FloridaSeven Marines and four soldiers were presumed dead after an Army Black Hawk helicopter crashed during a nighttime training mission off the Florida coast, where some human remains have washed ashore, 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 and officials did not immediately release information on what might have caused the crash. 

Local media showed images of what appeared to be helicopter parts and base officials asked nearby residents to report any findings. The military in the morning called the effort a "search and rescue mission." Heavy fog in the morning hampered the search.

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 about 2 a.m. EDT on Wednesday, the base said in a statement.

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. President Obamphoned military officials to express condolences, White House spokesman Josh Earnest told reporters, noting the president anticipated a detailed investigation.