Crews searched along the fog-shrouded Florida coast on Wednesday, but seven Marines and four soldiers were feared dead after an Army Black Hawk helicopter crashed during a nighttime training exercise.
A spokesman at the Air Force base near where the chopper crashed, however, said crews were still in search-and-rescue mode even though a Pentagon official has said the 11 military members aboard are presumed dead.
Eglin Air Force Base spokesman Mike Spaits had said the search will continue throughout the night, but an "operational pause" was announced just before 9 p.m. because of weather.
"There is always room for optimism," he said. "The fog has been hampering our search efforts, and more fog is continuing to roll in."
Andy Bourland, another spokesman for the base, said that human remains had been found and that aircraft debris had washed ashore. It was too soon to say what caused the crash, but he told NBC News that there were "weather issues" overnight.
The Marines were from a special operations regiment at Camp LeJeune in North Carolina, Bourland said. The helicopters and air crew were assigned to the Army National Guard unit at Hammond, Louisiana.
President Barack Obama called the commanders of Marine special operations and the Louisiana Guard to express condolences, the White House said.
Maj. Gen. Glenn H. Curtis of the Louisiana National Guard told reporters at an afternoon news conference that "this will remain a search-and-rescue operation until further notice."
He said that the pilots of both helicopters had extensive experience, and would have been aware of the weather when they took off. But "you can depart from one station and hit weather that you didn't expect," he said.
Curtis said the Louisiana Guard was supporting the Marines on the training mission, but he had no further details on the nature of it. The names of the 11 servicemembers were not immediately released.
If the worst fears are realized, it would be one of the greatest losses of life in military training in recent years. Seven Marines were killed in a mortar blast at an Army depot in Nevada in 2013, and seven were killed when two helicopters collided in Arizona in 2012.
The 11 servicemembers were aboard one of two UH-60 Black Hawks participating in the exercise. The second helicopter returned safely.
Mike Spaits, another spokesman for the base, confirmed that there were fatalities but told reporters "there's always hope" for survivors. The aircraft debris, including the tail rotor of the downed chopper, was recovered by a Coast Guard vessel, officials told NBC News.
The helicopter was believed to have crashed at about 8:30 p.m. ET in Santa Rosa Sound, a narrow strip of water that separates the Florida Panhandle from a barrier island, just east of Pensacola.
Kim Urr, who works at the a campground near where the helicopter went down, told The Associated Press that she heard a strange sound followed by two explosions at about that time.
"It sounded like something metal either being hit or falling over, that's what it sounded like," she said. "And there were two booms afterward, similar to what you hear with ordnance booms, but more muffled."
Pat Campbell, who lives in the nearby town of Navarre, said that he came home from dinner at about 8:30 and noticed a fuel smell, "just like the BP spill."
On Wednesday, he said, "It's very foggy. If we walk down on my dock, you see the glaze of the jet fuel on the water, and the smell is very strong."
In Washington, Defense Secretary Ashton Carter, who was testifying to a Senate committee on the fight against ISIS, said: "Our thoughts and prayers are with them and their families as the search and rescue continues."
The Marine Corps said it was working "closely with all parties involved to locate our Marines and the Army aircrew as soon as possible."
"Our thoughts and prayers are with the Marines, soldiers and family members of those involved in this mishap," the corps said in a statement.
The downed helicopter departed from Destin, Florida. Eglin Air Force Base is in Valparaiso, Florida, about 45 miles east of Pensacola.
Emmanuelle Saliba of NBC News contributed to this report.