"We're still putting together pieces of the puzzle," Ohio State Highway Patrol Lt. Edward Mejia told the AP. "We haven't determined the cause."

Mejia said the bus driver, Dwayne Garrett, 64, of Cincinnati, gave a statement to investigators and voluntarily had blood drawn for testing, the results of which won't be known until next week.

Greyhound Lines Inc. spokeswoman Alexandra Pedrini said Garrett was rested and has 15 years of experience "with a clean record." Greyhound officials told them that the bus recently had its annual inspection.

The Grand Rapids Press rweported that David Otte, 56, of Wyoming, Mich., was hospitalized after the crash but was released and back home Sunday. Otte told the newspaper he was returning from a visit with his brother in Alabama.

"I felt us hit the rumble strips, which woke me up, and I just thought 'get back on the road,'" said Otte. "The next thing I knew, we were down a bank and then we flipped three times."

Otte said the scene during the crash was "pure chaos," as he was hit in the head by a swinging bathroom door near the back of the bus.

"There were so many people screaming and yelling," Otte told the Grand Rapids Press. "I have bruises all over, which might be from getting bounced around or from being hit by the door."

The Detroit Free Press reported that Greyhound buses have been involved in 102 crashes over the past two years. Three of those accidents involved fatalities and 57 involved injuries. Nationally, in 2011 alone, there were 54,000 accidents involving buses, with 283 fatalities and 2,400 injuries.

Greyhound officials on Tuesday still had not identified a cause for the crash of its passenger bus near Cincinnati that left 35 injured, with six still hospitalized, after flipping over in a cornfield.