The Justice Department has asked a federal court in Pittsburgh to revoke the citizenship of Anton Geiser, 79, of Sharon.
The complaint alleges Geiser served as an SS guard at the Sachsenhausen concentration camp in Germany from January to November 1943, when he was transferred to the Buchenwald concentration camp.
Geiser declined comment and referred questions to his attorney, but refused to say who was representing him.
According to the Justice Department, Geiser was born in what is now Croatia and joined the Waffen SS guard in 1942. He came to the United States in 1956 and became a U.S. citizen in 1962.
U.S. immigration laws were amended in 1978 to allow deportation of anyone who assisted or participated in Nazi acts of persecution.
Since 1979, the Justice Department has had an office dedicated to investigating cases of former Nazis living in the United States and has won cases against 94 people.