Titan sub: Investigators board Polar Prince as it returns to harbour

Officials from the Transportation Safety Board (TSB) of Canada board the Polar Prince after it arrived back in St. John's

Investigators in Canada have boarded the support ship used to launch the Titan submersible in their bid to understand what caused the vessel's catastrophic implosion.

Flags on board the Polar Prince were at half-mast as it docked in St John's, Newfoundland, on Saturday.

Another boat was seen in the harbour towing the Titan's launch platform.

The Titan was on a dive to the wreck of the Titanic when it broke apart, killing all five people on board.

Locals in St John's gathered around the cannon on top of Battery Lookout at 08:00 (11:30 BST) to watch the Polar Prince return to port. As some passengers disembarked, investigators in hard hats and high-visibility jackets climbed aboard.

The Polar Prince was the Titan's support vessel

The Polar Prince was the Titan's support vessel and had towed the submersible out to the area in the North Atlantic where it carried out its dive on Sunday, about 400 miles from St John's.

On board were members of the support team and some family members of the victims. It was also involved in the search for the Titan once it lost contact about one hour and 45 minutes into its dive.

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Parts of the submersible were found on the ocean floor on Thursday, approximately 1,600ft (487m) from the bow of the Titanic wreck.

Canada announced on Friday that it was launching a safety investigation. Other countries' government agencies may join in, but it is unclear at this stage which will lead the investigation.

As well as the role of the Polar Prince, experts say officials will also look at the materials used to make the sub's outer walls.

Since news of the accident broke, industry experts have come out to say that they had previously raised questions about safety practises at OceanGate, the company that owned the Titan, and whose CEO Stockon Rush was on board at the time of the accident.

Emails seen by the BBC showed Mr Rush dismissing concerns from one expert as "baseless cries".

Also on board the Titan were Shahzada and Suleman Dawood, Hamish Harding, and Paul-Henry Nargeolet.