Friedrich Karl Berger arrived in Frankfurt on February 20th, “possibly the last” such expulsion by Washington of a former Nazi, a US official had said then.

Prosecutors in the city of Celle, who had previously halted their probe of the man, had reopened investigations over suspicion of complicity in murders on his return, as Berger had said he was willing to be questioned.

But “after exhausting all evidence, prosecutors at Celle have once again closed the investigation because of a lack of sufficient suspicion,” they said in a statement.

Berger, who had retained German citizenship, was deported for taking part in “Nazi-sponsored acts of persecution” while serving as an armed guard at the Neuengamme concentration camp system in 1945, the US Justice Department said.

He had been living in the US since 1959, and was stationed as a young man from January 28th, 1945 to April 4th, 1945, at a subcamp of Neuengamme, near Meppen, Germany.

German investigators had been examining whether during his time there, and in particular when “monitoring a march evacuating the sub-camp, he had contributed to the death of many detainees”.

More than 40,000 prisoners died in the Neuengamme system, records show.

Germany has been hunting down former Nazi staff since the 2011 conviction of former guard John Demjanjuk on the basis he served as part of the Nazi killing machine set a legal precedent.

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Since then, courts have handed down several guilty verdicts on those grounds rather than for murders or atrocities directly linked to the individual accused.

Among those who were brought to late justice were Oskar Gröning, an accountant at Auschwitz, and Reinhold Hanning, an SS guard at the same camp.

Both were convicted of complicity in mass murder at the age of 94 but died before they could be imprisoned.

In February, German prosecutors charged a 95-year-old who had been secretary at the Stutthof camp with complicity in the murders of 10,000 people, in the first such case in recent years against a woman.

Days later, a 100-year-old former guard at the Sachsenhausen camp, north of Berlin, was charged with complicity in 3,518 murders.