(AP) -- Searchers struggled Wednesday to find the seven Marines and four soldiers killed when a helicopter crashed, hampered by the same fog that plagued a nighttime training mission.

A second helicopter turned back safely shortly before the wreck, which left debris washing ashore along the Florida coast, officials said.

Military officials haven't said what caused the crash of the UH-60 Black Hawk, but the weather was bad enough for the other crew to return to land, said Maj. Gen. Glenn H. Curtis, adjutant general of the Louisiana National Guard.

The helicopter that crashed had a veteran crew from Hammond, Louisiana, that served multiple tours in Iraq and helped humanitarian missions after Gulf Coast hurricanes and the BP oil spill.

They were carrying unconventional warriors from the Marines Special Operations Command. Like the Army's Green Berets and the Navy's SEALs, they were highly trained to endure grueling conditions and sensitive assignments on land and at sea, from seizing ships to special reconnaissance missions and direct action inside hostile territory.

Tuesday night's training involved practicing "insertion and extraction missions," using small boats and helicopters to get troops into and out of a target site, said Capt. Barry Morris, spokesman for the Marine Corps Special Operations Command at Camp Lejeune, North Carolina.

Even the training can be dangerous.

"It has everything in it except for the bullets, so when they are actually training, it's like being in combat in a lot of ways," said Bruce Labrecque, the owner of the Mariners Inn in Hammond, Louisiana, near where the Guardsmen were based. The spot is popular for lunch for locals and military in the area.

President Barack Obama expressed his condolences to the families and said he's confident of a detailed and thorough investigation, said his spokesman, Josh Earnest.

"Our thoughts and prayers are with them and their families as the search and rescue continues," Defense Secretary Ash Carter said on Capitol Hill.

Kim Urr, 62, who works at the Navarre Beach campground near the Eglin Air Force Base training area, said she heard a strange sound, followed by two explosions around 8:30 p.m. Tuesday.

"It sounded like something metal either being hit or falling over, that's what it sounded like. And there were two booms afterward, similar to what you hear with ordnance booms, but more muffled," Urr said.

Human remains were found Wednesday before the weather deteriorated again, and all 11 service members were presumed killed. But it was still considered a search and rescue mission.

A small flotilla of boats searched the choppy water, airmen walked shoulder-to-shoulder down the beach, scanning the sand.

"There is always room for optimism," Eglin spokesman Mike Spaits said.

___

Associated Press contributors include Lolita C. Baldor in Washington; Jason Dearen in Gainesville, Florida; Freida Frisaro in Miami; Kevin McGill and Stacey Plaisance in Hammond, Louisiana; and Emery P. Dalesio at Camp Lejeune in North Carolina.