Greyhound Bus Overturns-More than 30 people were injured after a Detroit-bound Greyhound bus overturned on an Ohio interstate early Saturday, authorities said.

The bus was heading northbound on I-75 in Butler County, Ohio, when it overturned shortly after 4 a.m., Butler County Emergency Operation Center officials said in a news release.

"The bus started shaking...and we hit a tree then after we hit the tree we rolled... at least twice," said passenger Christopher Lake.

The bus was carrying 52 people. Thirty four people were transported to area hospitals, including six people who were air lifted.

"As we arrived, the injured and walking wounded were walking up the hill, seeking attention and we set up triage areas," said Liberty Township Fire Chief Paul Stump.

Four of the injured were in critical condition and 12 were in serious condition, according to authorities.

Ohio State Highway Patrol officials told ABC News that none of the injuries sustained in the crash appeared to be life threatening.

The cause of the accident has not been determined.