from assault to drugs.
Watts' mother told the Sacramento Bee she believes Sunday was his first charter, carrying mostly Laotian senior citizens to an Indian Casino for a night of gambling.
Watts remains in critical condition.
"The bus drifted off the road, crossed over to the other side, crossed back and then one final turn came and crossed back over the road way, struck the dirt embankment and that's when the bus flipped," Bell said.
Among those killed in the wreck was the owner of the tour bus company, Daniel Cobb.
ABC7 was unable to locate his office, but did locate the lot where he stored his busses, most of them in questionable condition.
The bus that crashed had an expired Texas license plate.
The Greyhound name is written on the bus, but Greyhound says they sold it two years ago.
The crash site was on a narrow county road, known to locals as a short cut to the casino from Interstate 5. The CHP says that route only saves two miles.