Rescuers in a remote area of the Atlantic Ocean raced against time Tuesday to find a missing submersible before the oxygen supply runs out for five people who were on a mission to document the wreckage of the Titanic.

Despite an international rescue effort, U.S. Coast Guard officials said the search covering 10,000 square miles (26,000 square kilometers) had turned up no signs of the lost sub known as the Titan, but they planned to continue looking.

Authorities reported the carbon-fiber vessel overdue Sunday night, setting off the search in waters about 435 miles (700 kilometers) south of St. John’s, Newfoundland. Aboard were a pilot, renowned British adventurer Hamish Harding, two members of an iconic Pakistani business family and a Titanic expert.

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 Expeditions, which oversaw the mission.

That means the oxygen supply could run out Thursday morning.

CBS News journalist David Pogue, who traveled to Titanic aboard the Titan last year, said the vehicle uses two communication systems: text messages that go back and forth to a surface ship and safety pings that are emitted every 15 minutes to indicate that the sub is still working.

Both of those systems stopped about an hour and 45 minutes after the Titan submerged.

“There are only two things that could mean. Either they lost all power or the ship developed a hull breach and it imploded instantly. Both of those are devastatingly hopeless,” Pogue told CBC on Tuesday.

The submersible had seven backup systems to return to the surface, including sandbags and lead pipes that drop off and an inflatable balloon. One system is designed to work even if everyone aboard is unconscious, Pogue said. Experts said the 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 rescuers could attach to the submersible.

By Tuesday morning, an area totaling 10,000 square miles had been searched, the U.S. Coast Guard tweeted.

The Canadian research icebreaker Polar Prince, which was supporting the Titan, was to continue conducting surface searches with help from a Canadian Boeing P-8 Poseidon reconnaissance aircraft, the Coast Guard said on Twitter. Two U.S. Lockheed C-130 Hercules aircraft also conducted overflights.

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

Concannon, who said he was supposed to be on the dive but could not go, said officials were also working to get a remotely operated vehicle that can dive to a depth of 3.7 miles (6 kilometers) 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 submersible.

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

Authorities have yet to formally identify those on board, though some names have been confirmed, including OceanGate CEO Stockton Rush, who, according to the company, was serving as a member of the crew.

Rush told The Associated Press in June 2021 that the Titan’s technology was “very cutting edge” and was developed with the help of NASA and aerospace manufacturers.

“This is the only submersible – crewed submersible – that’s made of carbon fiber and titanium,” Rush said, calling it the “largest carbon fiber structure that we know of,” with 5-inch-thick carbon fiber and 3.25-inch-thick titanium. Harding, who lives in Dubai in the United Arab Emirates, was one of the mission specialists, according to Action Aviation, a company where Harding serves as chairman.

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 descended 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. 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.

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 was also aboard, 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.

Greg Stone, a longtime ocean scientist based in California and a friend of Rush, called the lost submersible “a fundamentally new submarine design” that showed great promise for future research. Unlike its predecessors, the Titan was not spherical in shape.

“Stockton was a risk taker. He was smart ... he had a vision. He wanted to push things forward,” Stone said.

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 described the “mission support fee” for the 2023 expedition as $250,000 a person.

Recalling his own trip aboard the Titan, Pogue said the 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.”