A U.S. military helicopter crashed into water off northwest Florida Tuesday night, presumably killing all 11 people on board. 

Seven marines and four aircrew members seem to have died when their Army Black Hawk helicopter crashed during a night time training mission, CNN reported. The helicopter was reported missing around 8:30 p.m. Tuesday, but Eglin Airforce Base spokesman Andy Bourland said debris began washing ashore around 2 a.m. Wednesday around Okaloosa Island. 

The helicopter crashed east of the Navarre Bridge, which crosses the Santa Rosa Sound connecting mainland northern Florida and a barrier island . 

Bourland said the search and rescue operation was ongoing, but they have found human remains, according to NBC News. Fog and poor visibility continues to hamper rescue efforts. He said it is too soon to tell what caused the crash, but the weather may have contributed to it. The missing Marines came from a special operations regiment out of Camp LeJune in North Carolina. The helicopters and aircrew came from the Army National Guard unit out of Hammond, La. Officials have not released the names of the Marines or aircrew. 

The UH-60 Black Hawk helicopter was not alone during the training mission Tuesday, CNN reported. A second UH-60 helicopter was out, but it returned safely to the base in Hammond. 

The UH-60 Black Hawk was introduced into service in 1979 following the UH-1 Huey used in the Vietnam War. The two-engine helicopter can reach up to 173 mph, and it can lift an entire 11-man infantry and its equipment. The frame was designed to protect the crew and passengers by progressively crushing on impact.