GEOFF BENNETT: Good evening, and welcome to the "NewsHour."

The search for the missing submersible in the North Atlantic Ocean intensified throughout the day.

AMNA NAWAZ: And the U.S. Coast Guard says it's still treating the mission as a search-and-rescue operation, even as the supply of oxygen is likely dwindling for the five people aboard.

More ships and special equipment are heading to the area this evening.

The day began with a surprise, reports of banging noises detected by sonar.

The first glimmer of hope in the massive maritime search.

CARL HARTSFIELD, Woods Hole Oceanographic Institute: There have been multiple reports of noises, and every one of those noises is being analyzed, tracked, look for patterns and reported upon.

AMNA NAWAZ: A Canadian military plane picked up underwater noises in the North Atlantic on Tuesday, as the hunt for the missing submersible stretches to day four.

At a U.S. Coast Guard briefing in Boston today, officials said more sounds were heard today, and experts are reviewing the data.

CARL HARTSFIELD: They're trying to put all the pieces together.

The noises have been described as banging noises.

But, again, they have to put the whole picture together in context.

There are a lot of vessels in the area, and they each make noise, right?

So all of that has to be eliminated.

AMNA NAWAZ: The submersible vanished on Sunday during a deep sea tour of the Titanic shipwreck, the watery grave of one of the worst ocean disasters in the 20th century.

The Titan, roughly the size of a minivan, set off with about four days' worth of breathable oxygen.

Officials say that is enough to last until tomorrow morning.

For the international search-and-rescue operation the clock is ticking and the pressure growing.

Several countries are bringing in additional surface ships and underwater vessels to aid the effort.

Canadian ships have dropped sonar buoys to detect any underwater sounds in the search area about 400 miles off the southern coast of Newfoundland.

And following reports of the noise, French researchers dispatched an unmanned robotic vehicle set to arrive this evening capable of hooking the sup to a cable that could tow it to the surface.

Jules Jaffe is an oceanographer at the Scripps Institution.

JULES JAFFE, Research Oceanographer, Marine Physical Laboratory, Scripps Institution of Oceanography: The ocean is inherently a noisy place.

We have lots of marine mammals making sounds.

And there's a lot of instrumentation in the ocean there, geophysical explorations going on.

Sound travels pretty well underwater, and so I would give it a low chance of identification that, in fact, the banging noises that we heard were from the underwater submersible.

AMNA NAWAZ: As questions remain about how to reach the sub, past allegations and questions about the company that runs the voyage, OceanGate Expeditions, have surfaced.

In 2018, a terminated OceanGate employee sued the company, raising concerns about the safety of its submersible technology.

That same year, more than three dozen industry experts sent a letter with similar concerns to CEO Stockton Rush, warning of potential catastrophic outcomes.

Rush insisted at the time OceanGate took adequate safety measures.

Rush was piloting this latest trip and is now among the five passengers aboard the missing vessel.

PETER GIRGUIS, Harvard University: Oftentimes, when organizations want to push the technology envelope, they will use materials that are uncommon or take designs that have been -- haven't been tested.

AMNA NAWAZ: Peter Girguis is a professor at Harvard University, where his lab develops ocean instruments and samplers.

PETER GIRGUIS: I think what we're seeing, though, is more and more information coming out from documents from OceanGate and from others engaged with them that raise the possibility that they -- that there may have been oversights as they tried to get this experimental submersible in the water and on kind of active duty, if you will.

AMNA NAWAZ: Even as the estimated oxygen deadline looms, Coast Guard officials said they are not giving up.

CAPT.

JAMIE FREDERICK, U.S. Coast Guard: There are a lot of factors you consider.

And then after you consider all of those factors, 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.

But, again, we're not there yet.