Seven US Marines and four aircrew are missing, presumed dead after their army helicopter crashed during a night training exercise in Florida.
The UH-60 Black Hawk helicopter was reported missing Tuesday around 8:30pm, near Eglin Air Force Base.
The area was under heavy fog but it was not immediately clear if that played a part in the accident.
Base spokesman Andy Bourland said rescue teams located debris from the aircraft at around 2am and an investigation was ongoing.
"The thoughts and prayers of everybody here at the White House are with the families of those who were killed in this that occurred apparently overnight," spokesman Josh Earnest said.
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President Barack Obama had expressed his condolences in phone calls with the military commanders affected.
"The president reassured the commanders of the nation's deep appreciation for the many sacrifices that our men and women in uniform and their families make to protect and defend our country," Earnest added.
A second Black Hawk returned safely to base.
