It was a brutal act from another time, almost another world, when the Cold War was hot and Germany was divided: An officer of East Germany’s feared secret police shot and killed a man trying to cross into the West.

Half a century later, a German court on Monday found the 80-year-old former officer, Manfred Naumann, guilty of murder and sentenced him to 10 years in prison, one of the harshest penalties meted out for the reign of terror by the secret police, known as the Stasi.

Over several days in March and April of this year, the only known living witnesses to the shooting faced the defendant in a high-security courtroom in Berlin, testifying to what they saw on March 29, 1974.

The witnesses — then schoolgirls, now retired women — all said that seeing a killing so young affected them for the rest of their lives. Trim and neatly dressed, Mr. Naumann, who lived for years in comfortable anonymity in a house in Leipzig, Germany, looked on in silence.

The trial, almost 35 years after the fall of the Berlin Wall, was a reminder of the pervasiveness and impunity of the dreaded Stasi, and of the dark sides of the oppressive Communist regime, which many people in the East now view with some nostalgia.

In November 1989, when the wall fell, the Stasi had an estimated 91,000 employees and 180,000 part-time spies, using coercion and violence to keep the Communists in power for four decades. It pressed ordinary people into spying on their co-workers, neighbors, friends, even their families, and building dossiers on millions of people.