Polish-born Nathan Gasch, who survived the Sachsenhausen and Auschwitz camps during the Holocaust, found himself living next door to a former Sachsenhausen camp guard in a retirement home in Mesa, Arizona recently. Gasch realised that his neighbour, the Romanian-born Martin Hartmann, had a troubled past when he saw a picture of Hartmann in an SS uniform. Gasch chose not to reveal his finding to anyone. As he puts it, "I figured we were living in a community here. I just let it go."
However, after an investigation that took two years to complete, Hartmann was discovered by Office of Special Investigations of the US Department of Justice. It turned out that Hartmann had been a member of the SS Death's Head Guard Batallion, composed of volunteers, during the Second World War. He came to the US in 1955, and later became a US citizen under false pretences. Last month, Hartmann, who is now 88 years old, was stripped of his US citizenship and deported to Germany (BBC).
I think Hartmann ought to stand trial in Germany, but somehow I doubt that's going to happen.