Relatives and colleagues mourned on Friday five people, including two Pakistanis, who died when their submersible imploded in the North Atlantic during a deep dive to the Titanic wreck, triggering questions about safety rules for such adventures into the ocean depths.

Debris from the Titan submersible, which had been missing since Sunday, was detected on Thursday by a robotic diving vehicle deployed from a Canadian ship as part of an international rescue effort.

Remains of the submersible, which lost contact with a surface ship about one hour and 45 minutes into a two-hour descent, were discovered on the seabed about 488 metres from the bow of the Titanic wreck, about four kilometres below the surface, US Coast Guard Rear Admiral John Mauger said.

He told reporters on Thursday the debris was consistent with “a catastrophic implosion of the vehicle.”

“On behalf of the United States Coast Guard and the entire unified command, I offer my deepest condolences to the families,” Mauger said.

Authorities said they later learned the pieces included the sub’s tail cone and the 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 demobilising 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 sombre 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.

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, US founder and chief executive officer of OceanGate Expeditions, which operated the submersible and charged $250,000 per person to make the Titanic trip. He was piloting the craft.

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.

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.

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.

Nargeolet was nicknamed “Mr Titanic” for his frequent dives at the site. British Titanic explorer Dik Barton paid tribute to the work of his friend Nargeolet but noted issues raised with the design and maintenance of the craft. “Everyone’s wise after the event, but as we’re hearing before, unfortunately, there were many red flags flying here,” he 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.

A statement issued by the British Asian Trust on behalf of the Dawood family read: “We mourn the tragic loss of Shahzada and his beloved son, Suleman, who had embarked on a journey to visit the remnants of the legendary Titanic.”

The University of Strathclyde in Scotland, where Suleman studied, also issued a statement of condolence.

The Dawood Family through a Twitter post expressed their “profound grief”.

“Our beloved sons were aboard OceanGate’s Titan submersible that perished underwater,” the post said.

The family stated that they were grateful to the people who were involved in the rescue operations, adding that untiring efforts were a source of strength for them during this time.

“The immense love and support we receive continues to help us to endure this unimaginable loss,” the post said.

The family expressed condolences to the families of the other passengers who were on the submersible.

On Friday, the Dawood Foundation posted a brief statement grieving the loss of Shahzada and Suleman.

Engro Corporation also took to Twitter to offer their condolences to the Dawood family, grieving the “unthinkable loss” of their vice-chairman Shahzada Dawood.

“Our thoughts and prayers are with the Dawood family at this tragic time,” the conglomerate said in the Twitter post.

The government of Pakistan conveyed its “deepest condolences” to the Dawood family and the other passengers onboard the fateful submersible, according to Radio Pakistan.

According to the state broadcaster, Foreign Office Spokesperson Mumtaz Zahra Baloch, in a statement, said Pakistan appreciated the multinational efforts carried out over the last several days in search of the vessel.

“Every human eye is in tears over the tragic accident,” Foreign Minister Bilawal Bhutto Zardari said according to a statement issued by the PPP media cell.

The PPP Chairman stated that his heart goes out to the families of those grieved by the unfortunate and heartrending accident and also prayed for peace in the eternal life of all departed souls.

“I, on behalf of Pakistan and its people, also extend deepest sympathy and condolences to all those nations and the families whose citizens and loved ones were trapped in the unfortunate submarine,” he added.

President Dr Arif Alvi also extended his condolences to the Dawood family and offered prayers for them.

Questions about Titan’s safety were raised in 2018 during a symposium of submersible industry experts and in a lawsuit by OceanGate’s former head of marine operations, which was settled later that year.

Worldwide media coverage of the search overshadowed the aftermath of a far greater disaster from the wreck of a migrant vessel off Greece last week that killed hundreds of people.

International effort

Teams from the US, Canada, France and Britain had spent days scanning a vast swathe of open sea for the Titan.

A marine app showed the French research ship Atalante, which had raced to the area at the US Navy’s request, was still in the vicinity on Friday. It only reached the search area on Thursday with its deep-sea robot.

Mauger said it was too early to say when the Titan met its fate. He said sonar buoys used in the water for more than three days had not detected an implosion.

The position of debris relatively close to the wreck suggested it happened near the end of Sunday’s descent.

The US Navy said an analysis of acoustic data detected “an anomaly consistent with an implosion or explosion” near the submersible’s location when communication with Titan was lost.

A senior Navy official said in a statement first quoted by the Wall Street Journal that its information “while not definitive” was shared with commanders of the search mission.

The Journal, citing unnamed US defence officials, said the sound was picked up by a system used to detect enemy submarines.

Moviemaker James Cameron, who directed the 1997 Oscar-winning film “Titanic” which did much to revive global interest in the British ocean liner that sank in 1912, said he learned of the acoustic findings within a day and knew what it meant.

“I sent emails to everybody I know and said we’ve lost some friends. The sub had imploded,” Cameron, who has ventured to the wreck in submersibles, told Reuters.

He said he had sent those emails on Monday.

“There may be calls for more regulation, better understanding of how these sorts of vehicles are designed, built and certified for use in these deep-sea environments,” said Stefan Williams, professor of marine robotics at the University of Sydney.

Scientist and journalist Michael Guillen, who survived an expedition in 2000 that became trapped in the wreck’s propeller, said: “We need to stop, pause and ask this question, why do you want to go to the Titanic and how do you get there safely?”

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 lies about 1,450km east of Cape Cod, Massachusetts, and 640km south of St John’s, Newfoundland.

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.

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

The 6.5m Titan had been due to resurface seven hours after beginning its descent at 8am 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 around 20,000 square kilometres 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 on Tuesday and Wednesday. But Mauger said that ultimately the sounds did not appear to have any relation to the site of the debris.

Marine scientist and oceanographer David Mearns, who specialises 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.