An Army UH-60 Black Hawk helicopter went down during a training exercise about 2 miles off the Hawaiian island of Oahu, according to the US Coast Guard.
Responders are searching for five missing crewmen.
The aircraft went down Tuesday night off the largely uninhabited Kaena Point on the west side of the island.
Two Black Hawk air crews were training between Kaena Point and Dillingham Airfield, a few miles east, when communications were lost, the Coast Guard said.
Three helicopters, a Coast Guard cutter and response boat and the Honolulu Fire Department were among those responding.
The Coast Guard spotted a debris field near Kaena Point at 11:28 p.m. (5:28 a.m. ET), roughly 80 minutes after personnel at Wheeler Army Airfield in central Oahu lost communications with the helicopter.
Weather in the area is partly cloudy with winds of 15 to 25 mph and 2-foot seas.