Remotely operated vehicles have been moved to where the sounds were heard and more search equipment will be deployed as they arrive, US Coast Guard Captain Jamie Frederick said at a briefing in Boston on Wednesday. The noises have been picked up in the past two days by a Canadian aircraft with sonar devices, he said.

Time is running out for the missing crew, if they are still alive. The Titan, which started with about 96 hours of air in case of an emergency, is estimated to have about 1O hours left. Its crew is also believed to have limited rations on board, according to Frederick.

“We’re searching where the noises are and that’s all we can do,” Frederick said. “We don’t know what they are, to be frank.”

The unknown sounds have become the focus of the race to find the crew onboard the Titan, which went missing Sunday early into a dive to survey the remains of the Titanic. An armada of vessels from across the world, advised by submarine and ocean experts, are scouring the North Atlantic about 900 miles east of Cape Cod, Massachusetts, in what remains a search-and rescue mission.

The task of locating and identifying the sounds is complicated by the clutter of noise from the growing number of vessels on the surface. So far, searches by so-called ROVs have come up bare, according to to Frederick. At the same time. the constantly changing weather, ocean currents and sea conditions on the surface mean the search zone is growing by the hour. The area in question is already about twice the size of Connecticut and 2.5 miles (4 kilometers) deep.

A mother ship on the surface lost all communications with the Titan on Sunday, about 1 hour and 45 minutes after it began diving toward the Titanic, which sank in 1912 on its first trans-Atlantic voyage. The Titan, a 6.7-meter-long craft made of carbon fiber and titanium, is designed to operate at a maximum depth of 4,000 meters.

It’s not clear whether there’s enough time to retrieve the submersible — even if it was found immediately — before its air supplies deplete. On Wednesday, the US Coast Guard reiterated that the goal of the taskforce is to safely rescue the Titan’s crew, and the search will carry on.

“There is an enormous complexity associated with this case,” Frederick said at the briefing. But he added: “We have to remain optimistic and hopeful.”

“Sometimes we don’t find what we’re looking for,” he said. “Sometimes you’re in a position where you have to make a tough decision. We’re not there yet but if we continue to search, potentially we could be at that point. That’s a discussion that we will have with the families long before I’m going to discuss that in public.”

Watch: What it’s like to dive to the Titanic wreck

France, meanwhile, has dispatched a vessel equipped with an underwater robot that can go as deep as the Titanic site, about 4,000 meters below the surface. Three more ships arrived on the scene Wednesday morning. Underwater exploration company Magellan will also provide one of their ROVs “in the near future,” the US Coast Guard said. Magellan’s equipment can reach depths of up to 6,000 meters and has descended to the Titanic wreck several times.

Several privately owned vessels, one with a decompression chamber and some with underwater search devices, are also preparing to join the search. OceanGate Expeditions, operator of Titan and the Titanic survey trip, is leading the underwater search effort.

The crew onboard the Titan include Hamish Harding, who’s the founder of investment firm Action Group and an avid adventurer, and French maritime expert Paul Henry Nargeolet. The other three are OceanGate founder Stockton Rush, and Shahzada and Suleman Dawood, a father and son in one of Pakistan’s most prominent families.