The somber announcement ended a multinational search-and-rescue operation that captivated the world since the tiny tourist craft went missing in the North Atlantic four days ago.
Rear Admiral John Mauger told reporters in Boston that analysis showed debris found on the seafloor, 1,600 feet (500 meters) from the bow of the Titanic, was consistent with the implosion of the sub's pressure chamber.
"On behalf of the United States Coast Guard and the entire unified command, I offer my deepest condolences to the families," Mauger said.
On board were British explorer Hamish Harding, French submarine expert Paul-Henri Nargeolet, Pakistani-British tycoon Shahzada Dawood and his son Suleman, and Stockton Rush, CEO of the sub's operator OceanGate Expeditions.
OceanGate said its "hearts are with these five souls and every member of their families during this tragic time."
"These men were true explorers who shared a distinct spirit of adventure, and a deep passion for exploring and protecting the world's oceans," it said in a statement.
"We grieve the loss of life and joy they brought to everyone they knew."
The Coast Guard announced earlier on Thursday that an underwater robot had discovered a "debris field" in the search area.
Authorities said they later learned the pieces included the sub's tail cone and front and back ends of its pressure hull.
Mauger said the coast guard could not be sure when or why the vessel imploded and declined to be drawn on whether remains of the men would be retrieved.
"This is an incredibly unforgiving environment down there on the seafloor," he said.
The process of demobilizing personnel and vessels from the scene would soon begin but unmanned robots would continue operations on the seabed for now, Mauger added.
"We'll collect as much information as we can," he said.
The small sub named Titan disappeared on Sunday as it descended to the Titanic, which sits more than two miles (nearly four kilometers) below the ocean's surface and 400 miles off the coast of Newfoundland, Canada.
OceanGate Expeditions charged $250,000 for a seat on the sub. In a 2018 lawsuit, its former director of marine operations raised concerns about the "experimental and untested design" of Titan.
Harding was a billionaire and keen explorer with three Guinness Records to his name, while the Dawoods belonged to one of Pakistan's richest families. Nargeolet was nicknamed "Mr Titanic" for his frequent dives at the site.
The British and Pakistani governments expressed their "deepest condolences" to all then men's families.
Forum