The five people aboard a missing submersible are thought to be dead, including Engro Corp Vice Chairman Shahzada Dawood and his son Suleman, according to the company that owns the vessel, bringing a grim end on Thursday to the massive search for the vessel that was lost during a voyage to the Titanic.

Later, the US Coast Guard offered its "deepest condolences" to the bereaved families during a press briefing.

The submersible, owned by OceanGate Expeditions, lost contact on Sunday morning with its support ship about an hour and 45 minutes into what should have been a two-hour descent.

The minivan-sized Titan was carrying Shahzada and his son Suleman — who also have British citizenship — and British billionaire Hamish Harding. Also on board were Ocean Gate CEO Stockton Rush and deep sea explorer Paul-Henri Nargeolet.

"These men were true explorers who shared a distinct spirit of adventure, and a deep passion for exploring and protecting the world’s oceans," OceanGate Expeditions said in a statement. "Our hearts are with these five souls and every member of their families during this tragic time."

The statement from OceanGate came just minutes after CNN said that debris found on the ocean floor on Thursday near the wreck of the Titanic was assessed to be from the submersible, citing an internal US Coast Guard document.

An unmanned deep-sea robot deployed from a Canadian ship discovered a "debris field" near where the century-old wreck is located, two and a half miles below the surface, the US Coast Guuard said earlier on Twitter.

The detection of undersea noises on Tuesday and Wednesday using sonar buoys dropped from Canadian aircraft had temporarily offered hope that the people on board the submersible were alive and trying to communicate by banging on the hull.

But officials warned that analysis of the sound was inconclusive and that the noises might not have emanated from the Titan at all.

Even if the Titan was intact, the air supply on board was estimated at 96 hours when it entered the water on Sunday around 1200 GMT, meaning that the occupants likely would have run out of oxygen by Thursday morning.

The Titanic, which sank in 1912 on its maiden voyage after hitting an iceberg, killing more than 1,500 people, lies about 900 miles east of Cape Cod, Massachusetts, and 400 miles south of St John's, Newfoundland.

The expedition to the wreck, which OceanGate has been operating since 2021, cost $250,000 per person, according to OceanGate's website.

Questions about Titan's safety were raised in 2018 during a symposium of submersible industry experts and in a lawsuit by OceanGate's former head of marine operations, which was settled later that year.

Rescue teams from several countries spent days searching thousands of square miles of open seas with planes and ships for any sign of the 22-foot Titan.

The sweeping search has covered more than 10,000 square miles of ocean - about the size of the US state of Massachusetts.

On Thursday, the deployment of two specialised deep-sea unmanned vehicles expanded the effort to the ocean's depths, where immense pressure and pitch-black darkness had promised to complicate any rescue mission.

The missing submersible and subsequent hunt have captured worldwide attention, in part due to the mythology surrounding the Titanic. The "unsinkable" British passenger liner has inspired both nonfiction and fiction accounts for a century, including the James Cameron blockbuster 1998 movie, which rekindled popular interest in the story.