According to the California Highway Patrol, the driver of a casino-bound charter bus has been arrested on suspicion of driving under the influence of drugs.

The bus crashed and killed eight people.

California authorities are investigating whether prescription or nonprescription drugs were involved in the crash. The bus drifted off a two-lane road before the driver overcorrected and swerved back onto the road before tumbling into a ditch, ejecting some passengers. Other passengers on the bus were rolled and crushed. The bus also had invalid license plates.

“The roof collapsed down, the windows were broken out, and the bus was not only rolled over onto its side, it rolled completely over. It was facing the opposite direction,” said patrol spokesman Patrick Landreth.

This has been at least the eighth accident in the last three years that has involved buses carrying people to and from casinos that have caused death to the passengers. While the bus had “Greyhound” marked on its side, a Greyhound official said their company sold the bus more than two years ago.