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The US Coast Guard announced that the five personnel onboard a submersible lost near the Titanic disaster died -- most likely in an instant -- when their vehicle experienced what it called a "catastrophic implosion" at the ocean's depths.

The statement marked the conclusion of search and rescue efforts that began when the small tourist vessel vanished in the North Atlantic four days earlier.

US 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.

Catastrophic compulsion and "unforgiving environment"

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.

After the announcement, OceanGate stated 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 added in a statement.

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

Authorities said they later acknowledged that the pieces included the sub's tail cone and the front and back ends of its pressure hull.

The Coast Guard, according to Mauger, is unsure of how or why the ship collapsed and has chosen not to speculate on whether or not the men's bodies will be found.

"This is an incredibly unforgiving environment down there on the seafloor," he said.

Unmanned robots will continue to operate on the seabed for the time being, Mauger noted, even though the process of demobilizing men and boats from the site would shortly start.

"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, as per a report published on Thursday by the Wall Street Journal.

"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.

OceanGate’s submarine was lost on Sunday during an expedition to the Titanic wreckage, located about 900 miles east of Cape Cod in the North Atlantic and at a depth of approximately 13,000 feet.

Double standards

When the submersible, which was operating as a "turismo" submarine and with a crew of five millionaires went down at a depth of over 4,000 meters, the Andriana, a fishing boat carrying more than 700 migrants fled Libya was headed for the Italian shore. The boat capsized off the Greek coast, killing at least 81 people and leaving another close to 500 people missing. 

In the first incident, a multi-million dollar rescue operation was launched to find the vessel in the North Atlantic. In the second, the whole world stood watching.

After the migrants' tragedy, reports that showed a contradicting tale that led to the tragic event began to surface. In the first in-depth testimonies of survivors, Greece's coastguard was accused of neglect and cruelty, waiting hours before saving the migrants with a broken engine. 

The two incidents highlighted Western double standards in responding to catastrophic disasters.

Read more: Greece wreck survivors retell story of coastguard abuse, complicity