The five people aboard a submersible that went missing during a tourist expedition to the Titanic's wreckage died in an implosion after the loss of the pressure chamber, US Coast Guard officials said on Thursday.

The following is what we know so far:

Who were on board?

Hamish Harding: The British billionaire and chairman of aviation company Action Aviation was among those on the vessel. Dubai-based Harding had posted on social media that he was proud to be heading to the Titanic as a "mission specialist", adding: "Due to the worst winter in Newfoundland in 40 years, this mission is likely to be the first and only manned mission to the Titanic in 2023."

Harding was also on board the 2019 "One More Orbit" flight mission that set a record for the fastest circumnavigation of earth by aircraft over both geographic poles.

"He doesn't stand still. If he's not working hard, he's exploring hard," said Jannicke Mikkelsen, an explorer and friend of Hamish.

Shahzada Dawood and his son Suleman: Shahzada was vice chairman of one of Pakistan's largest conglomerates, Engro Corporation, with investments in fertilisers, vehicle manufacturing, energy and digital technologies. According to the website of SETI, a California-based research institute of which he was a trustee, he lived in Britain with his wife and two children.

Shahzada's interests included wildlife photography, gardening and exploring natural habitats, while Suleman was a fan of science fiction literature, according to a statement from the Dawood Group.

Paul-Henri Nargeolet: The 77-year-old French explorer was director of underwater research at a company that owns the rights to the Titanic wreck. A former commander in the French Navy, he was both a deep diver and a mine sweeper. After retiring from the navy, he led the first recovery expedition to the Titanic in 1987 and is a leading authority on the wreck site. In a 2020 interview with France Bleu radio, he spoke of the dangers of deep diving, saying: "I am not afraid to die, I think it will happen one day."

Stockton Rush: The founder and CEO of the vessel's U.S.-based operating company OceanGate was also on the submersible. "It is an amazingly beautiful wreck," Rush told Britain's Sky News of the Titanic earlier this year. "Rush became the youngest jet transport rated pilot in the world when he obtained his DC-8 Type/Captain’s rating at the United Airlines Jet Training Institute in 1981 at the age of 19," according to his biography on OceanGate's website.

What is the firm behind this?

* Based in Everett, Washington, OceanGate says it uses next-generation crewed submersibles and launch platforms to increase deep ocean access as far as 4,000 metres.

* "OceanGate has successfully completed over 14 expeditions and over 200 dives in the Pacific, Atlantic and Gulf of Mexico," its website says. "Following every mission, the team evaluates and updates the procedures as part as a continued commitment to evolve and ensure operational safety."

What was the vessel?

* Although popularly called a submarine, in marine terminology the "Titan" vessel was a submersible. While a submarine can launch itself from a port independently, a submersible goes down off a support ship.

The search

* An unmanned deep-sea robot deployed from a Canadian ship discovered the wreckage of the submersible on Thursday morning about 1,600 feet (488 meters) from the bow of the century-old wreck of the Titanic passenger ship, 2-1/2 miles (4 km) below the surface, U.S. Coast Guard Rear Admiral John Mauger said.

* Officials said they would continue to investigate the site of the debris field.