St. John's, Canada - A team of investigators has boarded the main support ship of the Titan submersible after it returned to the harbor following the deep-sea vessel’s fatal implosion.

Flags on board the Polar Prince were at half-mast as it arrived at the port in St. John’s in Newfoundland on Saturday, after four passengers and the pilot of Titan were killed in the incident near the wreckage of the Titanic.

Police and safety investigators could be seen on board the vessel after the Transportation Safety Board (TSB) of Canada announced it would be the subject of an investigation.

TSB officials could be seen boarding the Polar Prince shortly after it docked.

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Rib boats could be seen towing what appeared to be the Titan submersible’s launch platform away from the Polar Prince and further along the port.

Canadian Coast Guard (CCG) boats had already started to return to St. John’s harbor on Friday as the recovery operation began to wind down.

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

In a statement issued before ships began to return to the port, the CCG said the search and rescue operation had concluded.

The CCG said one of its vessels would remain on the scene and would "provide assistance and support to the recovery and salvage operations as requested by Maritime Rescue Coordination Center Boston."

The TSB said a team of investigators had been deployed to St. John’s to "gather information, conduct interviews and assess the occurrence."

In its own statement, the safety body said the investigation would be carried out "in accordance with the Canadian Transportation Accident Investigation and Safety Board Act and international agreements."

The TSB will not determine civil or criminal liability and conducts investigations for "the advancement of transportation safety."

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The investigation comes after the BBC reported that emails from Rush showed he had dismissed safety concerns over the Titan submersible.

In the exchanges with deep-sea exploration specialist Rob McCallum, OceanGate’s chief executive said he was "tired of industry players who try to use a safety argument to stop innovation."

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

In the days that followed the gone-missing report, the US Coast Guard said the vessel had a depleting oxygen supply that was expected to run out on Thursday.

A report from The Wall Street Journal said the US navy had detected a sound in the search area for the submersible on Sunday that was consistent with an implosion.

The Associated Press, citing a senior military official, reported that the navy passed on the information to the Coast Guard, which continued its search because the data was not considered by the navy to be definitive.