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(Updated at 1/8/2014 at 8:15 p.m.)

VIRGINIA BEACH - Two U.S. sailors died Wednesday and a third was missing after their minesweeper helicopter went down in the Atlantic Ocean during routine training off Virginia, the Navy said.

Two other crewmembers were being treated for injuries at a Norfolk hospital.

Officials said a Navy helicopter rescued four crewmembers about 20 miles off Cape Henry after their MH-53E Sea Dragon went down about 11 a.m. One was declared dead soon after arriving at Norfolk Sentara General Hospital, and the second sailor died hours later, the Navy said Wednesday night.

The search for the missing sailor was to continue throughout the night.

Navy officials said the helicopter was one of two Sea Dragons assigned to the Helicopter Mine Countermeasures Squadron 14 at Norfolk Naval Station that were training.

Officials said the 87-foot patrol boat Shearwater, out of Portsmouth, a small boat from Station Little Creek and two MH-60S helicopters responded. A Virginia Beach fire boat was assisting the military, officials said.

Meteorologist Evan Stewart said the water temperature was about 40 degrees and the air temperature about 30 degrees at the time of the crash.

The Sea Dragon, built by Sikorsky, is a modified version of the more common CH-53E Super Stallion and can operate from aircraft carriers or other warships. Primarily used to clear mines from shipping lanes, it also delivers heavy equipment to and from ships and can transport up to 55 combat troops.

At 99 feet long, the Sea Dragon is the largest helicopter in the Western world.

A July 2012 crash of another Sea Dragon killed two of five crewmembers during an attempt to retrieve a downed helicopter belonging to the Royal Air Force of Oman. A Navy investigation blamed the Norfolk-based crew for not conducting preflight safety checks and not having an established plan for aborting the mission in the mountains outside Muscat, the capital.

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