The files were the latest in over three million pages of intelligence material released under the Nazi War Crimes Disclosure Act of 2000.
The twenty files included those on Mengele, who carried out gruesome medical experiments at concentration camps; Gestapo chief Heinrich Mueller; Adolf Eichmann, the architect of the plan to exterminate Jews; and Klaus Barbie the "Butcher of Lyon."
"These files demonstrate as a body that the real winners of the Cold War were Nazi criminals, many of whom were able to escape justice because East and West became so rapidly focused after the (Second World) War on challenging each other that they lost their will to pursue Nazi perpetrators," said the Justice Department's Eli Rosenbaum.
The files had little new information on Hitler. But one document shows the Nazi leader's doctor told an informant before World War II that Hitler was a "border case between genius and insanity." He predicted in 1937 that Hitler would "end up as the craziest criminal the world had ever known."
Information about former U.N. secretary General Kurt Waldheim, who was barred from entering the U.S. while president of Austria in the 1980's, was also released. He was found not to have been used as a source by the American government, but may have had ties to Soviet intelligence.
Documents also indicated the OSS official Allen Dulles, later to become the head of the CIA, held secret negotiations in Switzerland with German officials who had committed war crimes. These officials later used their contacts with Dulles to protect themselves from prosecution after the war.
The declassified files show that at least five people were implicated in Nazi crimes.
"These materials show that the United States of America retained Nazi war criminals and there will be no question about it," said Thomas Baer, member of an interagency group that worked with the CIA to release the papers. "It's always been hinted at, but you're going to get some specifics.
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