The incident occurred in New Haven County on Thursday, June 20, at the Seymour Reservoir No. 2 in Oxford.
According to Will Healey, spokesman for the Department of Energy and Environmental Protection, DEEP Environmental Conservation (EnCon) Police, along with Oxford Police, Oxford Fire, and the Region 5 Dive Team, responded to a report of a swimmer who went underwater and did not resurface at the reservoir.
A 21-year-old man was recovered by the dive team unconscious and unresponsive and, following a period of CPR was declared dead on the scene, Healey said.
The incident remains under investigation by EnCon Police.
Healey said the State Office of the Chief Medical Examiner will determine the cause and manner of death.
The victim's name is being withheld pending notifications of family members.
Healey added that Seymour Reservoir No. 2 is not a designated swimming area.
This is a developing story. Check back to Daily Voice for updates.
Click here to follow Daily Voice Westport and receive free news updates.