The Titan submersible that has been missing for days in the Atlantic Ocean with five men on board suffered a “catastrophic loss” consistent with the implosion of the vessel, the United States Coast Guard has said.

US Rear Admiral John Mauger said on Thursday that a remotely operated underwater vehicle (ROV) found debris off the bow of the Titanic, the century-old shipwreck that was the intended destination of the expedition.

“The debris is consistent with a catastrophic implosion of the vessel,” Mauger told reporters.

The submersible, which had been missing since Sunday off the eastern coast of Canada, sparked a massive search spanning thousands of kilometres across the North Atlantic, pulling in US and Canadian agencies as well as other international assistance.

OceanGate, the company that owns the vessel, had announced on Thursday that the five crew members – four tourists and the company’s CEO, who was piloting the submersible – were believed to have died.

“We now believe that our CEO Stockton Rush, Shahzada Dawood and his son Suleman Dawood, Hamish Harding, and Paul-Henri Nargeolet, have sadly been lost,” the company said in a statement.

Dawood is a Pakistani-British businessman; his son is 19. Harding is a British billionaire and Gargeolet is a 77-year-old French explorer.

“These men were true explorers who shared a distinct spirit of adventure, and a deep passion for exploring and protecting the world’s oceans. Our hearts are with these five souls and every member of their families during this tragic time,” OceanGate said on Thursday.

Mauger said during Thursday’s news conference that an ROV first found the tail cone of the sub about 500 metres (1,600 feet) from the bow of the Titanic and later discovered five major pieces consistent with a “catastrophic loss of the pressure” in the vessel.

“Upon this determination, we immediately notified the families,” Mauger said.

“I offer my deepest condolences to the families. I can only imagine what this has been like for them. And I hope that this discovery provides some solace during this difficult time.”

Mauger said the undersea noises detected earlier in the week, which offered a glimmer of hope as crews raced to try to find the missing passengers, were not related to the vessel.

He said the Titan’s implosion would have produced a “significant broadband sound” that sonar buoys used by search crews would have picked up.

The Wall Street Journal and The Associated Press have since quoted unnamed officials in the US Navy saying the agency’s underwater sound monitoring equipment had indeed detected an “anomaly” shortly after Titan disappeared on Sunday. They indicated it was likely associated with the implosion.

The days-long search for the sub captured the world’s attention and drew a contrast between the efforts made to rescue the five men on board compared with what advocates described as global indifference to refugees who recently drowned in the Mediterranean.

The OceanGate expedition costs $250,000 per person. Its trip starts in St John’s, Newfoundland, on Canada’s east coast, before heading out to the Titanic wreckage site hundreds of kilometres to the southeast, the company’s website shows.

Dik Barton, a veteran Titanic explorer, told Al Jazeera earlier on Thursday that deep-sea expeditions are dangerous operations. “It’s a perilous place to go – dangerous, inhospitable, hostile,” Barton said. “And the circumstance at which this vessel disappeared was extremely odd.”

Speaking before the company released its statement confirming the deaths, Barton said it would be “crucial” to find the vessel and investigate what happened.

“There’s going to be a massive inquiry, and I’m sure legislation and regulation [will] become tighter and more scrutinising,” he said.

Mauger, of the US Coast Guard, acknowledged that there questions are swirling around how an incident like this could occur.

“This is something that happened in a remote portion of the ocean with people from several different countries around the world,” he said. “And so it is a complex case to work through, but I’m confident that those questions will begin to get answered.”

British Foreign Secretary James Cleverly described the death of the sub’s crew as “tragic”, noting that three of the people on board were from the United Kingdom.

“The UK government is closely supporting the families affected and expresses our deepest condolences,” Cleverly wrote on Twitter.

The family of two of those individuals, Shahzada Dawood and his son Suleman, issued a statement on Thursday thanking searchers for their efforts.

“We are truly grateful to all those involved in the rescue operations,” it said. “Their untiring efforts were a source of strength for us during this time.”