A key Stasi tactic was to use friends, colleagues and relatives to spy on each other.
Experts say that people became Stasi spies for various reasons: some because of their political beliefs, others for money or power. But some were compelled to do so, or were blackmailed to become involved.
While the ability to discover the identity of the betrayer has helped bring closure in some cases, in others it has led to bitter divisions between families and friends.
Lutz Trenkner and his wife Ursula were jailed by the East German dictatorship in May 1984 for consorting with enemies of the communist state – namely his older brother Ernst, who had escaped to the West before construction of the Berlin Wall in 1960.
In 1990 when Lutz gained access to his Stasi files he discovered that the person who informed on him was his other brother Henning.
Confused and angry he confronted his brother.
Family feud
"He wouldn't admit to it, but he wouldn't deny it either," he said.
It was an uncomfortable conversation. I didn’t speak to him after that.”
Now, 24 years later the brothers rarely communicate.
“We wish each other happy birthday once a year and that’s our only contact.”
The 70-year-old is brutally honest when asked if he has forgiven his brother nearly 25 years later.
“I once had a colleague who told me to see my brother as a victim, not a perpetrator.
“But even if I forgive my brother, even if I hug him, there’s always that thing that came between us, something he can never undo.”
The Detail contacted Lutz's brother to hear his account of how and why he became involved in Stasi activity.
His family said he was not in a position to comment on the events.