The gun, which was rusted inside and out and inoperable, was found at Nichols Elementary School in Fairfield County in the evening, Stratford Police said.
The caked-on dirt and debris on the weapon found in Stratford led police to believe it had been there for some time. It also matches a firearm that went missing from the scene of an incident in 2017, police said.
Authorities did not say what that previous incident was, but it happened near where the weapon was found. Detectives are investigating the gun's origins.
Police said there was no threat to the community.
Click here to follow Daily Voice Guilford and receive free news updates.