Rescuers searching for a tourist submersible near the Titanic wreck in the North Atlantic have heard “noises” in the area near where the vessel went missing on Sunday.
According to the US Coast Guard, a Canadian P-3 aircraft heard the sounds, which US Navy experts are analyzing.
Underwater operations have been relocated to explore the source.
But so far the remote operated vehicles [ROV] have “yielded negative results”, the Coast Guard said.
“Additionally, the data from the P-3 aircraft has been shared with our US Navy experts for further analysis which will be considered in future search plans,” the Coast Guard tweeted in the early hours of Wednesday, as the ROV searches continue”.
According to an internal US government memo seen by US media outlets, “banging” was heard at 30-minute intervals on Tuesday.
Additional sonar was used four hours later and noises could still be heard.
The BBC has contacted the Department of Homeland Security for comment.
Five people were on the vessel when contact was lost an hour and 45 minutes into its dive, or more than halfway down towards the wreck, on Sunday.
Search authorities estimate that the sub has fewer than 30 hours of oxygen left, meaning supplies are set to run out by about 10:00 GMT (06:00 EDT) on Thursday.
The five men on board include British businessman Hamish Harding, 58, British-Pakistani businessman Shahzada Dawood, 48, and his son Suleman, 18, French explorer Paul-Henry Nargeolet, 77, and Stockton Rush, 61, the chief executive of OceanGate.
The group was sealed inside the sub using external bolts, meaning they cannot escape from it by themselves even if it resurfaces.
Chris Brown, an explorer and friend of Mr Harding, said the reported banging sounds have “got them written all over it” and “just the sort of thing I would have expected Hamish to come up with”.
“If you made a continuous noise, that’s not going to get picked up, but doing it every 30 minutes, that suggests humans,” he told BBC Breakfast.
“I’m sure they’re all conserving oxygen and energy, because it’s cold and dark down there.”
After the sounds were detected, scientific society the Explorers Club, of which two of the men on board are members, shared an upbeat message.
“There is cause for hope that based on data from the field, we understand that likely signs of life have been detected at the site,” the organization’s president said in a statement.