The U.S. Coast Guard will lead an investigation into the catastrophic implosion of the Titan submersible, which broke apart with five aboard and sank to the ocean floor during a dive to the Titanic earlier this week.

All five passengers and crew were presumed dead Thursday, soon after a crew guiding a remotely operated vehicle spotted the Titan鈥檚 wreckage about 500 metres from the luxury liner鈥檚 bow, almost four kilometres below the surface of the North Atlantic.

The U.S. National Transportation Safety Board issued a statement Friday saying the U.S. Coast Guard had declared the loss of Titan a 鈥渕ajor marine casualty鈥� and, as a result, would lead the investigation.

Peter Knudson, a spokesman for the board, said Friday the NTSB had joined the probe.

Later in the day, the independent Transportation Safety Board of Canada issued a statement saying it, too, was launching an investigation, noting that Titan鈥檚 support ship, the Polar Prince, is a Canadian-flagged vessel.

鈥淭he TSB (is) the investigation authority of the flag state of the support vessel,鈥� the Canadian board said in a statement, adding that its investigation will focus on 鈥渢he circumstances of this operation conducted by the Canadian-flagged vessel Polar Prince.鈥�

The board said TSB investigators had already been dispatched to St. John鈥檚, N.L., where the Polar Prince and Titan were based prior to the ill-fated expedition. The ship, a former Canadian Coast Guard vessel, is owned by the Miawpukek First Nation in southern Newfoundland.

鈥淚n the coming days, we will co-ordinate our activities with other agencies involved,鈥� the TSB said. The board鈥檚 investigations, which can take years to complete, can鈥檛 assign fault or determine civil or criminal liability.

The TSB said there were 17 crew members and 24 people on board the Polar Prince when five of them boarded Titan on Sunday morning and submerged on a course toward Titanic, about 700 kilometres southeast of Newfoundland. About one hour and 45 minutes later, the support vessel lost contact. The trip to the bottom usually takes about two hours.

In all, five large pieces of the submersible were spotted Thursday.

Carl Hartsfield, a director of the Woods Hole Oceanographic Systems Laboratory, in Falmouth, Mass., said the location of the debris was consistent with where the vessel was expected to be when it lost contact with the surface ship.

Since the beginning of the search late Sunday, the U.S. Coast Guard has repeatedly referred to Titan as a Canadian vessel, though the company that operated the small craft, OceanGate Expeditions, is based in Washington state.

The company鈥檚 CEO, Stockton Rush, was piloting the submersible when it imploded under the extreme pressure near the sea floor. The passengers on board were British billionaire Hamish Harding, French explorer and Titanic expert Paul-Henry Nargeolet and Pakistani businessman Shahzada Dawood and his 19-year-old son, Suleman.

Marc Isaacs, a maritime lawyer in Toronto, said the Canadian and American safety boards will likely work together. 鈥淚 doubt two organizations like that are going to get into a turf war about who leads an investigation.鈥�

OceanGate, based in Everett, Wash., continues to face intense scrutiny over its safety practices.

Richard Garriott de Cayeux, president of the U.K.-based Explorers Club, said Friday he was grieving the loss of two friends who were on the Titan, but he also made a point of citing the 鈥渞egular criticism鈥� he had heard about the vessel. He said Titan was never inspected or certified by an industry authority, which he said made it the only deep-diving submersible to carry commercial passengers without that level of safety approval.

鈥淭his 鈥� made me uncomfortable to ride in the Titan, and I expressed my misgivings to friends who asked,鈥� Garriott de Cayeux wrote in a statement, adding he didn鈥檛 feel it was appropriate to say this during the search-and-rescue operation.

The Explorers Club president is founder of Space Adventures, the first company to arrange space flights for private citizens. Two of those who died on the Titan 鈥� Harding and Nargeolet 鈥� were club members.

鈥淲hile we should all appreciate efforts to innovate in order to push the boundaries of exploration, this must be done safely and sensibly,鈥� he said.

Marine engineers have also drawn attention to the fact that Titan, which had a carbon-fibre hull, was never 鈥渃lassed鈥� or certified by an independent third party to ensure it met certain safety standards. In 2018, a group of engineers wrote a letter warning that the company鈥檚 鈥渆xperimental鈥� approach could have 鈥渃atastrophic鈥� consequences.

In response, OceanGate explained on its website that Titan was not classed because the process could inhibit innovation.

Meanwhile, the U.S. navy confirmed late Thursday its acoustic sensors detected 鈥渁n anomaly consistent with an implosion鈥� in the deep area of the ocean where the doomed submersible was operating Sunday, several hours before it was reported missing.

In an email, a senior U.S. navy official said an analysis of the acoustic data was shared with the 鈥渦nified command鈥� leading the search.

The U.S. navy has long maintained a network of listening devices on the floor of the North Atlantic to detect hostile submarines.

The navy official said the finding was not definitive, adding that the information was considered with acoustic data provided by other partners in the search for the Titan, which was led by the U.S. Coast Guard. The official said the decision was made to continue with the search-and-rescue mission and 鈥渕ake every effort to save the lives on board.鈥�

But information about the anomaly wasn鈥檛 shared with the public until after the Titan鈥檚 wreckage was found and all five aboard were presumed dead.

The Canadian Armed Forces, which took part in the search, did not respond to a request for comment about the U.S. navy鈥檚 data.

In Halifax, a Canadian military official confirmed Friday that the Canadian Coast Guard vessel John Cabot would remain in the search area to help with the recovery operation, though no details were released.

The U.S. Coast Guard has yet to say whether any of the fractured pieces of the sunken submersible would be retrieved from the ocean floor.

Some of the other Canadian ships and aircraft used during the search were sent back to their bases Friday, including coastal defence vessel HMCS Glace Bay, based in Halifax, and a CP-140 Aurora long-range patrol aircraft, based at CFB Greenwood, in western Nova Scotia.

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