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.

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

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

Frederick said 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.

Authorities reported the carbon-fiber vessel overdue Sunday night, setting off the search in waters 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 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 1 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.

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.

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.

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. 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, known as “mission specialists.” They take turns operating sonar equipment and performing other tasks in the submersible.

These included Harding, who lives in Dubai in the United Arab Emirates, 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.

French explorer and Titanic expert Paul-Henry Nargeolet was also aboard, said David Gallo, a senior adviser for strategic initiatives and special projects at RMS Titanic. He identified Nargeolet, who has led multiple expeditions to the Titanic, in a CNN interview.