AUSTIN, Minn. (AP) — Investigators were starting to reconstruct a tour bus accident that killed two people and injured 21 in southern Minnesota, seeking answers to what caused the driver to swerve off a clear, dry interstate freeway and roll the bus into a ditch.

The bus was traveling east on Interstate 90 just west of Austin on Wednesday afternoon, carrying a group of mostly senior citizens on their way home from a day trip to an Iowa casino. It crossed the median into the westbound lanes, flipped over and ended up on its side in the ditch north of the freeway, said Minnesota Department of Public Safety spokesman Andy Skoogman.

Passenger Ardell Swanson, 71, of Austin, said she was just putting her head back to rest when the bus crashed.

“When I got myself organized there was all kinds of red and white and blue lights flashing,” Swanson said. “There was glass all over.”

The bus was carrying 23 people including the driver. Minnesota State Patrol Capt. Matt Langer said injuries ranged from minor to critical; details about those who died were not immediately released.

The driver was in serious condition Wednesday night at St. Marys Hospital in Rochester, where two passengers also were in critical condition. Langer said the driver had been interviewed.

Eight of 14 passengers taken to Austin Medical Center also remained hospitalized Wednesday night, as did three of six people taken to Albert Lea Medical Center. Hospital officials said no further information would be released until Thursday morning.

Langer said the bus had no seat belts, and when it overturned, some passengers were trapped underneath. Rescue crews were forced to pull off the bus’s windshield because the 47-passenger coach had settled on its right side, blocking the door.

Tammy Eggum of Hayward, who was driving in the other direction at the time of the crash, told the Austin Daily Herald that the bus briefly went airborne before landing on its side.

“It was like the movie ’Speed,”’ Eggum told the newspaper.

Swanson was treated at the Austin hospital for what she called minor injuries.

“I’m just going to be really sore,” she said.

The bus was operated by Strain Bus Line, which runs a bus every Wednesday from Rochester to Diamond Jo Casino in Northwood, Iowa, about 30 miles southwest of Austin. The owner of the company did not return messages from The Associated Press.

The company’s owner, Dalmer Strain, told the Star Tribune in Minneapolis that the bus was being operated by a veteran driver in his 50s who also had experiencing driving semitrailer trucks.

Bold Lines Inc., which does business under the name Strain Tours, is a small operator with six drivers and four buses and has had no accidents in the past two years, according to the Federal Motor Carrier Safety Administration Web site.