Des Plaines police said an officer was patrolling the Rivers Casino parking structure when someone flagged them down, asking for an ambulance just before 1 a.m.
Police said one person, identified only as male, suffered a gunshot wound to his leg. The victim, whose age was not immediately known, was transported to a local hospital.
Rivers Casino Des Plaines released a statement later Saturday afternoon, saying a patron accidentally shot someone inside their vehicle in the parking garage. They added that no weapons were ever inside their casino.
Police are investigating the shooting.