Connor Priesz, 18, of Crystal Lake, is in critical condition after being pulled by friends from the Fox River on Monday afternoon.
Priesz was swimming with a group of friends when he allegedly went under and did not re-surface. The friends pulled him to shore at about 4 p.m.
The McHenry County Sheriff Department's Marine Unit and Nunda Rural Fire Protection District responded to Timberland Road near the Fox River on the east side of Crystal Lake, Undersheriff Andrew Sinke said on Tuesday.
Timberland Road is off Behan Road near the Prairieview Education Center.
Priesz was unconscious, and may have suffered a seizure, according to the McHenry County Sheriff's Department. He was transported to Centegra Hospital in McHenry, where he was listed in critical condition, the Northwest Herald reports.