United States maritime officials say they will issue a report aimed at improving the safety of submersibles worldwide.

Investigators from the US, Canada, France and the United Kingdom are working closely together on the probe of the June 18 accident, which happened in an “unforgiving and difficult-to-access region” of the North Atlantic, US coast guard Rear Admiral John Mauger said.

Salvage operations from the sea floor are ongoing, and the accident site has been mapped, coast guard chief investigator Captain Jason Neubauer said on Sunday.

He did not give a timeline for the investigation.

Neubauer said the final report will be issued to the International Maritime Organisation.

“My primary goal is to prevent a similar occurrence by making the necessary recommendations to advance the safety of the maritime domain worldwide,” Neubauer said.

Evidence is being collected in the port of St John’s, Newfoundland, in co-ordination with Canadian authorities.

All five people on board the Titan were killed.

Debris from the vessel was located about 3810 metres underwater and roughly 488 metres from the Titanic on the ocean floor, the coast guard said last week.

One of the experts whom the coast guard has been consulting said on Monday that he does not believe there is any more evidence to collect.

“It is my professional opinion that all the debris is located in a very small area and that all debris has been found,” said Carl Hartsfield, a retired US navy captain and submarine officer who now directs a lab at the Woods Hole Oceanographic Institution that designs and operates autonomous underwater vehicles.

Authorities are still trying to sort out what agency or agencies are responsible for determining the cause of the tragedy, which happened in international waters.

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

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

A key part of any investigation is likely to be the Titan itself.

The vessel was not registered either with the US or with international agencies that regulate safety.

And it was not classified by a maritime industry group that sets standards on matters such as hull construction.

The investigation is also complicated by the fact that the world of deep-sea exploration is not well-regulated.

OceanGate CEO Stockton Rush, who was piloting the Titan when it imploded, had complained that regulations can stifle progress.

On Saturday, the Transportation Safety Board of Canada said it has begun an investigation and has been speaking with those who travelled on the Polar Prince.

Board chairperson Kathy Fox said officials will share information it collects with other agencies, such as the US National Transportation Safety Board, which is taking part in the overall investigation.

The Royal Canadian Mounted Police said on Saturday that it will conduct a full investigation only if it appears criminal, federal or provincial laws were broken.

-AAP