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(AFP) - All five people aboard a submersible missing near the wreck of the Titanic died -- likely in an instant -- after their vessel suffered what the US Coast Guard said Thursday was a "catastrophic implosion" in the ocean depths.

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.

The Coast Guard announced earlier Thursday that an underwater robot had discovered a "debris field" in the search area.

- 'Unforgiving environment' -

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 US military originally detected the likely implosion of the craft on secret underwater sound monitoring devices shortly after it went missing on Sunday, the Wall Street Journal reported Thursday.

"The US Navy conducted an analysis of acoustic data and detected an anomaly consistent with an implosion or explosion in the general vicinity of where the Titan submersible was operating when communications were lost," an unnamed senior Navy official told the Journal.

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.

Harding's family paid tribute to the aviation tycoon in a statement, saying he was a "passionate explorer" as well as a "loving husband and a dedicated father to his two sons."

"What he achieved in his lifetime was truly remarkable and if we can take any small consolation from this tragedy, it's that we lost him doing what he loved," the family said.

The Dawoods' loved ones also expressed their "profound grief" at their loss in a brief statement.

The British and Pakistani governments expressed their "deepest condolences" to all the men's families.

- Titanic's lure -

The 21-foot (6.5-meter) Titan had been due to resurface seven hours after beginning its descent at 8:00 am on Sunday.

But the craft lost communication with its mothership less than two hours in.

Ships and planes from the US and Canadian coast guards, as well as a robot sent from France, scoured 10,000 square miles (around 20,000 square kilometers) of surface water -- roughly the size of the US state of Massachusetts -- for the vessel.

The search honed in on areas where underwater banging noises were detected late Tuesday and Wednesday. But Mauger said that ultimately the sounds did not appear to have any relation to the site of the debris.

The Titanic hit an iceberg and sank in 1912 during its maiden voyage from England to New York with 2,224 passengers and crew on board. More than 1,500 people died.

It was found in 1985 and remains a lure for nautical experts and underwater tourists.

The pressure at that depth as measured in atmospheres is 400 times what it is at sea level.

Marine scientist and oceanographer David Mearns, who specializes in deep water search and recovery operations, said earlier the debris discovery indicated a rapid breakup of the submersible.

"The only saving grace about that is that it would have been immediate, literally in milliseconds, and the men would have had no idea what was happening," Mearns, who was friends with two of those onboard, told Sky News.

© Agence France-Presse