The pilot and four passengers of a missing Titanic submersible are now believed to be dead after the U.S. Coast Guard said the vessel suffered a "catastrophic implosion."

OceanGate Expeditions said in a statement Thursday that all five people – pilot CEO Stockton Rush, Shahzada Dawood and his son Suleman Dawood, Hamish Harding and Paul-Henri Nargeolet – have "sadly been lost."

The debris is consistent with a catastrophic loss of the pressure chamber," Rear Adm. John Mauger of the First Coast Guard District announced during a news conference. “Upon this determination, we immediately notified the families. On behalf of the United States Coast Guard, and the entire Unified Command, I offer my deepest condolences to the families. I can only imagine what this has been like for them and I hope this discovery provides some solace during this difficult time.”

Paul Hanken, who Mauger described as an “undersea expert,” said the first piece of debris that was found was the nose cone.

“That was the first indication that there was a catastrophic event,” Hanken said, adding that they soon found a second smaller debris field. “Within that debris field, we found the other end of the pressure hull.”

Mauger said the debris found from the sub, known as the Titan, “is consistent with a catastrophic implosion of the vessel." The location of the Titan was in an area that was roughly 1,600 feet from the Titanic wreck, he added.

The BBC reported that a dive expert and friend who said he's friends with two passengers said the sub's "landing frame and a rear cover" were among the debris.

Mauger thanked additional agencies for their “robust” efforts during what he described as a “highly complex search operation.” He said remotely operated vehicles (ROVs) will remain on scene to gather more information and attempt to construct a timeline.

The Coast Guard refused to comment about whether the remains of the men will be retrieved. Stockton's passengers are a British adventurer, two members of a Pakistani business family and a Titanic expert.

The Titan had a four-day oxygen supply when it set off around 6 a.m. Sunday to view the wreckage of the Titanic. The Canadian research boat Polar Prince lost contact with the sub later that day during a dive approximately 900 miles east of Cape Cod, Massachusetts. Coast Guard crews began by searching the surface of the water, as well as underwater, for the 21-foot vessel.

The Titan weighs 20,000 pounds (9,071 kilograms). The U.S. Navy’s Flyaway Deep Ocean Salvage System is designed to lift up to 60,000 pounds (27,216 kilograms), the Navy said on its website.

Unlike submarines that leave and return to port under their own power, submersibles require a ship to launch and recover them. OceanGate Expeditions hired the Polar Prince to ferry dozens of people and the submersible craft to the North Atlantic wreck site.

Rescuers have rushed ships, planes and other equipment to the site of the disappearance. Coast Guard officials said underwater noises – described as banging sounds – were detected in the search area Tuesday and Wednesday.

On Thursday, the Coast Guard said an undersea robot sent by a Canadian ship had reached the sea floor, while a French research institute said a deep-diving robot with cameras, lights and arms also joined the operation.

Breathable air on the Titan passed the critical 96-hour mark Thursday. Experts emphasized that this was an imprecise estimate and could be extended if passengers had taken measures to conserve breathable air.

Newly uncovered allegations suggest there had been significant warnings made about vessel safety during the submersible’s development. OceanGate has also recently been criticized for the use of a $30 video game controller to steer the Titan.

James Cameron, who directed the 1997 movie "Titanic," spoke about what he called similarities between the sinking of the ship in 1912 and the Titan submersible.

“I’m struck by the similarity of the Titanic disaster itself, where the captain was repeatedly warned about ice ahead of his ship and yet he steamed at full speed into an ice field on a moonless night and many people died as a result,” Cameron told ABC News.

Read the full statement from OceanGate below:

"We now believe that our CEO Stockton Rush, Shahzada Dawood and his son Suleman Dawood, Hamish Harding, and Paul-Henri Nargeolet, have sadly been lost.

These men were true explorers who shared a distinct spirit of adventure, and a deep passion for exploring and protecting the world’s oceans. Our hearts are with these five souls and every member of their families during this tragic time. We grieve the loss of life and joy they brought to everyone they knew.

This is an extremely sad time for our dedicated employees who are exhausted and grieving deeply over this loss. The entire OceanGate family is deeply grateful for the countless men and women from multiple organizations of the international community who expedited wide-ranging resources and have worked so very hard on this mission. We appreciate their commitment to finding these five explorers, and their days and nights of tireless work in support of our crew and their families.

This is a very sad time for the entire explorer community, and for each of the family members of those lost at sea. We respectfully ask that the privacy of these families be respected during this most painful time."

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Editor's note: The Associated Press contributed to this report.