Three people were injured after shots were fired from a single vehicle in the parking lot, according to Six Flags. The vehicle then immediately drove away.
Park security and officers from the on-site Gurnee Police Department Substation responded immediately, park officials said.
Two people were transported by emergency services for evaluation. A third person declined treatment, the park said.
"The park closed early with guests and team members departing under direction of the Gurnee Police Department," Six Flags said in a statement.
People can be seen in video huddling together inside the park. Another video shows police lights in the distance.
3 injured after shooting in parking lot at Six Flags Great America in Illinois, park says
The amusement park is located in Gurnee, Illinois, approximately 45 miles north of Chicago.
Social media reports of an active shooter have been dispelled by the Lake County Sheriff's Office, which is assisting the Gurnee Police Department with the investigation. A spokesperson saying there is no evidence of that at this time.
No further details have been provided by police.
This is a developing story and more information will be provided when available.