The US Coast Guard will lead the investigation into the implosion of the Titan submersible which killed five people.

The US National Transportation Safety Board will assist in the investigation, while Canadian investigators are examining Titan’s Canada-flagged support ship, the Polar Prince.

Details about the ongoing investigation come as ships involved in the round-the-clock recovery efforts have started returning to shore.

The Canadian Coast Guard (CCG) confirmed two vessels were en route to St John’s harbour on Friday evening with one remaining on site.

The Polar Prince is due to arrive and dock in the early hours of Saturday morning.

A scaled-back search continued on Friday as the robots – remotely operated vehicles, known as ROVs – continued to scan the sea floor for evidence that might shed light on what occurred in the deep Atlantic water.

The bill for the five-day search for Titan will easily stretch into millions of dollars, experts say.

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The massive international effort by aircraft, surface ships and deep-sea robots began on Sunday when the Titan was reported missing.

Before it was clear the submersible suffered a catastrophic implosion, searchers raced against a 96-hour clock in the desperate hope to find and rescue the vessel’s occupants before their oxygen supply ran out.

The search area spanned thousands of miles in waters 2.5 miles deep, and involved agencies including the US Coast Guard, the Canadian Coast Guard, the US Navy and other agencies and private entities.

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The aircraft alone from these agencies have an hourly cost of tens of thousands of dollars, according to the Government Accountability Office.

Turboprop P-3 Orion and jet-powered P-8 Poseidon sub hunters, along with C-130 Hercules, were all used in the search.

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Some organisations will seek payment for their services but the US Coast Guard – whose bill alone will reach millions of dollars – is generally prohibited by federal law from collecting reimbursement pertaining to any search or rescue service.

The Titan operated in international waters, far from the reach of US or other national laws.

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It was not registered as a US vessel or with international agencies that regulate safety, nor was it classified by a maritime industry group that sets standards on matters such as hull construction.

British adventurer Hamish Harding and father and son Shahzada and Suleman Dawood were killed on board Titan, alongside the chief executive of the company responsible for the vessel, Stockton Rush, and French national Paul-Henri Nargeolet.

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The submersible lost contact with the tour operator an hour and 45 minutes into the two-hour descent to the wreckage of Titanic, with the vessel reported missing eight hours after communication was lost.

It is unclear exactly when or where the implosion occurred, but a US Navy acoustics system detected an “anomaly” on Sunday that was likely the Titan’s fatal end.

The Titan, owned by undersea exploration company OceanGate Expeditions, had been detailing the Titanic’s decay and the underwater ecosystem around the sunken ship in yearly voyages since 2021.

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