The payments, underwritten by American taxpayers, flowed through a legal loophole that gave the US Justice Department leverage to persuade Nazi suspects to leave the US, an Associated Press investigation has found.
If they agreed to go, or simply fled before deportation, they could keep their Social Security, according to interviews and internal US government records.
Among those receiving benefits were armed SS troops who guarded the network of Nazi camps where millions of Jews perished; a rocket scientist who used slave labourers to advance his research in the Third Reich; and a Nazi collaborator who engineered the arrest and execution of thousands of Jews in Poland.
Hartmann moved to Berlin in 2007 from Arizona just before being stripped of his US citizenship.
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Denzinger fled to Germany from Ohio in 1989 after learning denaturalisation proceedings against him were underway. He soon resettled in Croatia and now lives in a spacious apartment on the right bank of the Drava River in Osijek.
Denzinger would not discuss his situation when questioned by an AP reporter; Denzinger's son, who lives in the US, confirmed his father receives Social Security payments and said he deserved them.
But internal US government records obtained by the AP reveal heated objections from the State Department to OSI's practices.
Social Security benefits became tools, US diplomatic officials said, to secure agreements in which Nazi suspects would accept the loss of citizenship and voluntarily leave the United States.
"It's absolutely outrageous that Nazi war criminals are continuing to receive Social Security benefits when they have been outlawed from our country for many, many, many years," said US Rep. Carolyn Maloney of New York, a senior Democratic member of the House Oversight and Government Reform Committee. She said she plans to introduce legislation to close the loophole.