The passengers on the missing Titan submersible are dead, Coast Guard and operator OceanGate say

• An international search effort was launched after the Titan submersible went missing Sunday.
• Officials found debris Thursday which they said was consistent with a "catastrophic implosion."

The five passengers on board the Titan submersible traveling to the Titanic wreckage are believed to be dead, the vessel's operator and the US Coast Guard said on Thursday.

They reached the conclusion after a days-long search found debris from the sub on the ocean floor, indicating a failure of the vessel's hull that would be impossible to survive.

The debris, found not far from the wreck of the Titanic, was "consistent with the catastrophic loss of the pressure chamber" and then a "catastrophic implosion," said US Coast Guard Rear Admiral John Mauger, who led the search.

Around the same time that Mauger announced his findings in a press conference, OceanGate Expeditions, which operated the sub, gave a statement of its own confirming the deaths.

Five men were on board: the British billionaire Hamish Harding, Shahzada Dawood and his 19-year-old son Suleman, former French navy diver Paul-Henri Nargeolet, and OceanGate CEO Stockton Rush.

What happened to the submersible

The Titan submersible, a 21-foot-long vessel of titanium and carbon fiber, set off Sunday to explore the remains of the RMS Titanic located nearly 13,000 feet underwater, hundreds of miles off the coast of North America.

But an hour and 45 minutes into the expedition, the Titan lost communications with its mothership at the surface.

Eight hours elapsed before OceanGate contacted the Coast Guard, which launched a wide-ranging rescue effort that lasted until Thursday.

Hoping the vessel was still intact, Coast Guard officials estimated the sub had 96 hours of breathable oxygen when it first left for its destination. That deadline passed without resolution on Thursday morning, though the debris was located later that day.

Evidence to emerge since suggests that the implosion took place within hours of the sub setting off, meaning the passengers would have been dead before anybody really started looking for them.

Hopes were raised on Wednesday after searchers said they detected a banging sound in the general search area.

In hindsight it was a false lead. The Coast Guard said on Thursday there didn't seem "to be any connection between the noises and the location on the sea floor."

They said they don't know if passengers' bodies can be recovered, but that the search would continue.

Loved ones react

Friends and relatives of the passengers spoke to the media while the search was underway, and were quick to pay tribute once the deaths were announced.

Nargeolet's daughter spoke to Reuters before the debris was discovered, saying: "What he liked the most was to be in a submarine, [near] the Titanic. He is where he really loved being. I would prefer him [dying] at a place where he is happy."

David Mearns, a diving expert and friend of two passengers on the Titan, told Sky News his "worst fears have now been realized" and "two friends of mine are gone."

Suleman Dawood's aunt told NBC News her nephew was "terrified" before the trip but went to bond with his dad over Father's Day weekend.

Richard Garriott de Cayeux, the president of the Explorers Club, said in a statement Thursday that he and club members are "heartbroken."

"Hamish Harding is a dear friend to me personally and to The Explorers Club," he wrote. "He holds several world records and has continued to push dragons off maps both in person and through supporting expeditions and worthy causes." He also called Nargeolet was "one of the foremost experts on submersible expeditions to the Titanic."

He added: "We're heartbroken for the families, friends and colleagues of those who were lost. Their memories will be a blessing and will continue to inspire us in the name of science and exploration."