(Reuters) - The wreckage of the Army Black Hawk helicopter that crashed during a nighttime training mission off Florida's Gulf Coast has been found and the 11 service members on board are believed to be dead, a military official said on Thursday.

"At this point we are not hopeful for survivors," said Colonel Monte Cannon of Eglin Air Force Base, adding the search effort is now in recovery mode.

Seven Marines and four soldiers were on the UH-60 Black Hawk helicopter reported missing during a training exercise Tuesday night. The cause of the crash has not been released, although heavy fog was reported in the area at the time of the exercise.

The Marines were part of a special operations unit from Camp Lejeune in North Carolina. They were conducting training involving "helicopter and boat insertion and extraction," with the Army air crew providing the helicopter support, a Marine Corps spokesman said.

The soldiers and the helicopter were part of the Louisiana National Guard assigned to an Army unit based in Hammond, Louisiana.

The search focused on the Santa Rosa Sound, a stretch of water along the Florida Panhandle where the helicopter went down.

(Reporting by Colleen Jenkins; Editing by Bill Trott)