Federal authorities are assisting Ohio investigators Monday with the probe into the Greyhound bus crash near Cincinnati that occurred Saturday morning.

The U.S. Department of Transportation, the Ohio State Highway Patrol and the Butler County Sheriff’s Office are investigating the bus crash that injured 35 people when the Greyhound flipped over on Interstate 75. The bus was heading to Detroit from Cincinnati when it veered off I-75 and hit a tree before rolling over in a cornfield.

Authorities said Monday they have still not figured out why the bus crashed, although witnesses reported that the bus driver, 64-year-old Dwayne Garrett of Cincinnati, dozed off. A Greyhound spokeswoman told the Cincinnati Enquirer that the bus driver was “fully rested.”

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

The bus driver was trapped in the overturned bus and was injured in the crash, Mejia said. Investigators interviewed Garrett, and he consented to having his blood drawn. The results won’t be known for at least a week.

Greyhound spokeswoman Alexandra Pedrini told the AP that Garrett had a clean record and had 15 years of experience.

Investigators were particularly interested in the brakes of the bus, along with other parts, according to the wire service. Pedrini said the bus had just had its annual inspection.

Officials said the severity of the injuries could have been a lot worse, and no one sustained life-threatening wounds.

"We are very lucky," Liberty Township Fire Chief Paul Stumpf told USA Today, adding that the “excellent response” of first responders helped the situation. "The injuries are not as serious as they could have been."

As part of the investigation, the probe will also focus on surveillance video captured from inside the bus moments before the crash, Lt. Anne Ralston of Ohio Highway Patrol told the AP.