Riders were trapped upside down for more than two hours Saturday when the Demon roller coaster at Six Flags Great America got stuck in a loop.

Four of the 23 riders were treated and released from area hospitals.The train got stuck around 11 a.m. Firefighters and park personnel needed more than two hours to rescue all the riders, park spokesman Scott Kirn said.

Park officials were investigating how the train got stuck, Kirn said.

The park has not opened for the season but was rented out to U.S. Steel for a party, Kirn said.