Rescuers searching for the missing Titan submersible said all of the five people on board the vessel are believed to be dead, the US Coast Guard said on Thursday.
The company that operated the missing sub, OceanGate, said that it believed all those on board "have sadly been lost."
Earlier on Thursday, the US Coast Guard said that rescuers uncovered a "debris field" within the search area where teams are searching for the missing Titan submersible that was traveling to the wreck of the Titanic.
A US Navy official said on Thursday that an acoustic "anomaly" that could have been an implosion was heard on Sunday shortly after the vessel was reported missing.
What is the latest?
Experts recovered five pieces of debris that belonged to the Titan submersible. Following the discovery, the Coast Guard alerted the family members of those who were on board, saying that there was little chance of survivors.
The debris found indicates "a catastrophic implosion of the vessel," US Coast Guard Rear Admiral John Mauger said in a press conference in the US city of Boston.
The US Navy said that it had detected an acoustic "anomaly" shortly after the submersible was declared missing on Sunday, an unnamed senior US Navy official told the Associated Press and the Wall Street Journal.
"The US Navy conducted an analysis of acoustic data and detected an anomaly consistent with an implosion or explosion in the general vicinity of where the Titan submersible was operating when communications were lost," the official said.
The navy had passed on the information to the US Coast Guard, which continued its search until Thursday.
"On behalf of the United States Coast Guard and the entire unified command, I offer my deepest condolences to the families," he said.
The Titan debris is approximately 1,600 feet (488 metres) away from the bow of the sunken Titanic on the ocean floor, Mauger said.
Search teams will continue to investigate the debris field, but said it was too early to tell when the implosion on the submersible happened.
The Titan, a 21-foot (6.5-meter) tourist submersible, lost communication with its mothership less than two hours into its trip on Sunday.
Who was on board the Titan?
Prominent Pakistani businessman Shahzada Dawood and his son, Suleman, were on board the vessel in the North Atlantic, their family said in a statement.
Dawood is the vice chairman of one of the largest conglomerates in Pakistan, Engro Corporation, which has stakes in fertilizers, vehicle manufacturing, energy and digital technologies.
British billionaire Hamish Harding, 58, was also among the passengers, according to a social media post from a family member. Harding, an aviation tycoon and holder of three Guinness World Records, had earlier posted about his expedition on Instagram, saying that he was proud to join OceanGate's Titanic mission.
OceanGate's founder and CEO Stockton Rush was also confirmed by the company to be on board.
The fifth member on board was a French national — 77-year-old French explorer and oceanographer Paul-Henri Nargeolet.
What has the reaction been?
The British government offered its condolences to the families of those on board. Three of the victims were British citizens.
“The UK government is closely supporting the families affected and expresses our deepest condolences,” Foreign Secretary James Cleverly wrote on Twitter.
Pakistan's government praised the search effort and offered its condolences to the relatives of Shahzada Dawood and his son Suleman. The Dawoods belong to one of Pakistan's most prominent families.
“Our deepest condolences to the Dawood family and the family of the other passengers on the sad news about the fate of the Titanic irreversible in the North Atlantic,” the Ministry of Foreign Affairs said.
"We appreciate the multinational efforts over the last several days in search of the vessel."
The family of Hamish Harding and his company Action Aviation said in a statement that they were "united in grief with the other families who have also lost their loved ones on the Titan submersible."
The statement said Harding was "a passionate explorer — whatever the terrain — who lived his life for his family, his business and for the next adventure."
"What he achieved in his lifetime was truly remarkable, and if we can take any small consolation from this tragedy, it's that we lost him doing what he loved," the statement added.
What happened with the search on Thursday?
Time pressure was running high on Thursday, with experts concerned that the five people on board might have run out of oxygen if the vessel was still intact.
Rescue organizers rushed more ships and vessels to the site of the disappearance, hoping to locate the tiny vessel after detecting underwater sounds for a second straight day.
Those on board the Titan had a four-day supply of oxygen when they set off. But by Thursday, the estimated timeline of 96 hours had passed.
Ships and planes searched 10,000 square miles (around 20,000 square kilometers) of surface water — roughly the size of Slovenia — in an effort to find the vessel some 400 nautical miles (740 kilometers) off the coast of Newfoundland, Canada.
The rescuers relocated two remotely operated vehicles capable of searching under the water and one surface vessel with sonar capability after a Canadian P-3 aircraft detected sounds possibly coming from Titan.
The operating company, OceanGate Expeditions, charges $250,000 (€227,610) for a place on the submersible.