OAK RIDGE, Tenn. — It is perhaps fitting that the decades-long search for Nazi collaborators living on United States soil may have reached its conclusion — or something close to it — in a small city, in an unremarkable ranch house on an equally unremarkable cul-de-sac.

By many accounts, the man living inside that house was also seemingly unremarkable — not unlike the dozens of other under-the-radar Nazi collaborators who have been found and prosecuted over the last half-century.

The man, 94-year-old Friedrich Karl Berger, was ordered by a federal judge this week to return to Germany, where he remains a citizen and where he had served as a guard in a Nazi concentration camp during World War II.

Like in so many cases before his, the revelation about the friendly old man — a thoughtful father and husband who had raised his children in that house and had cared for his ailing wife until her death — came as a stunning surprise to his neighbors.

Ed McCleary, a retired school principal, recalled years of pleasant chitchat with Mr. Berger.

The revelations about his past, he said, “has not been circulated here.” But, he added, “It would be concerning to me to find out he had been involved in such things.”

Few cases like Mr. Berger’s remain.

Since 1979, the Justice Department has made a special point of hunting down collaborators in Nazi war crimes, with the intention of deporting them back to their home countries. But from the beginning, it has been a race against their natural life spans.