More than 50 years after Lionel "Buster" Crabb squeezed into his wetsuit for the last time and submerged himself in Portsmouth harbour, another supposed solution to the mystery of the diver's death has surfaced.
Buster was an underwater explosives disposal expert during the second world war, and MI6 employed him to investigate the hulls of visiting Soviet ships for mine hatches.
Crabb disappeared in 1956, the day he was sent to check for underwater equipment on a Soviet cruiser that had brought the Russian leaders Nikita Khrushev and Marshal Nikolai Bulganin to the UK.
Now a former Russian frogman has described in a Russian documentary how he cut the British diver's throat after catching him placing a mine on the hull of the leaders' cruiser.
Eduard Koltsov, a retired sailor, said he needed to tell the truth about the cold war mystery before he died. Koltsov, who was 23 at the time, was investigating suspicious activity around the ship when, he said, he saw Crabb fixing a mine to the hull.
In the documentary, he showed off the dagger he claims to have used to kill Crabb and the red star medal he was awarded.
"I saw a silhouette of a diver in a light frogman's suit, who was fiddling with something at the starboard next to the ship's ammunition's stores," he said. "I swam closer and saw that he was fixing a mine."
The headless body of a frogman was found in Chichester, West Sussex, in June 1957, and a coroner later ruled it was that of Crabb, but the diver's family dismissed Koltsov's claim.
Lomond Handley, 61, one of his few living relatives, said he found the remarks "astonishing and hard to believe".
"For any of Her Majesty's navy to endanger a visiting ship would have been unthinkable," he added. "Any explosion would have embarrassed our government and destroyed the relationship between Britain and the Soviet Union that the government was trying to build up.
"Crabb may have gone down to see whether there were mines, and it may have been his duty to get rid of them."
Crabb, a Royal Navy commander, was rumoured to have been an inspiration for Ian Fleming's James Bond, who had the same rank.
He won the George medal for his work as a frogman specialising in removing German limpet mines from merchant ships. Handley has spent much of her life attempting to find out what happened to Crabb, but has "come up against brick walls".
Secret documents relating to the controversy were released to the public at the National Archives in Kew, south-west London, last year. They reveal the determination of officials to cover up what really happened, even rejecting a request for maintenance from Crabb's wife, Margaret.
The then prime minister, Sir Anthony Eden, told the Commons it would "not be in the public interest" to disclose the circumstances of Crabb's death.
He added that "what was done was done without the authority or knowledge of Her Majesty's ministers".
The cover-up prompted wild speculation for years, including claims that Crabb was alive and well and living in Russia and others that the Soviets had killed him.
The secret documents revealed that, five months after Crabb's death, WH Lewin, the head of naval law, wrote in a memo: "If this came out ... it would not seem to square very well with our statement that Crabb had been out of the navy for over a year at the time of his death."
The official admiralty line following the incident was that Crabb had been "specially employed in connection with trials of certain underwater apparatus" and was missing, presumed drowned.
"The government told lie after lie, successive governments too, not only the government of the day," Handley said. "No government has come out with the truth."