The Lake County Coroner's Office identified the boy as Brian Monzanga of Round Lake.
Chief Christopher Covelli with the Lake County Sheriff's Office said he either jumped off a boat or fell into the water in Myers Bay, which is part of Pistakee Lake, and never resurfaced.
Authorities said the swimmer's friends jumped in and tried to find him, but were unable to.
That's when the 911 call came in around 6 p.m. and minutes later, emergency crews arrived on scene near north suburban Fox Lake.
Divers and search teams conducted what they hoped would be a rescue operation - before the search shifted to a recovery.
The missing teen was found nearly two hours later by an underwater drone-device. He was not wearing a life jacket.
"Just recreational swimming on the water, went into the water and never came back up. At that point, we were contacted via 911 and started our operations and response," Fox Lake Fire Deputy Chief Samuel Santiago said. "This is a great location to come bring your families and that's what they were just trying to do. So just from that aspect, from the organization to the family, we're very sorry for your loss."
The incident remains under investigation by the Illinois Department of Natural Resources Conservation Police and the Lake County Coroner's Office.