“If you had told me then I would be in court cross-examining a Nazi in 2020, I would have said, ‘Not happening. Not possible for anyone to be doing that in 2020.’ But there you go,” said Mr. Rosenbaum, the director of Human Rights Enforcement Strategy and Policy at the department.

He declined to say how Mr. Berger’s case came to the department’s attention, but he said such cases typically develop through a tip from a foreign government or from proactive research by the department, which has developed a list of names of suspected Nazi war criminals and methodically searched for them for 40 years.

In Mr. Berger’s case, he said, the department received important investigative help from law enforcement and archival authorities in Germany, England, Denmark, Poland and Russia.

Mr. Berger, he said, was a regular soldier in the German Navy who was assigned by the SS to guard a concentration camp at Meppen, where forced laborers — many of them Russian civilians — were taken out each day in the dead of winter to dig anti-tank fortifications. He said Mr. Berger was tied to the operation through one critical piece of evidence — his name appeared on a set of SS cards identifying the guards in the Neuengamme system along with their basic service information.

The cards were found amid skeletons and other wreckage in the Thielbek, one of three German ships that were carrying thousands of prisoners when they were bombed by the British Royal Air Force in the Bay of Lübeck in 1945. In about 1950, Mr. Rosenbaum said, the Thielbek was raised from the sea and the cards were recovered and transcribed.

“Fortunately, they could make out almost everything for Berger,” he said, including when he went to Meppen and when he left. “It’s an incredibly intricate jigsaw puzzle, but this one was one of the most important pieces of evidence.”

Mr. Berger, he said, had apparently been living quietly in Oak Ridge, where he blended in easily with his suburban neighbors. “It’s a nation of immigrants,” Mr. Rosenbaum said, “and we’re all accustomed to encountering people of different background from overseas.”

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