Rescuers in a remote area of the Atlantic Ocean raced against time early Tuesday to find a missing submersible carrying five people on a mission to document the wreckage of the Titanic.

The carbon-fiber submersible named the Titan, part of a mission by OceanGate Expeditions, carried a pilot, a renowned British adventurer, two members of an iconic Pakistani business family and a Titanic expert. Authorities reported the vessel overdue Sunday night about 435 miles south of St. John’s, Newfoundland, according to Canada’s Joint Rescue Coordination Center.

Every passing minute, however, puts the Titan’s crew at greater risk. The submersible had a 96-hour oxygen supply when it put to sea at roughly 6 a.m. Sunday, according to David Concannon, an adviser to OceanGate. That means the oxygen supply could run out by approximately 6 a.m. Thursday.

“It is a remote area — and it is a challenge to conduct a search in that remote area,” said Rear Adm. John Mauger, a commander for the U.S. Coast Guard, which also is searching for the Titan.

Mauger told NBC’s “Today” show on Tuesday that his crews were working to prioritize underwater search efforts and get equipment there that can be helpful to the search.

“We’re working very, very hard to make sure that we bring all assets … to bring leading technical experts to understand what capability is available, what capability we can get to the team,” he said.

The Canadian research icebreaker Polar Prince, which was supporting the Titan, reportedly lost contact with the vessel about an hour and 45 minutes after it submerged. The Polar Prince was to continue to do surface searches throughout the night and a Canadian Boeing P-8 Poseidon reconnaissance aircraft will resume their surface and subsurface search in the morning, the U.S. Coast Guard said on Twitter. Two U.S. Lockheed C-130 Hercules aircraft also have conducted overflights.

The Canadian military also dropped sonar buoys to listen for any possible sounds from the Titan.

Ship-tracking satellite data from MarineTraffic.com analyzed by The Associated Press showed the Polar Prince some 430 miles southeast of St. John’s on Tuesday morning. The Bahamas-flagged cable layer Deep Energy was nearby as well, likely assisting in the surface search.

In an earlier email to the AP, Concannon said he was supposed to be on the dive but could not go. He said officials were working to get a remotely operated vehicle that can reach a depth of 3.7 miles to the site as soon as possible.

OceanGate’s expeditions to the Titanic wreck site include archaeologists and marine biologists. The company also brings people who pay to come along, known as “mission specialists.” They take turns operating sonar equipment and performing other tasks in the five-person submersible.

The Coast Guard said Monday that there was one pilot and four “mission specialists” aboard. However, OceanGate’s website suggests that the fifth person aboard may be a so-called “content expert” who guides the paying customers.

OceanGate said its focus was on those aboard and their families.

“We are deeply thankful for the extensive assistance we have received from several government agencies and deep sea companies in our efforts to reestablish contact with the submersible,” it said in a statement.

OceanGate CEO Stockton Rush told The Associated Press in June 2021 that each dive would typically include a pilot, a researcher and three mission specialists. He said the mission specialists would operate sonar equipment and cameras as well as handle communications and possibly drive the Titan.

“So, we have a text messaging system — actually two of them. So, someone will be on communication, typing messages back and forth. And then everybody, conditions permitting, will get to drive the sub. So, assuming that the visibility is pretty good and the currents aren’t bad, and we’re not next to the wreck, they’ll have a chance to drive the sub.”

Rush called the Titan’s technology “very cutting edge,” and said it was developed with the help of NASA and Boeing and a number of aerospace manufacturers.

“This is the only submersible – crude submersible – that’s made of carbon fiber and titanium; largest carbon fiber structure that we know of. It’s 5-inch-thick carbon fiber, three-and-a-quarter inch thick titanium,” Rush said.

Authorities have yet to formally identify those on board, though some names have been confirmed.

British businessman Hamish Harding, who lives in Dubai in the United Arab Emirates, was one of the mission specialists, according to Action Aviation, a company for which Harding serves as chairman. The company’s managing director, Mark Butler, told the AP the crew set out Friday.

“There is still plenty of time to facilitate a rescue mission, there is equipment on board for survival in this event,” Butler said. “We’re all hoping and praying he comes back safe and sound.”

Harding is a billionaire adventurer who holds three Guinness World Records, including the longest duration at full ocean depth by a crewed vessel. In March 2021, he and ocean explorer Victor Vescovo dived to the lowest depth of the Mariana Trench. In June 2022, he went into space on Blue Origin’s New Shepard rocket.

Also on board were Pakistani nationals Shahzada Dawood and his son Suleman, according to a family statement sent to the AP. The Dawoods belong to one of Pakistan’s most prominent families. Their eponymous firm invests across the country in agriculture, industries and the health sector.

“We are very grateful for the concern being shown by our colleagues and friends and would like to request everyone to pray for their safety while granting the family privacy at this time,” the statement said. “The family is well looked after and are praying to Allah for the safe return of their family members.”

Shahzada Dawood also is on the board of trustees for the California-based SETI Institute that searches for extraterrestrial intelligence.

French explorer and Titanic expert Paul-Henry Nargeolet also was on board, according to David Gallo, a senior adviser for strategic initiatives and special projects at RMS Titanic. Gallo identified Nargeolet, a friend who has led multiple expeditions to the Titanic, on Tuesday during an interview with CNN.

The expedition was OceanGate’s third annual voyage to chronicle the deterioration of Titanic, which struck an iceberg and sank in 1912, killing all but about 700 of the roughly 2,200 passengers and crew. Since the wreckage’s discovery in 1985, it has been slowly succumbing to metal-eating bacteria.

OceanGate’s website had described the “mission support fee” for the 2023 expedition as $250,000 a person.

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

The expedition was scheduled to depart from St. John’s, Newfoundland, in early May and finish up at the end of June, according to documents filed by the company in April with a U.S. District Court in Virginia that oversees Titanic matters.

CBS journalist David Pogue, who went on the trip last year, noted his vessel got turned around looking for the Titanic.

“There’s no GPS underwater, so the surface ship is supposed to guide the sub to the shipwreck by sending text messages,” Pogue said in a segment aired on CBS Sunday Morning. “But on this dive, communications somehow broke down. The sub never found the wreck.”

The submersible, named Titan, is capable of diving 2.4 miles (4 kilometers) “with a comfortable safety margin,” OceanGate said in its court filing.

It weighs 20,000 pounds (9,072 kilograms) in the air, but is ballasted to be neutrally buoyant once it reaches the seafloor, the company said.

Experts said Monday that rescuers face steep challenges.

Alistair Greig, a professor of marine engineering at University College London, said submersibles typically have a drop weight, which is “a mass they can release in the case of an emergency to bring them up to the surface using buoyancy.”

“If there was a power failure and/or communication failure, this might have happened, and the submersible would then be bobbing about on the surface waiting to be found,” Greig said.

Another scenario is a leak in the pressure hull, in which case the prognosis is not good, he said.

“If it has gone down to the seabed and can’t get back up under its own power, options are very limited,” Greig said. “While the submersible might still be intact, if it is beyond the continental shelf, there are very few vessels that can get that deep, and certainly not divers.”

Even if they could go that deep, he doubts they could attach to the hatch of OceanGate’s submersible.