A search is under way in the US for seven Marines and four soldiers after their army helicopter came down while training in Florida.
The crash happened near Pensacola during a night training exercise at Eglin Air Force Base.
An Eglin spokesman told the BBC human remains have washed ashore, but officials still consider it a search and rescue mission.
Debris from the aircraft was found early Wednesday morning.
During a hearing in Washington on Wednesday, Defence Secretary Ashton Carter said "all our thoughts and prayers are with them and their families as the search and rescue operation continues".
Fog has made the search for the UH-60 helicopter - also known as a Black Hawk - difficult.
Much of the area was enveloped in fog from Tuesday evening, said Katie Moore with the National Weather Service in Tallahassee, with visibility at two miles (3.2km) or less.
Unnamed military officials have told US media that all 11 service members are presumed dead.
A second helicopter participating in the routine training mission returned safely.
Both aircraft are assigned to the 1-244th Assault Helicopter Battalion in Hammond, Louisiana.
The seven Marines involved in the accident were part of a special operations group based at Camp Lejeune, North Carolina.
The soldiers were from a National Guard unit based in Hammond, Louisiana. Their names are not being released while relatives are notified.
The beach, water and bay are said to provide an ideal training area for special operations units from all branches of the military.