An Army UH-60 Black Hawk helicopter went down during a training exercise about two 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.
The UH-60 is an Army utility helicopter used in air assaults, air cavalry and as a medical evacuation unit.
Other aircraft tragedies
The crash of the UH-60 is the latest episode in a series of recent US military aircraft crashes.
Earlier this month, three Marines were declared dead after their aircraft crashed off the coast of Queensland, Australia. Of the 26 personnel onboard the MV-22B Osprey tiltrotor aircraft, 23 were rescued.
The bodies of 1st Lt. Benjamin Cross, Cpl. Nathaniel Ordway and Pfc. Ruben Velasco were never found, according to a Marine Expeditionary Unit spokeswoman.
In July, 15 Marines and one Navy corpsman died after their KC-130T aircraft crashed in Mississippi. The military transport plane was moving personnel and equipment from North Carolina to a Naval Air Facility in El Centro, California. The troops were then to be taken to a base in Yuma, Arizona, for pre-deployment training.
Both crashes prompted the US Marine Corps to order all of its aircraft grounded for a 24-hour period this month so unit commanders could focus “on the fundamentals of safe flight operations, standardization, and combat readiness.”
The aircraft were not all grounded at the same time.
The Army lost another UH-60 Black Hawk in April when it went down on a golf course in Leonardtown, Maryland, during a training exercise. One crew member was killed.