Five people on board the tourist submarine that disappeared on an expedition to explore the Titanic shipwreck over the weekend did not survive a "catastrophic loss of the pressure chamber," officials said Thursday.

The announcement came after the U.S. Coast Guard said the massive search underway in the North Atlantic had located a debris field on the sea floor, which was confirmed to be pieces of the missing sub.

"The debris field is consistent with a catastrophic implosion of the vessel," Rear Adm. John Mauger of the Coast Guard said at a briefing, offering "deepest condolences to the families." A spokesperson for OceanGate Expeditions, the company behind the voyage, told reporters that the passengers, including OceanGate CEO Stockton Rush, "have sadly been lost."

Here's what we know so far about the submersible craft and what led up to this point.

What happened?

A five-person crew on a submersible named Titan, owned by OceanGate Expeditions, submerged on a dive to the Titanic wreckage site Sunday morning, and the crew of the Polar Prince research ship lost contact with the sub about an hour and 45 minutes later, the Coast Guard said. 

The Coast Guard first alerted mariners about the missing sub Sunday night, saying a "21 foot submarine" with a white hull was overdue and giving its last known position. "VESSELS IN VICINITY REQUESTED TO KEEP A SHARP LOOKOUT, ASSIST IF POSSIBLE," the alert message read.

The sub was lost in an area about 900 miles east of Cape Cod, in the North Atlantic, in water with a depth of about 13,000 feet, which is about level with the depth of the Titanic wreck. Amid growing concern about its dwindling supply of breathable air, search and rescue efforts by a unified command composed of several international agencies ramped up accordingly.

The five people aboard included an operator — later identified as Stockton Rush, the CEO of OceanGate Expeditions — and four mission specialists, a term the company uses for its passengers, who paid up to $250,000 for a seat.

For days, the fate of the sub and its passengers was a mystery.

But after the debris was found, a U.S. Navy official said the Navy had detected "an acoustic anomaly consistent with an implosion" shortly after the sub lost contact with the surface Sunday, CBS News national security correspondent David Martin reported. The information was relayed to the Coast Guard, which used it to narrow the radius of the search area, the official said.

Such an implosion, under the intense pressure of the depths of the sea, would have destroyed the vessel almost instantly, experts explained.

"in a fraction of a second, it's gone," Will Kohnen, chairman of the professional group the Marine Technology Society Submarine Committee, told the Reuters news agency. 

"It implodes inwards in a matter of a thousandth of a second," Kohnen said. "And it's probably a mercy, because that was probably a kinder end than the unbelievably difficult situation of being four days in a cold, dark and confined space. So, this would have happened very quickly. I don't think anybody even had the time to realize what happened." 

The Coast Guard is leading the investigation into the incident, and the National Transportation Safety Board said Friday it will assist.   

Who were the passengers aboard the sub? 

CBS News confirmed that the five people aboard the submersible were Hamish Harding, a 59-year-old British billionaire, business owner and explorer; British-Pakistani businessman Shahzada Dawood and his son, Suleman; French explorer Paul-Henri Nargeolet, who had made multiple dives over the years to explore the Titanic; and Stockton Rush, the CEO of OceanGate Expeditions, who was serving as pilot.

Just ahead of the Coast Guard briefing Thursday afternoon, a statement issued by OceanGate spokesperson Andrew Von Kerens offered condolences to the families of the Titan crew and recognized that all five people on board the submersible were believed to be dead.

"These men were true explorers who shared a distinct spirit of adventure, and a deep passion for exploring and protecting the world's oceans," the company said in the statement. "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."

When the Coast Guard confirmed the sub's likely implosion on Thursday, Mauger said they were communicating with consulates general in both the U.K. and France.

The Dawood family, of the large Pakistan-based global business conglomerate Dawood Group, issued a statement Tuesday confirming their family members were on the expedition.

"Please continue to keep the departed souls and our family in your prayers during this difficult time of mourning," the Hussain and Kulsum Dawood family said Thursday in a statement through the Dawood Foundation. "We are truly grateful to all those involved in the rescue operations. ... The immense love and support we receive continues to help us endure this unimaginable loss."

Nargeolet, a renowned French explorer and former diver for the French Navy who was part of the first expedition to visit the Titanic wreck in 1987, was returning for another dive aboard the Titan submersible. 

In a Facebook post on Monday, Rory Golden, an explorer who became the first Irish diver to visit the Titanic wreckage in 2000, said he was part of the voyage but was not on the submersible that went missing.

Search and rescue efforts

Authorities said early Thursday morning that a Canadian vessel, Horizon Arctic, had deployed a remotely operated underwater vehicle that reached the sea floor. The ROV ultimately located what the Coast Guard originally described as a debris field on the sea floor, which included identifiable pieces of the sub, authorities confirmed that afternoon.

"This morning, an ROV, or remote operated vehicle, from the vessel Horizon Arctic, discovered the tail cone of the Titan submersible approximately 1,600 feet from the bow of the Titanic on the sea floor," said Mauger at a news briefing. "The ROV subsequently found additional debris. In consultation with experts from within the unified command, the debris is consistent with the catastrophic loss of the pressure chamber."

"Upon this determination, we immediately notified the families," he added. "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 that this discovery provides some solace during this difficult time."

Mauger said authorities were "still working to develop the details for the timeline involved with this casualty and the response," and referenced the "incredibly complex operating environment along the sea floor, over two miles beneath the surface."

Paul Hankins, an undersea expert for the U.S. Navy, explained during the news conference that crews discovered "five different major pieces of debris that told us that it was the remains of the Titan." These pieces included, initially, the nose cone, which was outside of the pressure hull. 

"We then found a large debris field," Hankins said. "Within that large debris field, we found the front end bell of the pressure hull. That was our first indication that there was a catastrophic event."

A second, smaller debris field was located shortly after, and the debris found there "comprised the totality of that pressure vessel," Hankins said. 

"The debris field is consistent with a catastrophic implosion of the vessel," he said, adding that the team will continue to map the debris field area.

Asked by a reporter what the prospects were for recovering the passengers, Mauger said, "This is an incredibly unforgiving environment down there on the sea floor, and the debris is consistent with a catastrophic implosion of the vessel. So we'll continue to work and continue to search the area down there, but I don't have an answer for prospects at this time."

Discovering the Titan debris came after multiple agencies from the U.S. and Canada spent days scouring thousands of square miles of open ocean in search of the missing sub.

The U.S. Coast Guard announced Wednesday that underwater noises were detected in the search area and that searches involving ROVs were focusing on the area where the noises were heard.

On Wednesday, three more vessels had arrived to join the search, including one with side-scan sonar capabilities designed to create images of large sections of the sea floor, the Coast Guard said in a tweet. That vessel began conducting search patterns alongside at least two others, as multiple military and other agencies worked together under a unified command. 

Frederick said Wednesday there were five "surface assets" involved in the search, and another five were expected to join the operation within the next 24 to 48 hours. He said the team also had two ROVs "actively searching," with several more due to arrive to join the search Thursday.

The Coast Guard said it had C-130 aircraft searching for the sub, and that the Rescue Coordination Center Halifax was assisting with a P-8 Poseidon aircraft, which has underwater detection capabilities. Canadian P-3s were also involved in the operation and deployed sonar buoys.

Just after midnight Wednesday, officials said aircraft had detected underwater noises in the search area, and underwater search operations were relocated as a result, though the origin of the noises remained unknown. The sounds were picked up several times Tuesday night and Wednesday morning, according to the Coast Guard. 

"With respect to the noises, specifically, we don't know what they are, to be frank with you," Frederick said. "The P-3 detected noises, that's why they're up there, that's why they're doing what they're doing, that's why there are sonar buoys in the water."

News of the vanished submersible and subsequent rescue mission originally broke Monday morning. At the time, Lt. Jordan Hart of the Coast Guard in Boston told CBS News that personnel there were leading the rescue mission, and focusing on waters off Newfoundland in eastern Canada. 

The Boston Regional Coordination Center was managing the rescue operation, as the location of the Titanic shipwreck falls within the Boston coordination center's territory, according to a map of jurisdictions along the East Coast of North America.

That combined search area grew to about twice the size of the state of Connecticut, and the subsurface search extended down as far as 2 and a half miles deep, Frederick said, stressing that the search and rescue teams were dealing with an incredibly complex set of circumstances.

"We also have to factor in the ever-changing weather conditions, currents and sea states that expand the search area every hour," he said earlier in the week. "There's an enormous complexity associated with this case due to the location being so far offshore and the coordination between multiple agencies and nations. We greatly appreciate the outpouring of support and offers to provide additional equipment."

What caused the noises?

Frederick acknowledged that the sounds detected underwater by Canadian aircraft could have been caused by multiple sources. 

Following the discovery of the sub debris on the sea floor, a U.S. Navy source told CBS News that the implosion would be inconsistent with banging noises heard at 30-minute intervals. Those noises, the official said, are now assessed as having come from other ships in the area.

Carl Hartsfield, an expert in underwater acoustics and the director of the Woods Hole Oceanographic Institution, which is on-site at the search area as a consultant, explained that it can be challenging to differentiate between "human sounds" and "nature sounds" coming from beneath the surface.

"The ocean is a very complex place, obviously, human sounds, nature sounds, and it's very difficult to discern what the sources of those noises are at times," Hartsfield said. 

Before the sub was found, Chris Roman, an associate professor of oceanography at the University of Rhode Island, told CBS News that, technically, it was possible that sounds from inside a submersible could have been detected, but that wasn't the only potential source of the noise.

"Sound travels very efficiently underwater. If people were intentionally making noises within the sub, it's very likely they could be detected with a sound buoy, and that position can be translated into a new search area," Roman said. But he also noted that, as Frederick mentioned in his briefing, "there's a lot of other things in the ocean that make noises."

The submarine

The unique submersible craft that disappeared was owned by OceanGate Expeditions, a company that deploys manned submarines for deep sea exploration and has in the past advertised this particular sub's voyages to carry tourists down to the wreckage of the RMS Titanic for $250,000 per seat. 

More than a century after the Titanic sank in April 1912, the wreck lies on the ocean floor about 400 miles southeast of the Newfoundland coast. 

OceanGate said recently on its website and on social media that its expedition to the shipwreck was "underway," describing the seven-night trip as a "chance to step outside of everyday life and discover something truly extraordinary." In addition to one ongoing expedition, the company had planned two others for the summer of next year, according to the site. 

Because of the sub's oxygen capacity, it can only be fully submerged for a portion of the weeklong voyage. The sub has emergency oxygen and a 96-hour sustainment capability if there's an emergency aboard, Mauger said.

In a statement Monday after news broke of the missing sub, OceanGate confirmed the missing submersible was theirs and that a rescue operation had been launched to find and recover it. The company said it was "exploring and mobilizing all options to bring the crew back safely." 

"For some time, we have been unable to establish communications with one of our submersible exploration vehicles which is currently visiting the wreck site of the Titanic," said Andrew Von Kerens, a spokesperson for OceanGate. "We pray for the safe return of the crew and passengers, and we will provide updates as they are available."

Inside the Titan

Dubbed the Titan, OceanGate's deep sea vessel, was said to be the only five-person submersible in the world with the capabilities to reach the Titanic's depth, nearly 2 and a half miles beneath the ocean's surface, CBS "Sunday Mornings" correspondent David Pogue reported last year. 

BBC News reported that the vessel typically carries a pilot, three paying guests and another person described as a "content expert" by the company. OceanGate's site says the Titan, weighing around 23,000 pounds, has the ability to reach depths of up to 4,000 meters — over 13,000 feet — and has about 96 hours of life support for a crew of five people.

Last summer, Pogue accompanied the Titan crew on the journey from Newfoundland to the site where the Titanic as lost. Several dive attempts had to be canceled when weather conditions indicated it may not be safe. At the time, he described the Titan as a one-of-a-kind submersible craft made from thick carbon fiber and coated on both ends by a dome of titanium. 

In 2018, a former employee of OceanGate Expeditions, submersible pilot David Lochridge, voiced concerns about the safety of the Titanic tour sub and filed a lawsuit against the company. 

Lochridge, who was fired by OceanGate and sued by the company for allegedly disclosing confidential information in a whistleblower complaint to the Occupational Safety and Health Administration, said in a court filing that the Titan would carry passengers as deep as 4,000 meters even though that depth had never been reached in a sub with its type of carbon fiber hull. According to his claim, he learned the vessel was built to withstand a certified pressure of 1,300 meters, although OceanGate planned to take passengers to 4,000 meters.  

Lochridge was not the only skeptic. The same year his complaint was filed, other industry leaders approached OceanGate with questions about the safety of its submersible. William Kohnen, president and CEO of Hydrospace Group, outlined his concerns in a 2018 letter to OceanGate, originally published by The New York Times, that warned of potentially "catastrophic" issues with the "experimental" sub, which was not certified. Kohnen told CBS News on Wednesday that although he did not send it, the letter was leaked to OceanGate and prompted the company to "amend a number of details that made sure the public knew" the submersible had not received its certification.

"The letter to Oceangate was meant as a professional courtesy to the CEO expressing industry concerns that the company was not following a traditional classification route for the certification of the submersible," Kohnen said. "The industry operates along an established and dynamic set of safety regulations and protocols that have served the submersible industry worldwide."

Ahead of his planned dive last summer, Pogue recalled signing paperwork that read, in part, "This experimental vessel has not been approved or certified by any regulatory body, and could result in physical injury, emotional trauma, or death." 

Space inside the submarine was similar to the interior of a minivan, and, with just one button and a video game controller used to steer it, the vessel "seemed improvised, with off-the-shelf components," Pogue said.

On his voyage, the sub was lost for a few hours, Pogue said.

"There's no GPS underwater, so the surface ship is supposed to guide the sub to the shipwreck by sending text messages," he reported at the time. "But on this dive, communications somehow broke down."

Were conditions right for the dive?

G. Michael Harris, founder of RMS Titanic, Inc. — a company that salvages artifacts from the Titanic wreckage — told CBS News on Tuesday evening that Titanic expeditions are generally conducted within a "three-month weather window" between the end of June and September, when the ocean waters are at their calmest.

Harris, who has led several expeditions to the wreckage site, questioned why the Titan's dive was conducted as early as Sunday.

"Right now, it's really early in the season. I'm not sure why OceanGate went out this soon," Harris said.

Harris also noted that when he conducts diving expeditions, he uses a transponder system, something that he believed the Titan likely did not have.

"It's a net that we navigate in so that we know where we are at all times on the wreck of the Titanic," Harris said. "We're in constant communication with the vessel up top."

Harris said the Titan was "put on a sled and dumped in the water and their only navigation is from the support ship up top."

"I don't adhere to that myself, personally," Harris said. 

Harris noted that he has worked with Nargeolet, who is listed as director of underwater research for RMS Titanic, for the past 30 years, describing him as an "all-around good guy."

Who was Hamish Harding?

Harding, the first of the passengers to be publicly identified, had previously posted on social media about joining the Titanic shipwreck expedition.

In a post shared to his Facebook page on Saturday, Harding wrote: "I am proud to finally announce that I joined OceanGate Expeditions for their RMS TITANIC Mission as a mission specialist on the sub going down to the Titanic."

"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's Facebook post continued. "A weather window has just opened up and we are going to attempt a dive tomorrow. We started steaming from St. Johns, Newfoundland, Canada yesterday and are planning to start dive operations around 4am tomorrow morning. Until then we have a lot of preparations and briefings to do."

That post was Harding's most recent social media update related to the submarine trip. It included multiple photographs of him, including one that showed Harding signing his name on a banner that read "Titanic Expedition Mission V" and another that pictured the submersible vessel itself.

Richard Garriott de Cayeux, president of The Explorers Club, where Harding helped found the board of trustees, said they had spoken just a week earlier about the expedition. 

"When I saw Hamish last week at the Global Exploration Summit, his excitement about this expedition was palpable. I know he was looking forward to conducting research at the site," he said in a letter to club members after the sub's disappearance.

Harding was a veteran adventure tourist who also traveled to space aboard a Blue Origin rocket last year. Two years ago, he made it to the deepest part of the ocean, traveling with U.S. explorer Victor Vescovo to the floor of the Mariana Trench, 35,876 feet below the sea surface. That trip, in a $48 million submersible, earned both explorers the Guinness World Record for the longest distance traveled at the deepest part of the ocean by a crewed vessel.

"It was potentially scary, but I was so busy doing so many things — navigating and triangulating my position — that I did not really have time to be scared," Harding told The Week after that excursion.

This is an updated version of an article originally published on Monday, June 19. Reporting contributed by Emmet Lyons, Roxana Saberi, Alex Sundby, Aimee Picchi, Aliza Chasan, Li Cohen, Caroline Hinson, Anna Noryskiewicz, Analisa Novak and other CBS News staff.

In:
• Shipwreck
• Rescue
• Newfoundland
• RMS Titanic
• Submarine
• United States Coast Guard
• Boston
• Canada