Legendary Soviet spy Aleksandr Feklisov, who helped the Soviet Union get the secret formula for an atomic bomb, has died in Moscow at the age of 93. Feklisov was responsible for stealing nuclear technology from the U.S. His efforts cut by half the amount

The KGB handler also played a key role in bringing the USSR and the U.S. back from the brink of a nuclear conflict during the Cuban missile crisis in the 1960s.

A year before his death he spoke out about his experiences as a spy in the nuclear race after the Second World War.

“I’ll never forget that meeting was I think in 1948 in November, and a colleague came in and said – Kurchatov’s team is heading for its goal at great speed.  I said – what goal?  Well, he said, soon the Baby will be born and will make its voice heard,” he wrote in his book.

The baby was the nuclear bomb, the Soviet answer to the U.S nukes that killed more than 100,000 Japanese at Hiroshima and Nagasaki. 

The “great speed” of its evolution was partly due to Aleksandr Feklisov.

Born in 1914, Feklisov joined the Soviet secret service, the NKVD, in 1940. A handler who recruited agents and gathered information, he was sent to Washington and then London to track down the nuclear formula.

In his wake, he left a string of high profile moles based in London, including leading German nuclear scientist Klaus Fuchs, whose information provided Soviets with a crucial missing link.

Although he willingly gave up the secrets, both handler and mole knew MI5 could pick up their scent at any moment.

“It was difficult of course that sometimes he didn’t turn up to our meetings. Those were torturous moments.  You always wondered why he didn’t come, whether he mixed up the place? Or worse, maybe something happened to him, maybe surveillance is after him and he decided not to go out, or even worse – maybe he's been arrested,” Feklistov wrote.

For Fuchs himself it was also a personal sacrifice.

“Klaus, I said, why don’t you get married. He said, I know that by meeting with you I’m walking through a minefield, one wrong foot, and you and I will simply take off into the air,” Mr Feklisov said.

One false step by another Feklisov mole led to the most dramatic spy trial of the 20th century.

In 1943 Julius Rosenberg began to pass on secret bomb designs. Ten years later he and his wife were executed for treason. 

But what came at a high price for the agents brought Feklisov a promotion. He was posted back to the U.S where he became KGB station chief in Washington. 

At the height of the Cuban missile crisis, it was Feklisov who came up with the compromise that brought the world back from the brink of nuclear war.

At 93 he was the last relic of a generation of Soviet spies, a role he played to the end. Immortalized in several Hollywood films, his profile rose as more secrets from the Cold War period came to light.

But it is believed Feklisov will take many secrets with him to his grave.   He remained loyal to his informants, people that he'd got to know well.