Seven Marines and four soldiers were presumed dead after an Army helicopter crashed during a nighttime training mission off the Florida coast, where some remains have washed ashore and search efforts were hampered by heavy fog, U.S. military officials said on Wednesday.
A spokeswoman for the Eglin Air Force Base in north Florida did not provide details on the remains.
"This is still considered a search and rescue mission," spokeswoman Sara Vidoni said in a statement, adding that heavy fog hampered search efforts.
One of two UH-60 Black Hawk helicopters participating in the routine exercise crashed near the base 50 miles (80 km) east of Pensacola, and rescue workers discovered debris around 2 a.m. on Wednesday, base spokesman Andy Bourland said.
A U.S. military official, speaking on condition on anonymity, said the 11 service members aboard were presumed dead in what could be among the deadliest domestic military training accidents in years.
Bourland said the helicopter was believed to have gone down over water during the mission. He said he did not know how fog in the area affected visibility.
The Marines were part of a special operations unit from Camp Lejeune in North Carolina, while the air crew and helicopter belonged to the Louisiana Army National Guard, Bourland said.
Four crew members were part of the Louisiana National Guard, according to a news release from the state governor's office.
The second helicopter landed safely, Bourland said. Names of the missing troops were being withheld pending notification of next of kin, said Bourland.
"Our thoughts and prayers are with them and their families as the search and rescue continues," Defense Secretary Ash Carter said at the start of his testimony before a congressional committee in Washington.
Major General Joseph Osterman, commander of U.S. Marine Corps Forces, Special Operations Command, said: "We are working closely with all parties involved to locate our Marines and the Army air crew as soon as possible."
In February 2012, seven Marines were killed when two helicopters collided during a nighttime training exercise along the California-Arizona border.
The following year, another seven Marines died in an explosion at a Nevada munitions depot, after a mortar round detonated prematurely during a live-fire training exercise. Eight other servicemembers were injured in that incident.
The latest incident occurred at an air force base spanning 464,000 acres in the Florida Panhandle that is used extensively for training.
(Reporting by Suzannah Gonzales, Colleen Jenkins, Phil Stewart, Letitia Stein and Curtis Skinner; Editing by Susan Heavey, Doina Chiacu and Lisa Lambert)