Update: The U.S. Coast Guard said a deep-sea robot has discovered a "debris field" in the search area. Read the latest here. Our earlier story is below.

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The frantic search near the Titanic wreckage site for the submersible that went missing carrying five people ramped up and expanded on Thursday with two ships deploying deep-sea robots — and at least one of the robots has already reached the ocean floor, officials said. 

The U.S. Coast Guard said in a tweet that the French vessel L'Atalante had reached the area and deployed their ROV, or remote operated vehicle. The Coast Guard also said that the Canadian ship Horizon Arctic had deployed an ROV "that has reached the sea floor and began its search for the missing sub."

The Coast Guard added that winds in the area were at 14 mph with gusts up to 19mph. There were sea swells reaching up to five feet and the air temperature was 50° Fahrenheit.

There has been no confirmed contact from OceanGate Expeditions' Titan submersible since it lost contact with its support ship on Sunday morning, but search planes detected banging noises at roughly half hour intervals both Tuesday and Wednesday, which they said were coming from under the water.

Five people are on the sub, including three tourists, a veteran French explorer and the owner of OceanGate Stockton Rush, who was piloting the sub.

Search crews were still hopeful Thursday morning that they could find the five people alive on the Titan submersible, but as the search area expanded to about double the size of Connecticut, fear was mounting that oxygen in the vessel may already have run out.

The Titan is believed to have carried enough oxygen to last the crew 96 hours, which means it could run out Thursday morning if it hasn't already. But the co-founder of OceanGate said he was still holding out hope.

"Today will be a critical day in this search and rescue mission, as the sub's life support supplies are starting to run low," Sohnlein said in a personal statement posted on Facebook. 

"I firmly believe that the time window available for their rescue is longer than what most people think," Sohnlein wrote. "I would encourage everyone to remain hopeful for getting the crew back safely."

Rescuers turned their focus to a remote area of the North Atlantic where the underwater banging noises were detected.

"We're searching where the noises are. That's all we can do at this point," U.S. Coast Guard Capt. Jamie Fredrick said Wednesday, stressing that it was still "a search and rescue mission — 100%."

There were no further reports on Thursday of noises being detected from under the water, and while there were many questions, there was no doubt that both time and oxygen were running out.

"In any search and rescue case, you look at all the different factors," Fredrick told CBS News. "How long we've been searching, the survivability, the oxygen on board — it's kind of a complex set of data we look at, but again, there is more than just one piece and right now we're focusing on the search."

More rescue vessels and equipment were joining the effort, but searching for the 21-foot-long Titan in the near-freezing, pitch-black conditions on the seafloor in an area with depths of up to 2.5 miles is undoubtedly challenging.

"If I were a family member, I would remain hopeful," Capt. David Marquet, who commanded the U.S. Navy submarine USS Santa Fe, told CBS News. "But people generally do not come back from the bottom of the ocean."

He said it was "ominous" that there had been no communication whatsoever from the Titan since early Sunday.

"It's a signal that crew is incapacitated," he said of the sub's, which apparently relied on simple text messages to communicate with its support ship on the surface during dives.

"It's a commercial organization," Marquet said. "They're trying to be really innovative and cutting edge… on the, our submarines, we would have more devices… but we have a nuclear submarine that costs $2 billion. So, we have a luxury of having all those kinds of things."

OceanGate has faced criticism for years for its seemingly maverick approach to highly-risky deep-sea exploration.

"This company decided to self-certify" its submersible, engineer Bart Kemper, who was part of the Marine Technology Society that expressed concerns in 2018, told CBS News. "It follows no code and follows no jurisdiction."

Kemper said the company agreed to be more transparent with its passengers after his organization raised its serious concerns, but continued to operate the experimental vessel without what the group said was typical certification for such manned underwater vehicles.

He said the global community of explorers and engineers who build such non-military vessels had never previously lost a submersible, "and that's the thing that we've lost. We've lost it."

"We've had failures, we've had, we've had fatalities," he said. "But we've never had this type of incident happen before."

In:
• Rescue
• RMS Titanic
• Submarine
• Atlantic Ocean