A Greyhound bus drove off an interstate highway in southwest Ohio early Saturday, struck a tree and a fence and flipped on its side before sliding to a stop in a cornfield, injuring at least 35 people.

None of the injuries was considered life-threatening, though several people were trapped and had to be extricated by firefighters and paramedics, the State Highway Patrol said in a statement.

Patrol Sgt. Pete Combs said an investigation was underway and he could not comment on a possible cause for the crash, which happened around 4 a.m. on I-75 in Liberty Township, about 25 miles north of Cincinnati.

Passenger Christopher Lake, of Michigan, told WCPO-TV in Cincinnati he thinks the drive may have suffered a medical problem. He said he saw the driver slumped over and heard a woman scream “Wake up! Wake up!”

Lake told the TV station the driver seemed fine when he boarded the bus. He said the bus rolled over at least twice after hitting the tree.

Jeff Galloway, director of the Butler County Emergency Management Agency, said 35 people were taken to hospitals, six by helicopters and 29 by ambulance. Their injuries ranged from minor to severe, officials said.

The bus, which left Cincinnati bound for Detroit, was carrying 51 passengers and the driver. Those passengers who were not injured and those who were treated and released from hospitals were transported back to Cincinnati.

At least eight people remained hospitalized Saturday night. Their conditions were not released.

Lake, who was not injured, told WCPO he saw some children on the bus and thought some passengers had suffered broken arms and legs.

Passenger William Brown told Hamilton (Ohio) Journal-News that he was asleep in the bus when it overturned. He was headed to Detroit to visit his family.

“I never thought that I would be part of an incident that occurred, especially today, just trying to get home to see my family,” Brown said. “I’m just glad that a lot of people are still alive and okay.”

The driver, who has been with the company for almost 15 years, had been on duty for only an hour at the time of the crash and was fully rested, said Kim Plaskett, a spokeswoman for Dallas-based Greyhound Lines Inc.

The driver was among the injured, but she said Plaskett could not release his name or medical condition due to medical privacy laws.

She said she also could not discuss any details of the crash or the possible cause. The company was cooperating with investigators and will talk to the driver and conduct an internal investigation to try to determine what happened, Plaskett said.

The bus just had its regular major annual inspection 14 days ago, Plaskett said. She said drivers also do pre-trip inspections to make sure buses are fit for travel.

According to the Federal Motor Carrier Safety Administration, Greyhound in the two years prior to Friday had reported 102 crashes, 57 with injuries and three of them fatal.

Plaskett said Greyhound sent a crisis-response team to the scene of Friday’s crash to help the customers and authorities as soon as the company was notified of the crash.

A telephone hotline was set up for friends and family members seeking information about the passengers on the bus. The phone number is 800-972-4583.

The skies were clear in the region early Saturday morning, according to the National Weather Service.