Officials from the UK and US picked up a "large acoustic signal" indicating an implosion of the Titan submersible days ago but couldn't make the information public because it was 'classified'.
The implosion was picked up by a "classified military device" in the North Atlantic and assessed by officials from the UK and US shortly after communications with the vessel went dark, according to a UK naval source.
That information was marked as classified and not handed down to the public, or some members of the rescue teams searching for the missing vessel due to the secrecy of the information and the device which picked it up.
It comes after the US Coast Guard confirmed on Thursday evening that the Titan suffered a "catastrophic implosion" and that all five passengers are believed to be dead.
A UK Naval source said that "by definition" the information was classified and couldn't be released to the public or the media. They confirmed that the signal was analysed by personnel from the UK and US.
"I am sure you can imagine what those sensors are supposed to track and we share that information with our allies as required," the source told i. "The Brits have people and leadership roles in that organisation. So hence, we knew."
The new revelation suggests that there were early indications that a rescue mission was unlikely to succeed but this was not shared due to the classification of the information.
"Whilst it would appear that classified systems may have heard noises detailing an implosion. as with all searches, you have to remain optimistic until you can categorically declare to the contrary," the UK Naval source said.
The UK naval source said the UK Foreign Office had been in regular contact with the victims' families, who "might have been told different" to what was said to the world's media.
The source added: "I would like to say that the family's expectations were managed accordingly."
On Tuesday, i learned that US military equipment had picked up a "large acoustic signal" with the "signature of an implosion" around the time the Titan went dark - about one hour and forty five minutes into its dive to the Titanic wreck site on Sunday.
Reporters from i pressed the US Coast Guard repeatedly on the suggestion of an implosion since Tuesday but requests were either ignored or met with "no comment".
Officials from the US Navy, UK Ministry of Defence, and Canadian military were also asked about on reports that the large signal had been picked up soon after communications with the vessel went dark. All either refused to comment or directed questions back to the US Coast Guard.
"In regards to this so-called implosion which has been reported, that would have been from a classified military system", the naval source told i. "By definition the word classified means we don't detail that information."
The source said that officials from Five Eyes - an intelligence alliance comprising Australia, Canada, New Zealand, the United Kingdom, and the United States - would have been made aware of the signal but officials had remained optimistic until there was "visible observation of wreckage."
On Thursday, Rear Admiral John Mauger - who led the search - confirmed in a news conference that the Titan had suffered a "catastrophic implosion" after a remotely operated vehicle had discovered the nose cone of the lost submersible about 487m (1,600ft) from the bow of the Titanic on the seafloor.
Despite the information, hours before the debris was found, Rear Admiral Mauger said he remained "hopeful" about the chance of rescue, adding that a medical team with a hyperbaric chamber used for decompression sickness had been brought to the site.
The five victims on board were British businessman and adventurer Hamish Harding; UK-based businessman Shahzada Dawood, his son Suleman Dawood, a University of Strathclyde student; Stockton Rush, the chief executive and founder of OceanGate, which runs the sub; and French submersible pilot Paul-Henri Nargeolet.