Seven Marines and four soldiers were missing and presumed dead Wednesday after their military helicopter crashed in waters off the Florida Panhandle during a training exercise, military officials said.

Nearly 12 hours after the craft was reported missing, all 11 service members were presumed dead, a Pentagon official told the Associated Press.

The Army UH-60 Black Hawk, carrying Marines from a Camp Lejeune, N.C.-based Special Operations group and soldiers from a Louisiana National Guard unit, was on a night training exercise when it crashed late Tuesday. Officials said the aircraft went down in foggy conditions but cautioned that it was too soon to determine the cause of the crash.

At a news conference, Maj. Gen. Glenn Curtis, the adjutant general of the Louisiana National Guard, said the pilots of the Black Hawk had thousands of hours of flight experience and were "instructor pilots," indicating that they were experienced and qualified enough to train other pilots.

A second Black Hawk participating in the exercise returned to base after takeoff because of the weather conditions, Curtis said. That helicopter landed safely and all personnel on board were accounted for.

"One of them started to take off and then realized there was a weather condition and turned around and came back," said Curtis, speaking from Hammond, La.

Some human remains washed ashore late Wednesday, as local authorities, the Florida Fish and Wildlife Conservation Commission and the Coast Guard conducted search operations in the vicinity of Eglin Air Force Base, near Valparaiso in the Florida Panhandle.

Andrew Bourland, a spokesman for the base, said fog in the search area had made the operation more difficult.

The Black Hawk that crashed had a flight data recorder, the Army said, which if located could help investigators determine what caused the crash.

In an appearance on Capitol Hill, Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told lawmakers that the crash highlights the risk faced by U.S. military personnel, "whether in training or in combat."