NAVARRE, Fla. - Military search crews have found the core of the Black Hawk helicopter that crashed amid dense fog during a Florida training mission, killing 11 people. The flight recorder is still in the wreckage, which settled below about 25 feet of water in the middle of Santa Rosa Sound, authorities said.

Late Thursday, the Louisiana National Guard said the bodies of two soldiers have been recovered.

With the discovery of the copter's shattered core, which has been inspected by divers, the response officially had changed from rescue to recovery, Col. Monte Cannon, vice commander of the 96th Test Wing, said at Eglin Air Force Base.

"It was certainly a high-impact crash," said Eglin Fire Chief Mark Giuliano.

"Very, very, very dense fog" still complicates recovery efforts, Giuliano added. There's almost no visibility at the crash site, and search crews in boats are moving as slowly as they can in the rough surf to avoid running into each other or wreckage from the crash, he said.

The military is not yet releasing the names of those killed, nor any details about them, Cannon said.

Neither has it described the cause of the crash, which happened Tuesday night in weather conditions so bad that another helicopter turned back.

The same fog and crashing gulf waves surrounded a pier where a large gathering held vigil Wednesday night, creating a somber backdrop to the songs, tears and prayers of people with strong ties to the military and Eglin Air Force Base.

Dozens of airmen walked the shores of Santa Rosa Sound on Thursday, and the Coast Guard searched for debris in the water, said Mike Spaits, a base spokesman. Pieces of clothing and bits of the UH-60 Black Hawk helicopter have washed ashore, and homeowners have contacted the military to pick it up, he said.

Jack Cullen, a National Weather Service meteorologist in Mobile, Ala., said that Thursday's dense sea fog could persist through Friday, which is common when warm southern air meets cold water this time of year.

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

The helicopter was carrying "unconventional warriors" from the Marines Special Operations Command.

The copter crashed in a strip of water between the mainland of the Florida Panhandle and a long barrier island facing the gulf. Military officials said search crews were focused on a six-mile stretch of the sound.

President Obama has promised a thorough investigation.