By MELISSA NELSON-GABRIEL 
Associated Press
    Human remains washed ashore in heavy fog Wednesday after seven Marines and four soldiers were killed in an Army helicopter crash during a night-time training mission off a Florida beach.
    All 11 service members were presumed dead, according to a Pentagon official who spoke with The Associated Press on condition of anonymity for lack of authority to be identified in the media.
    Kim Urr, 62, who works at the Navarre Beach campground near where the helicopter went down, said she heard a strange sound followed by two explosions around 8:30 p.m. Tuesday.
    "It sounded like something metal either being hit or falling over, that's what it sounded like. And there were two booms afterward, similar to what you hear with ordnance booms, but more muffled," Urr said.
    "We knew immediately that something was not right. We listened for sirens, but there were no sirens. Then this morning, we heard a lot of sirens," she added.
    Despite the presumption of death and the discovery of human remains, the military still considers it a rescue mission, said Sara Vidoni, a military spokeswoman for Eglin Air Force Base, outside Pensacola.
    "Our thoughts and prayers are with them and their families as the search and rescue continues," Defense Secretary Ash Carter said on Capitol Hill.
    Fog in the area reduced visibility to two miles or less when the UH-60 Black Hawk from the Army National Guard was reported missing Tuesday night, said Katie Moore with the National Weather Service in Tallahassee.
    The fog remained heavy Wednesday morning, as about a dozen airmen wearing fatigues walked shoulder-to-shoulder down the beach, scanning the sand. Searchers with dogs joined them, along with area law enforcement and rescue crews.
    Meanwhile, somewhere just offshore in the narrow sound that separates Santa Rosa Island from the Florida Panhandle mainland, search boats could be heard but not seen, blasting horns as their crews peered into the water.
    The Marines were part of a special operations group based in Camp Lejeune, North Carolina. The National Guard soldiers were from a unit based in Hammond, Louisiana. None were immediately identified so that families could be told first.
    The Army helicopter had taken off from an airport in nearby Destin to join other aircraft in the training area, which includes 20 miles of pristine beachfront under military control since before World War II.
    The military sometimes drops trainees into the water in the area, to make their way ashore from boats or helicopters.
    Test range manager Glenn Barndollar told The AP in August that the site is an ideal training area for special operations units from all branches of the military to practice over the water, on the beach and in the bay.