OceanGate CEO Stockton Rush emerges from the hatch atop the OceanGate submarine Cyclops 1 in the San Juan Islands, Wash., in 2018. Rush is one of five people aboard the submersible Titan that is missing after embarking on a mission to the wreckage of the Titanic. Alan Berner/The Seattle Times via Associated Press

In a race against the clock on the high seas, an expanding international armada of ships and airplanes searched Tuesday for a submersible that vanished in the North Atlantic while taking five people down to the wreck of the Titanic.

U.S. Coast Guard officials said the search covered 10,000 square miles but turned up no sign of the lost sub known as the Titan. Although they planned to continue looking, time was running out because the vessel had less than two days of oxygen left.

“This is a very complex search, and the unified team is working around the clock,” Cpt. Jamie Frederick of the First Coast Guard District in Boston told a news conference.

Frederick said the crew had about 41 hours of oxygen remaining as of midday Tuesday. He added that an underwater robot had started searching in the vicinity of the Titanic and that there was a push to get salvage equipment to the scene in case the sub is found.

Three C-17s from the U.S. military have also been used to move a commercial company’s submersible and support equipment from Buffalo, New York, to St. John’s, Newfoundland, to aid in the search, a spokeswoman for U.S. Air Mobility Command said.

The Canadian military said it provided a patrol aircraft and two surface ships, including one that specializes in dive medicine.

Authorities reported the carbon-fiber vessel overdue Sunday night, setting off the search in waters about 435 miles south of St. John’s. At the helm was pilot Stockton Rush, the CEO of the company leading the expedition. His passengers were British adventurer Hamish Harding, two members of a Pakistani business family and a Titanic expert.

The submersible had a 96-hour oxygen supply when it put to sea around 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.

The Titan submersible, shown in 2021, had a 96-hour oxygen supply when it put to sea around 6 a.m. Sunday. OceanGate Expeditions via Associated Press

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 remains of a coat and boots in the mud on the sea bed near the Titanic’s stern. Institute for Exploration, Center for Archaeological Oceanography/University of Rhode Island/NOAA Office of Ocean Exploration

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.

Eric Fusil, director of the University of Adelaide’s Shipbuilding Hub, said there are other scenarios that could have cut communications, including an electrical fire that could create toxic fumes and render the crew unconscious.

Another possibility is that Titan became entangled in the wreck of the Titanic and is stuck there, Fusil said.

“What I would like to believe … is that Titan suffered from a power loss, but they could still go back to the surface” and be spotted by aircraft and surface ships, he 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.”

“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.”

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.

OceanGate’s expeditions to the Titanic wreck site include archaeologists and marine biologists. The company also brings people who pay to come along. They take turns operating sonar equipment and performing other tasks in the submersible.

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.

Hamish Harding receives his Blue Origin astronaut pin after a successful flight to space on June 4, 2022, in Van Horn, Texas. Harding is one of the mission specialists onboard a missing submersible. Felix Kunze/Blue Origin via Associated Press

Passengers included Harding, who lives in Dubai in the United Arab Emirates; Pakistani nationals Shahzada Dawood and his son Suleman, whose eponymous firm invests across the country; and French explorer and Titanic expert Paul-Henry Nargeolet.

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. The wreckage was discovered in 1985 and 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.”

Associated Press writers Danica Kirka, Jill Lawless and Sylvia Hui in London, Rob Gillies in Toronto, Olga R. Rodriguez in San Francisco, Jon Gambrell in Dubai, United Arab Emirates, and Munir Ahmed in Islamabad contributed to this report.