A sub that went missing while carrying five people to the wreckage of the Titanic has less than 40 hours of breathable air left as of Tuesday evening, as the U.S. Coast Guard says search efforts continue. The sub had about 96 hours of oxygen at most onboard, officials said.

A Canadian research vessel lost contact with the submersible during a dive Sunday morning about 900 miles east of Cape Cod, Massachusetts, and U.S. and Canadian authorities have been searching for it.

Coast Guard Capt. Jamie Frederick told reporters during a news conference Tuesday afternoon that "about 40 hours of breathable air left" was an estimate based off of the vessel's original 96 hours of available oxygen.

Chief Petty Officer Robert Simpson, a Coast Guard spokesman, said there wouldn't be a "hard-and-fast" transition from a search-and-rescue mission to a recovery operation when those hours are up, saying there were several factors that could extend the search.

Frederick said authorities were working around the clock on the search in the Atlantic for the missing sub, calling the effort "an incredibly complex operation."

"We will do everything in our power to effect a rescue," Frederick said. "...There is a full-court press effort to get equipment on scene as quickly as we can."

Pakistani businessman Shahzada Dawood, his son Suleman, British explorer Hamish Harding and French explorer Paul-Henri Nargeolet were on the sub, along with Stockton Rush, the CEO of OceanGate Expeditions, the U.S.-based company that planned the voyage.

If the sub is found in time, Frederick said it was difficult to describe what a deep-sea rescue would exactly entail.

"That's a question that then the experts need to look at what is the best course of action for recovering the sub, but I think it's going to depend on that particular situation," he said.

The Coast Guard said the last recorded communication from the sub was about an hour and 45 minutes into Sunday's dive.

Since the sub went missing, the Coast Guard, Canadian coast guard, U.S. Navy and Air National Guard have searched a combined area of about 7,600 square miles, an area larger than the state of Connecticut, Frederick said.

Search efforts continued Monday night and into Tuesday, he said. A pipe-laying vessel arrived in the search area Tuesday and sent a remotely operated vehicle into the water to look for the sub at its last-known position.

With search flights scheduled to fly over the area throughout the day, a Canadian coast guard vessel was expected to arrive Tuesday evening, Frederick said. Several other Canadian vessels and a U.S. Coast Guard cutter were en route to the area.

The U.S. Navy was working on deploying military assets to aid the search, Frederick said.

In:
• RMS Titanic
• Submarine
• United States Coast Guard
• Live Streaming

Alex Sundby

Alex Sundby is a senior editor for CBSNews.com

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