Franklin County and Illinois Department of Natural Resources dive teams and boats equipped with sonar equipment combed Rend Lake Monday and Tuesday for Emilie Jo Turrentine, 18, of Carmi who disappeared while boating with friends late Monday morning.
Her body was found by a dive team in about 10 feet of water shortly after 2 p.m. Tuesday.
Franklin County Sheriff Don Jones said the missing teen had just celebrated her 18th birthday on Aug. 29.
"This is really a tragedy," Jones said. "There is no sign of foul play and no alcohol or drugs are involved.
Franklin County Coroner Marty Leffler said an autopsy was underway Tuesday afternoon.
Authorities have no suspicion of foul play in the death.
"Five people were on a pontoon boat just north of Route 154 west of Sailboat Harbour," Jones said. "The four people with her watched her take a deep breath, then she dove under the water and didn't resurface. She did not seem to be in distress. Everyone was just having a good time."
Jones said Benton firefighters in a search and rescue boat, the Rend Lake Search and Rescue Team under the direction of the Sheriff's Department, the Illinois Department of Natural Resources, the U.S. Army Corps of Engineers and the Sesser Fire Protection District searched for the missing swimmer. "The call came in at 11:14 a.m. on Labor Day," Jones said. "They searched until 10:30 p.m. when fatigue overtook the divers. The search resumed at 8 a.m. on Tuesday."