The first atomic spy, Briton Alan Nunn May, was recruited to work for the Russians by fellow spy Engelbert Broda, documents reveal.
Nunn May eventually married Broda's ex-wife Hildegarde
It had not been known how physicist Alan Nunn May - who passed secrets on the Anglo-US programme to build the first atomic bomb - had been recruited.
Files show Nunn May, imprisoned in 1946, met Austrian spy Broda when they worked at a Cambridge laboratory.
Upon his release in 1952, Nunn May married Broda's ex-wife Hildegarde.
Broda, who had been an active communist in Austria, worked at the Cavendish Laboratory, Cambridge, at the same time as Nunn May - from April to November 1942 - before the Briton moved to Canada.
Upon his return from Canada, he came to the attention of MI5 after a tip-off from the Canadian authorities.
It later became clear Nunn May had passed on secrets from the "Manhattan Project" to the Russians.
After his imprisonment, Nunn May was interviewed by agent William Skardon in Wakefield Prison on 21 March 1949.
Mr Skardon noted: "The only real information that I could obtain was that his recruitment, if that be the right word, took place literally a few hours before he left England for Canada."
Nunn May had said "the individual for whom I might be looking in this connection was well out of my reach", Mr Skardon added.
It was later established that Nunn May and Broda had worked at the same laboratory ahead of the Briton's move to Canada.
"Nunn May... said that he was recruited to the Russian service only a very short while before he left this country for Canada and that the individual who recruited him was no longer still within reach," the documents conclude.
"In March 1949 when Nunn May said this, Broda was no longer in the country."
The mystery of Nunn May's recruitment is revealed in papers released on Friday by the National Archives.