Canadian government regulators have opened a safety investigation into the undersea implosion of a tourist submersible that killed all five people aboard during a voyage to the century-old wreck of the Titanic.

A debris field from the submersible Titan was found at the bottom of the North Atlantic on Thursday by a robotic diving vehicle deployed from a Canadian search vessel, ending an intense five-day international rescue effort.

Fragments of Titan, which lost contact with its surface support ship about one hour and 45 minutes into a two-hour descent on Sunday, were discovered on the seabed about 488 metres from the bow of the Titanic wreck, about four kilometres below the surface, US coast guard Rear Admiral John Mauger said.

He told reporters the debris was consistent with “a catastrophic implosion of the vehicle”, meaning the 6.7m long vessel ultimately collapsed and was crushed under the immense hydrostatic pressure at that depth.

The five who died included Stockton Rush, pilot of the doomed craft and US founder and chief executive officer of OceanGate Expeditions, which operated the submersible and charged $US250,000 ($A374,000) per person to make the Titanic trip.

The others were British billionaire and explorer Hamish Harding, 58; Pakistani-born businessman Shahzada Dawood, 48, and his 19-year-old son, Suleman, both British citizens; and French oceanographer Paul-Henri Nargeolet, 77.

History repeats itself

The company behind the uncertified submersible that imploded with five on board was warned it was mirroring the overconfidence of the “unsinkable” Titanic amid widespread industry alarm over its safety.

But frustrated CEO Rush rejected those warnings in a letter, saying: “We have heard the baseless cries of ‘you are going to kill someone’ way too often.

“I take this as a serious personal insult.”

That exchange of emails was prompted by deepsea explorer Rob McCallum, who wrote to Mr Rush in March 2018 to express his concern about OceanGate Expedition’s safety.

Chillingly, he warned that the company was mirroring the overconfidence of the famous shipwreck it was seeking to explore underwater.

“I think you are potentially placing yourself and your clients in a dangerous dynamic,” the BBC reports he wrote.

“In your race to Titanic you are mirroring that famous catch cry: ‘She is unsinkable’”.

Investigation launched

In a statement on Friday, Canada’s Transportation Safety Board said it was launching a “safety investigation regarding the circumstances of this operation” because Titan’s surface support vessel, the Polar Prince, was a Canadian-flagged ship.

Guillermo Sohnlein, who co-founded OceanGate with Rush in 2009, reject6ed the accusation that Rush had a dismissive attitude to safety, saying his friend was “keenly aware” of the dangers of exploring the ocean depths.

“Stockton was one of the most astute risk managers I’d ever met,” said Sohnlein, who left the company in 2013, retaining a minority stake.

“He was very risk-averse.”

British Titanic explorer Dik Barton paid tribute to the work of his friend Nargeolet but noted issues raised with the design and maintenance of the craft.

“Everyone’s wise after the event, but as we’re hearing before, unfortunately, there were many red flags flying here,” he said.

Teams from the US, Canada, France and Britain had spent days scanning a vast expanse of open sea for the Titan.

The US coast guard’s Mauger said it was too early to say when the Titan met its fate.

The position of debris relatively close to the wreck suggested it happened near the end of Sunday’s descent.

The US navy monitors that part of the Atlantic for submarine activity and said an analysis of acoustic data detected “an anomaly consistent with an implosion or explosion” near the submersible’s location when communication with Titan was lost.

The acoustic data was shared immediately with the unified command led by the US coast guard, navy officials who spoke on condition of anonymity said.

It was decided the acoustic data was not definitive and the search and rescue mission should continue.

Moviemaker James Cameron, who directed the 1997 Oscar-winning film Titanic, said he learned of the acoustic findings within a day of the submersible disappearing and knew what it meant.

“I sent emails to everybody I know and said we’ve lost some friends,” Cameron, who has ventured to the wreck in submersibles, told Reuters.

“The sub had imploded.”

-with AAP