Josias Kumpf settled in the US after World War II
The US has deported to Austria a former SS man it says was involved in the Nazi killing of some 8,000 Jews, shot in a single day and buried in pits.
Josias Kumpf, 83, left Austria in 1956, settled in Wisconsin and became a US citizen in 1964.
The US justice department sued to strip him of his citizenship in 2003.
Confirming the deportation, Vienna said he could not be prosecuted in Austria because the statute of limitations had expired in 1965.
"We repeatedly indicated it to the United States," justice ministry spokeswoman Katharina Swoboda told AFP news agency.
"Our hands are tied."
'Orders to shoot'
Mr Kumf was found to have served as a guard at Sachsenhausen concentration camp in Germany and Trawniki in Nazi-occupied Poland, where the mass shooting took place in 1943.
"Josias Kumpf, by his own admission, stood guard with orders to shoot any surviving prisoners who attempted to escape an SS massacre that left thousands of Jews dead," said acting US Assistant Attorney General Rita Glavin.
His assignment had been to watch for victims who were still "halfway alive" or "convulsing" and prevent their escape, the US justice department said.
Josias Kumpf, born in Serbia, joined the SS Death's Head guard forces at Sachsenhausen in 1942 and served there for about a year before transferring to Trawniki, it added.
He also served at slave labour sites in Nazi-occupied France where prisoners built launching platforms for Germany's V-1 and V-2 rockets, the justice department said.
There was no immediate comment from Mr Kumpf or his lawyer, Peter Rogers. They have in the past denied that Mr Kumpf had a role in any atrocities.