According to a Six Flags Great America representative, safety sensors stopped the ride and riders were escorted off.
The Gurnee Fire Department was asked not to assist in the incident.
The Six Flags statement said in full:
"Safety sensors on one of our coasters stopped the coaster during the initial ascent. All guests were safely escorted off the ride and continued with their visit. The safety of our guests is always our highest priority and the ride will be closed for a thorough inspection before re-opening."
No injuries have been reported and no reason for the ride malfunction has been given.