The US Coast Guard is still treating the hunt for the missing Titan submersible as an "active search and rescue", despite hopes fading as the 96-hour oxygen supply dwindled.
More ships and expertise were scrambled to help find the missing deep-sea vessel, which lost communication on Sunday while about 435 miles south of St John's, Newfoundland, during a voyage to the Titanic shipwreck off the coast of Canada.
OceanGate Expeditions estimated the oxygen supply on the 6.7m (22ft)-long vessel, which has British billionaire adventurer Hamish Harding on board, would last the crew of five 96 hours, giving rescuers a deadline of around midday on Thursday.
But Rear Admiral John Mauger confirmed the main focus of the US Coast Guard and the unified command in charge of the operation was to retrieve the vessel in what is still being seen as an active search and rescue.
He told Sky News: "We continue to keep the crew members and the families in our thoughts as we proceed with this search and rescue while we're cognisant of the time and we've factored in a lot of data and information into the search.
"This is still an active search and rescue at this point and we're using the equipment that we have on the bottom right now, the remote operated vehicles to expand our search capability, and then also to provide rescue capability as well."
When questioned about the noises detected by the sonar buoys dropped into the ocean, he revealed initial reports found that it was "ocean background noise".
He said: "We've taken that information and shared it with top leading experts from the US Navy and the Canadian Navy, and they're working on the analysis of that information, they're continuing to work on the analysis of that information.
"The initial reports is that there's a lot of the sounds that were generated were from background ocean noise, but they continue to . look for all available information there.
"What's important to me, and what's important as the unified command, is that we've continued search in the areas where noise was detected with the ROVs that we have from the time of that detection, so we're not waiting for this analysis to take action.
"The analysis is really helpful to our overall search-and-rescue efforts, but we're not waiting on it, we've moved the remote operated vehicles that we've had on site to those areas where noise was detected."
In a statement published online Guillermo Sohnlein, co-founder of OceanGate, thanked the people involved in the search and rescue, as well as confirming the pilot to be his "co-founder and friend".
He also claims that the time window the people on board have is larger than the estimated 96 hours.
In the statement, he said: "Today will be a critical day in this search and rescue mission, as the sub's life support supplies are starting to run low.
"I'm certain that Stockton and the rest of the crew realized days ago that the best thing they can do to ensure their rescue is to extend the limits of those supplies by relaxing as much as possible.
"I firmly believe that the time window available for their rescue is longer than what most people think.
"I would encourage everyone to remain hopeful for getting the crew back safely."
Also in the craft are UK-based businessman Shahzada Dawood, his son Suleman Dawood, a student at the University of Strathclyde, OceanGate's chief executive and founder Stockton Rush, and French submersible pilot Paul-Henri Nargeolet.
In a statement, the University of Strathclyde said: "We are deeply concerned about Suleman, his father and the others involved in this incident.
"Our thoughts are with their families and loved ones, and we continue to hope for a positive outcome."
The Royal Air Force (RAF) confirmed it received a request overnight for assistance with movement of additional commercial equipment.
Two RAF planes, a C-17 Globemaster and A400 Atlas, departed RAF Lossiemouth in north-east Scotland on Thursday.
A British submariner and equipment from a UK firm has been sent to help the search at the request of the US Coastguard, Downing Street said.
Royal Navy submariner Lieutenant Commander Richard Kantharia, who was on exchange with the US Navy, has been seconded to the search and rescue team.
Experts said the chances of finding the sub and rescuing those inside was diminishing.
Former Royal Navy submarine captain Ryan Ramsey told the PA news agency: "The outlook is bleak, that's the only word for it as this tragic event unfolds and almost the closing stages of where this changes from rescue to a salvage mission."
Dr Jamie Pringle, a reader in forensic geosciences at Keele University, said: "I think there's always hope with these things but you know about the golden first 24 hours and we're well past that stage.
"So there's always a chance, it's never zero, but I think obviously the longer the time elapses the lower the chance of success."
Titan is believed to be about 900 miles east and 400 miles south of Newfoundland.
It is not known how deep the vessel is, with the seabed being around 3,800m from the surface.
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