Two men who were not wearing life vests were thrown from a tube and failed to surface on Douglas Lake near McGuire Creek in Jefferson County.
The victims were being towed by a pontoon boat.
The Jefferson County 911 office received a call at 6:30 p.m. on Thursday and rescuers were dispatched. The search was suspended because of darkness at 9:30 p.m. and resumed at 9 a.m. Friday. The search and investigation are ongoing.