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That letter, made public Monday, is part of an update the safety board released on its investigation into the June 18, 2023, dive by the Titan submersible in the waters southeast of Newfoundland that killed all five people on board.

The investigation has learned that before and after June 2023, a number of submersibles with people on board operated in ºÃÉ«tv waters and that while some are registered in Canada or another country, others are not.

"The investigation has been unable to confirm whether these submersibles have been subject to oversight by (Transport Canada) or other certifying bodies," the safety board says in its letter dated Thursday.

The board says it issued the letter to Transport Canada "for whatever followup action is deemed appropriate," adding it would like to be advised about how the federal government responds to the issue.

A Transport Canada spokesperson was not immediately available for comment. The board says on its website that safety information letters are sent to regulatory or industry stakeholders "to advise them of potentially unsafe acts or conditions identified during an investigation that pose low risks and do not require immediate remedial action."

Titan was not registered or certified in Canada or elsewhere to ensure that it met safety standards, and marine engineers had warned that the "experimental" approach taken by the vessel's owner — OceanGate — could have catastrophic consequences.

The ºÃÉ«tv-flagged cargo ship Polar Prince towed the Titan to its dive site, about 600 kilometres off Cape Race, N.L. About one hour and 45 minutes after it began its descent, the team on the Polar Prince lost contact with the submersible. The safety board notes that for previous Titan expeditions in 2021 and 2022, the submersible was not towed but was transported to the dive site on the deck of another ºÃÉ«tv ship, the Horizon Arctic.

On June 22, 2023, the United States Coast Guard said a catastrophic implosion had ripped apart the Titan submersible, killing all five people aboard, and that it had found pieces of the Titan on the ocean floor near the Titanic wreck. Those killed were the vessel's pilot, OceanGate Expeditions CEO Stockton Rush; British billionaire Hamish Harding; French explorer and Titanic expert Paul-Henry Nargeolet; and Pakistani businessman Shahzada Dawood and his 19-year-old son, Suleman.

Coast guard experts said the expanse of the debris field indicated that the Titan came apart in the water column above the steamship, which sank on April 15, 1912.

In 2018, a group of engineers and industry professionals had written to the Titan's owners to express their concern that the vessel had not been certified. In a 2019 blog post, OceanGate explained that Titan — made from carbon fibre and titanium — was not classed because the process could inhibit innovation.

This report by ºÃÉ«tvwas first published June 17, 2024.