A UH-60 Black Hawk helicopter had five crew members on board when it reportedly went down two miles west of Kaena Point, Oahu, according to a Coast Guard statement.
Several aircraft and boats are being used in the search.
Evelyn Mallory of Ko Olina said she was on the second floor of her residence when she heard a loud "vibration" sound at about 9:30 p.m. Tuesday.
Air and sea vessels from the Coast Guard, Army, Marine Corps and Honolulu Fire Department have been searching for survivors and debris near Kaena Point since late Tuesday night. Curt Kellogg, a 25th Infantry Division spokesman, during a news conference at the search's command center in Haleiwa Harbor.
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Three helicopters, a Coast Guard cutter and response boat and the Honolulu Fire Department were among those responding. Night training offshore is routine, he said.
The 25th Combat Aviation Brigade is based in Hawaii.
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Five Army aviators are missing after a UH-60 Black Hawk helicopter they were flying went down Wednesday near the island of Oahu. The incident occurred between Kaena Point, northwest of Hawaii's capital, Honolulu, and Dillingham Airfield.
This is the second major crash the U.S. military has experienced this month. The crash occurred at about 10:40 p.m., during a training mission.
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