Debris from the Titan sub – which could provide the first clues as to what caused its deadly implosion – has been brought back to port.

A ship that has been searching for the wreckage returned to St Johns, Newfoundland, on Wednesday (local time), and unloaded large sections of the doomed submersible.

Photos from the wharf show several pieces being lifted from the Horizon Arctic, including the nose cone with its distinctive circular window.

Other pieces included the landing frame and rear cover.

The experimental deep-water tourism vessel was constructed with two titanium end caps and a carbon fibre cylinder between.

The Titan was destroyed in a catastrophic implosion during its June 18 descent to view the Titanic site almost four kilometres below the surface of the sea, killing all five passengers and crew.

The US coast guard declared the men dead on Friday (AEST) after a remotely operated vehicle spotted the Titan’s wreckage about 500 metres from the bow of the sunken Titanic.

Debris from the Titan submersible is unloaded in St Johns, Newfoundland. Photo: AAP

One expert who assisted the US Coast Guard with the search said analysing the recovered debris could reveal important clues about what happened to the Titan.

And there could be electronic data, said Carl Hartsfield of the Woods Hole Oceanographic Institution.

“Certainly all the instruments on any deep sea vehicle, they record data,” he said.

“They pass up data. So the question is, is there any data available? And I really don’t know the answer to that question.”

The Canadian-flagged Horizon Arctic used an ROV to search the ocean floor for the remains of the submersible, about 700 kilometres south of Newfoundland.

The owners of the ROV, US-based Pelagic Research Services, confirmed that its team had successfully completed offshore operations.

The company said it was removing its equipment from the Horizon Arctic after a lengthy retrieval operation.

“They have been working around the clock now for ten days, through the physical and mental challenges of this operation, and are anxious to finish the mission and return to their loved ones,” the company’s statement said.

The US coast guard is leading the investigation into why the submersible imploded during its June 18 descent.

The US National Transportation Safety Board has said the Coast Guard has declared the loss of the Titan submersible to be a “major marine casualty” and the coast guard will lead the investigation.

OceanGate Expeditions, the company that owned and operated the Titan, is based in the US but the submersible was registered in the Bahamas.

OceanGate is based in Everett, Washington. It closed when the Titan was found.

Meanwhile, the Titan’s mother ship, the Polar Prince, was from Canada, and those killed were from Britain, Pakistan, France and the US.

Killed in the implosion were Ocean Gate CEO and pilot Stockton Rush; two members of a prominent Pakistani family, Shahzada Dawood and his son Suleman Dawood; British adventurer Hamish Harding; and Titanic expert Paul-Henri Nargeolet.

The operator charged passengers $US250,000 ($376,500) each to participate in the voyage.

The implosion of the Titan has raised questions about the safety of private undersea exploration operations.

The US coast guard also wants to use the investigation to improve safety of submersibles.