(CNN) -- Authorities in Germany are investigating whether they can prosecute a former Nazi camp guard who has been living in the United States for 60 years, they said Monday.
Prosecutors are probing the case of Johann Breyer, who admits to having been a guard at the notorious Auschwitz camp, where more than 1 million people, most of them Jews, were killed during World War II.
Breyer, 87, says he was only a perimeter guard and did not persecute anyone, according to legal papers related to his battle to keep his U.S. citizenship.
"He was a trained, paid, uniformed armed Nazi guard who patrolled the perimeters of two such camps with orders to shoot those who tried to escape," a court ruled in 1994.
Breyer won his fight to remain in the United States in 2003, when a court ruled that he was not responsible for having joined a Nazi unit because he was only 17 years old at the time.
But German authorities are investigating evidence against him, and expect to have preliminary results in October, said Gert Schaefer, a spokesman for the chief public prosecutor in the Bavarian town of Weiden.
Breyer emigrated to the United States in 1952 and claimed citizenship as a displaced person.
In the 1990s, the government fought to strip him of his citizenship, arguing that Nazis were not eligible.
Breyer did not dispute having been an SS Totenkopf (Death's Head) guard, responsible for guarding Nazi concentration camps, according to the U.S. Court of Appeals of the Third Circuit, which heard one stage of his legal battle in 1994.
Therefore, "Breyer assisted in persecution," the court ruled.
"The Department of Justice proved in federal court that John (Johann) Breyer participated in Nazi-sponsored acts of persecution while serving as an SS guard at the Buchenwald Concentration Camp and the Auschwitz Death Camp during World War II," DOJ spokeswoman Rebekah Carmichael said Monday in a statement.
But, she added, findings about his mother's birth and the date of his SS enlistment made it legally impossible to deport him.
His case is similar to that of John Demjanjuk, the Ukrainian-born Ohio man who spent more than two decades fighting charges that he was a Nazi war criminal before he died this year.
CNN's Fred Pleitgen and Janina Bembenek contributed to this report.