Former senior legal colleagues Kevin Egan and Clive Grossman think they know what might have happened to Gary Alderdice on June 24, 1994, in Vladivostok.
Mr Egan said a man with an indeterminable accent he had met in a private club in Hong Kong about 18 months ago had told him the most convincing story.
'He said it was a dreadful accident in the sense that common criminals had got access to the intelligence that Gary was arriving with money,' Mr Egan said.
'They had gone around to the woman's flat - they obviously knew her and she was there. They treated them as they would have treated their own - in other words, brutally. He was shot, she was then tortured and also shot.
'When the mafia in Vladivostok found out what had happened, who had been killed and the dreadful ramifications that it might have, the two men involved were taken out and ended up in a ditch outside Vladivostok with bullets in their heads.'
Mr Grossman also adheres to the robbery theory. The murder occurred a few years after the collapse of the Soviet Union and the fall of communism and Vladivostok was a wild port city, full of sailors who had not been paid for a long time.
'Word got around that somebody was coming from Hong Kong who must be rich, so basically it was just a robbery,' Mr Grossman said.