By Steve Rosenberg
BBC News, Berlin
In a tiny government office in Zirndorf, Georg Schleyer takes handfuls of ripped up bits of paper out of a sack and arranges them on his desk.
The software could process the files in five years instead of 400
As more and more scraps emerge from the bag, the jumble of fragments grows bigger and more intriguing. I can make out the code names of secret agents, there are torn-up photos and twisted lengths of microfilm.
These are the files of the Stasi - the former East Germany's Ministry of State Security. They are secrets the Stasi had tried to destroy, but which are now being pieced back together.
It is all very low-tech. To help him, Georg has a magnifying glass, some sticky tape and an iron to smooth out the creases.
But with the help of these items, over the last 12 years Georg has uncovered East German informers. He has learnt a lot about the Stasi's use of drugs in sport and discovered that the Stasi tried to cover up a train crash in which 75 children were killed.
"Sometimes I get so excited with my work, I miss my lunch break," Georg says.
"Once I even forgot to go home. What I'm doing is important. I'm helping to bring the history of East Germany back to life. So that former citizens of the GDR will have the chance to find out who spied on them, and why."
No secret police force in history has ever spied on its own people on a scale like the Stasi.
Some calculations have concluded that in East Germany there was one informer to every seven citizens. Back-to-back, the files would have stretched more than 100 miles (161km).
So when the Berlin Wall fell in 1989, panic-stricken Stasi officers had mountains of classified files to destroy. Unluckily for then, the shredding machines could not cope with the sheer volume of paper and broke down. So the Stasi resorted to ripping files up by hand.
But the secrets did not die. More than 600 million scraps were recovered, put in sacks and stored. Georg and his team of puzzlers have pieced some back together. But it would take them more than 400 years to finish the job by hand.
Now, though, computers may speed up the solution.
At the Fraunhofer Institute in Berlin, scientist Jan Schneider shows me how the new software works. As he feeds Stasi scraps into a scanner, the pieces appear on a giant monitor ready to be sorted and matched.
"It's like a grand jigsaw puzzle," Jan explains.
The complete history of the Stasi may soon be known
"Just like in a jigsaw, where you sort the pieces into sky, trees and so on, so here too in the computer you sort the snippets - according to their background colour, the colour of the writing, whether they're typewritten or handwritten. After that the search space is small enough to puzzle."
Then Jan presses a button and, almost magically, the different fragments dance around the screen before joining together into a single document.
There are more than 16,000 sacks of Stasi scraps to get through.
Experts believe that hidden inside are some of the Stasi's darkest secrets - untold stories of spies and informers, of undercover operations in the West and repression at home. It is thought that computers could process the entire contents within five years.
In her living room, former dissident Ursula Poppe shows me some of her Stasi files, which have already been pieced back together.
She has discovered that more than 80 people had been spying on her in the GDR. Now Ursula wants all the Stasi files repaired - to remind Germans how repressive the East German regime was.
"More and more people feel nostalgic because many of them feel the time now is not better than the time in the past," Ursula tells me.
"They forget the bad situations, the repression and the gap of freedom. So I think it's very important to get a realistic view of the GDR past."
The Stasi had torn that past into shreds. But with computers handling the pieces, the biggest and most sinister jigsaw puzzle in history may soon be completed.