Ohio State Highway Patrol Lt. Edward Mejia said the driver, who was injured when he was trapped in the overturned bus, but was "conscious and alert" when authorities responded to the scene.
Driver Dwayne Garrett, 64, also willingly had blood drawn for testing for illegal substances.
“We’re still putting together pieces of the puzzle,” said Highway Patrol Lt. Edward Mejia. “We haven’t determined the cause.”
Investigators checked the bus's brakes and mechanical systems Monday, even though Greyhound said the bus had recently undergone its annual inspection.
The Detroit-bound bus overturned about 4 a.m. on I-75, after running off the highway between the a rest area and Ohio 63 exit to Monroe, landing on its side in a cornfield.