Northwest Daily News, Devon Ravine, Associated Press
An Okaloosa County ambulance sits at the Eglin Air Force entrance in Fort Walton Beach, Fla., Wednesday, March 11, 2015.

EGLIN AIR FORCE BASE, Fla. — Seven Marines and four soldiers aboard an Army helicopter that crashed over waters off Florida during a routine night training mission were presumed dead Wednesday, and crews found human remains despite heavy fog hampering search efforts, military officials said.

A Pentagon official said all 11 service members were presumed dead and that the Coast Guard found debris in the water. The official spoke to The Associated Press on condition of anonymity because the official wasn't authorized to speak on the record.

Human remains had washed ashore, but crews still considered it a search-and-rescue mission, said Sara Vidoni, a military spokeswoman for Eglin Air Force Base, outside Pensacola.

The helicopter — a UH-60 Black Hawk from the Army National Guard — was reported missing around 8:30 p.m. Tuesday, and crews found debris around 2 a.m., said Andy Bourland, a spokesman for Eglin Air Force Base, outside Pensacola.

Much of the area was enveloped in fog from Tuesday evening to Wednesday morning, said Katie Moore with the National Weather Service in Tallahassee. Much of that time, the visibility was at two miles or less, she said.

The fog created low visibility even as the sun came up, and the area was under a fog advisory.

Local law enforcement agencies vehicles gathered Wednesday at the crash scene, near a remote swath of beach between Pensacola and Destin. The beach is owned by the military, and as part of the Eglin base, it is used for test missions.

From the beach, search boats could be heard blasting horns as they combed the water but could not be seen through the fog. The Coast Guard had secured the waterways, Vidoni said.

Base officials said the Marines were part of a Camp Lejeune, North Carolina-based special operations group. The soldiers were from a Hammond, Louisiana-based National Guard unit. Names of those involved were not immediately released, pending notification of next of kin, Bourland said.

Bourland said the Army helicopter took off from a nearby airport in Destin and joined other aircraft in the training exercise.

The training area includes 20 miles of pristine beachfront that has been under the control of the military since before World War II. Military police keep a close watch on the area and have been known to run off private vendors who rent jet skis or paddle boards without permission.

Test range manager Glenn Barndollar told The AP in August that the beach provides an ideal training area for special operations units from all branches of the military to practice over the water, on the beach and in the bay.

The military sometimes drops trainees over the water using boats or helicopters and the trainees must make their way onshore.

Associated Press reporters Lolita C. Baldor in Washington and Freida Frisaro in Miami contributed to this report.