Searchers looking for a submersible that went missing carrying five people to the wreck of the Titanic detected “banging” sounds every 30 minutes near the area where the vessel vanished, according to the U.S. Coast Guard.

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An internal U.S. government memo obtained by Rolling Stone and CNN said crews searching for the submersible Tuesday heard banging at 30-minute intervals, and a Canadian P3 aircraft also located a white rectangular object in the water.

However, according to the Coast Guard, subsequent searches “yielded negative results” as the clock ticked down to the last 24 hours of the craft’s presumed oxygen supply.

“Canadian P-3 aircraft detected underwater noises in the search area. As a result, ROV (a remotely-operated vehicle) operations were relocated in an attempt to explore the origin of the noises. Those ROV searches have yielded negative results but continue,” the Coast Guard said in a tweet. “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.”

Four hours later and after additional sonar devices were sent to the area, the banging sound was still heard being detected, the memo, which was sent by the Department of Homeland Security, said.

“Additional acoustic feedback was heard and will assist in vectoring surface assets and also indicating continued hope of survivors,” according to that update.

The Associated Press reports that the news of the banging noises has sparked hope among rescuers and members of a group called The Explorers Club. Two of the club’s members — Hamish Harding and Paul-Henri Nargeolet — are on the subversive.

Richard Garriott de Cayeux, the president of the club, wrote an open letter to his club’s adventurers saying that they had “much greater confidence” now after they spoke to officials in Congress, the U.S. military and the White House about the search.

In addition to Harding and Nargeolet, Shahzada Dawood and his son Suleman Dawood and Stockton Rush, the CEO of OceanGate, the company that owns the craft, are on board the submersible.

Named Titan, the 21-foot-long submersible, operated by U.S.-based OceanGate Expeditions, lost contact with its mother ship on the surface of the North Atlantic on Sunday morning about one hour and 45 minutes into what was a two-hour dive to the site of the wreck of the RMS Titanic, which sank in April 1912 after the ship hit an iceberg.