It has long been suspected that East Germany ran a clandestine network of 12,000 spies over the other side of the German border during the 40 years of separation.
What was unclear‚ thanks to the deliberate and almost total destruction of the most pertinent secret service files‚ was who they were and what exactly they were up to.
Now, though, research has shone a light on the activities of more than a thousand of the so-called Unofficial Collaborators (IM), ordinary citizens recruited by East German agents to spy on their friends and colleagues in West Germany.
The most controversial discovery has been parts of the file belonging to an IM called "Erich Neu" – better known to his parishioners in the West German town of Munster as Pastor Josef Frindt.
Two years after his death, at 81, it has been alleged that Frindt was not just doing the work of God, but also that of the East German ministry of state security, better known as the Stasi.
It seems that in between carrying out christenings, weddings and funerals, Frindt found time to submit 95 spy reports to his Stasi handlers. At least one of them was about a promising young colleague called Joseph Ratzinger, better known these days as Pope Benedict XVI.
Twenty-two years after the fall of the Berlin Wall, such revelations continue to trickle out of the BStU, the federal body set up in 1990 to look after the 69 miles of files the Stasi left behind.
Every month in Germany there are stories in the media accusing some public figure or other of collaborating with the Stasi. But until now, the work of the secret service's most sensitive department – the so-called Hauptverwaltung A (HVA), the Stasi's foreign wing – has remained largely a mystery.
That's because, after the Berlin Wall fell on 9 November 1989, while most East German citizens were celebrating the end of the cold war by rushing west, Stasi employees in the Berlin HQ were busy shredding the files they knew could get them in the most trouble.
Top of the list was the work carried out in West Germany by the HVA, which was run by the debonair Markus Wolf, widely believed to be the inspiration for the spymaster Karla in John le Carré's Tinker Tailor Soldier Spy (erroneously, according to Le Carré himself).
Between December 1989 and June 1990, almost every HVA paper file was destroyed, making it very difficult to assess the extent of the operation.
Now, though, a researcher from the BStU has painstakingly pieced together remaining scraps to build up the best picture yet of the Stasi's work in West Germany.
Helmut Müller-Enbergs, visiting professor at Gotland University in Sweden, has managed to find out more about over a thousand of these people, using data found on the so-called Rosenholz discs.
These are rudimentary CD-Roms containing snapshots of around 350,000 hugely sensitive Stasi records which somehow ended up in the hands of the CIA, before finally being returned to Germany in 2003 – albeit only those files pertaining to German citizens, a continuing source of frustration for the BStU.
According to Müller-Enbergs, by the end of 1988 there were 3,000 IMs working actively for the Stasi in West Germany. The biggest number of these – 542 – were based in West Berlin, followed by 149 in the then West German capital, Bonn.
Why Pastor Frindt was recruited in Munster is unclear from the remaining files but, says Müller-Enbergs, "we know that both his father and sister lived in the east".
Perhaps he hoped for a better life for them both. It is believed that before his death in 2009, Frindt was asked about the alleged Stasi links and denied even knowing Ratzinger, let alone spying on him.
The future pope was a professor of theology at Munster University between 1963 and 1966, though Müller-Enbergs said Frindt informed on Ratzinger in later years, while Frindt was a member of a small branch on Amnesty International in Dorsten, near Munster.
The Stasi were desperate for information from the church during the cold war because they suspected clergy in the west had links with their East German colleagues – and the church, in the officially atheist east, was a hotbed of resistance, playing a key role in the peaceful protests that eventually brought down the iron curtain.