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, 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, neighbours, friends, even their families, and building dossiers on millions of people.

The case also reopened the age-old German question of individual guilt in State-run terror machinery. “It’s a very courageous and important verdict,” said Stefan Appelius, a political scientist who has spent decades researching state crimes committed in East Germany, and whose research into this case ultimately led to Monday’s conviction.

Yet for all its resonance, the killing of Czeslaw Kukuczka went largely unnoted for years, until 2016, when Appelius found clues in the Stasi’s own meticulous records.

After publishing his research both in an academic journal and in Super-Illu, a magazine with a mostly East German readership, he quickly realised that almost no one had heard of the case.

Initially, it was not clear who had carried out the killing. Later, the public attorney dropped the case on the premise that the charge would be manslaughter, not murder, so the time limit for prosecution would have lapsed.

But then the story got traction in the victim’s native Poland, where his widow and children had not been told what had happened. (His widow later died, and his children became plaintiffs in the criminal case.) Polish prosecutors issued an international arrest warrant for Naumann, which helped push German authorities into preparing an indictment for murder.

Calling Monday’s verdict a landmark, Appelius said: “I think this a really good signal for the many victims and their families.”

On the morning of his last day, in 1974, Kukuczka entered the Polish embassy in East Berlin, holding a bag that, according to photos shown in court 50 years later, held some wires and a crude electrical switch. Kukuczka told the embassy staff that the bag contained a bomb and that he was prepared to blow himself up if he was not allowed to leave the country for the West.

Kukuczka had been a firefighter and a labourer in the village of Kamienica, in the south of Poland. He had spent some time in prison in the 1950s for embezzlement, according to the Polish historian Filip Gańczak, who researched the victim’s life.

A Polish officer at the embassy, Maksymilian Karnowski, promised Kukuczka passage to the West before leaving the room and calling the Stasi, according to historical records. The embassy gave Kukuczka temporary papers and even some West German money, and then Stasi officers drove him to the train station at Friedrichstrasse.

The station was located at what was, in 1974, the most important crossing point in the fortified, militarised border between East and West Berlin. There, train passengers could pass through the Iron Curtain, after being cleared through a border checkpoint known as the Tränenpalast, or Palace of Tears.

Kukuczka was allowed to clear all of the checks there and walked into a tunnel leading to trains bound for West Berlin, where he was shot. Naumann, as well as other officers involved, were later given medals for their role in the operation. Surviving records said that because of their work, “a serious border provocation was prevented and the terrorist neutralised”.

What the Stasi did not count on were three girls from Bad Hersfeld in West Germany, who were visiting East Berlin and were in the same station. They returned home and spoke both with Bild, Germany’s most important tabloid, and with West German federal prosecutors collecting testimony on crimes committed in the East. Their accounts did not lead to any direct action, but they ensured that some West Germans knew about the shooting, and that there was a record.

But the case, unsolved, faded from popular memory. Naumann was promoted and enjoyed the perks of a nearly three-decade Stasi career, rising to the rank of major.

In their testimony this year, the three witnesses said they saw a man in ragged clothes and greasy hair who had cleared passport control and walked into a tunnel. They said he passed another man, wearing sunglasses and a civilian sports coat, who, in what one witness described as a “smooth motion” pulled a gun and shot the victim in the back, from about six feet away.

The witnesses remembered the victim holding his back where he was hit before he sank to the floor.

East German border officials rushed to close the doors to the tunnel to bar the view of people standing in line at passport control. The girls didn’t know until long after the Berlin Wall fell that the man died hours later at a Stasi prison hospital.

New York Times News Service