It remains one of the great riddles of the war. How could Hitler’s ruthless henchman, a forbidding figure described by another top Nazi as having hands like “strangler’s claws”, simply disappear and evade capture for decades?

Müller was a man of immense power. As boss of the Gestapo, he oversaw the implementation of Hitler’s murderous policies against the Jews. He was one of the planners at the Wansee conference where the Nazi leadership decided the “Final Solution of the Jewish Question”.

Adolf Eichmann, who was hanged by Israel in 1960 for implementing the Holocaust, was acting under Müller’s orders.

It was Müller, too, who at the end of March 1944 gave the order to shoot 50 British and allied prisoners of war after the mass break-out known as “the great escape”. He would certainly have been hanged had he been captured and put on trial at Nuremberg.

His disappearance was as mysterious as that of Martin Bormann, Hitler’s deputy, and Josef Mengele, the concentration camp doctor. Many investigators concluded that Müller must have perished in the ashes of Berlin. But there was never any proof and Simon Wiesenthal, the Nazi hunter, always kept Müller at the top of his most wanted list.

The CIA, too, went on investigating, prompted by tantalising reports that he had tried to contact one of his former mistresses in West Germany.

Now, six decades on, the truth seems to be coming to light. Two former senior intelligence officers, a Frenchman and an American, have unveiled a secret that they have been keeping to themselves for many years.

If what they were told is correct, and they believe it is, Müller was not what he seemed: he played a cynical double game serving as a secret policeman to two of the 20th century’s greatest tyrants. When he tried to escape the grip of his real masters they pursued him across the world to silence him. And who were these real masters? Not the Nazis but that other scourge, the Soviet communists.

EVEN at the height of his powers during the the second world war, when he had the rank of SS Gruppenführer, Müller was a mysterious as well as a feared figure. Born into a policeman’s family in Munich in 1900, he fought briefly as a teenage pilot in the fledgling German air force at the end of the first world war and was decorated for gallantry.

In the chaotic years after Germany’s defeat he joined the Munich police force and rose rapidly through the ranks, bettering himself socially by marrying the daughter of a local printer who published the newspaper of Bavaria’s political moderates.

It was a time of growing political extremism. Müller specialised in tracking communist agitators and he found favour with the Nazi leadership, still based in Munich before their rise to power in Berlin.

What brought him to their attention was the discreet manner in which he handled the “suicide” of Hitler’s young niece Geli Raubal in September 1931. Müller was the first policeman on the scene when she was found shot with a service revolver in the Munich flat in which Hitler kept her (and where he had reputedly subjected her to his sexual advances).