Crystal Lake police responded to a report of a firearm found at South Elementary School's playground at 601 Golf Road after class had ended for the day.
Investigators believe 45-year-old Crystal Lake man George Kiska dropped a gun around 3:50 p.m. while he was at the playground with his child.
Another child found the gun and told their parent. Kiska was still there with his child when the gun was found, police said.
Police said Kiska was cited for Unlawful Possession of a Firearm in a School Zone.
Police asked anyone with information to call them at 815-356-3620 or send an anonymous tip by texting "CLPDTIP," along with the tip information to 847411(tip411).
Further information was not immediately available.