(CNN) -- The driver of a charter bus that overturned on a Minnesota freeway, killing two women, may have had a medical emergency, a spokesman for the State Patrol said Thursday.
"One factor being considered is whether the driver suffered a medical episode while behind the wheel," Patrol Capt. Matt Langer said.
The driver, 52-year-old Edwin Erickson of Elgin, was hospitalized in serious condition, authorities said. Langer said he had a valid driver's license.
The women who died, both from Minnesota, were identified Thursday as Rhonda Hill, 52, of Plainview and Pamela Holmquist, 56, of Kasson.
The accident happened Wednesday after the bus left a casino in Rochester, Minnesota, headed for Northwood, Iowa, Langer said.
Watch video of tour bus crash site 
Erickson and 21 passengers were injured when the bus went out of control outside Austin, Minnesota, authorities said. They said two people were in critical condition, and one person was unhurt.
Andy Skoogman, spokesman for the Minnesota Department of Public Safety, said the bus was eastbound on Interstate 90 when it veered across a median and westbound lanes before overturning and landing in a ditch on the side of the road. The wreck happened three miles west of Austin.
No other vehicles were involved.
Authorities said weather conditions didn't appear to be a factor in the accident.
