SACRAMENTO, CA --- Toxicology reports have concluded that the driver of a casino-bound bus that crashed last week killing nine people was not driving under the influence of drugs or alcohol, the Sacramento Bee reported.

Colusa County District Attorney John Poyner said that toxicology reports on Quintin J. Watts show he had no alcohol in his system and had not taken drugs at the time of the accident. Immediately after the crash, CHP officials said they were detaining Watts on suspicion of DUI.

CHP officials told the Sacramento Bee the accident investigation could still take months.

Watts was driving a bus with 42 passengers bound for the Colusa Casino Resort when the bus veered several times across a rural road and rolled into a ditch just after 6 p.m. on a Sunday. Nine passengers died, including Daniel Cobb, owner of the bus and of Cobb's Bus Service.

Watts was being held by state corrections officials Tuesday afternoon at Deuel Vocational Institute in Tracy, Calif., pending an investigation into whether he violated parole by driving the bus without proper licensing, state corrections officials said.

"We're just beginning the process of looking at that now, whether or not to revoke his parole," state corrections spokeswoman Terry Thornton told the Sacramento Bee.

Watts is on parole for a 2007 conviction in San Joaquin County for possession of a firearm by an ex-felon. He did not have state authorization to drive a passenger bus, according to Department of Motor Vehicle records.