NEW YORK - Declassified CIA files reveal that U.S. intelligence officials went to great lengths to protect a Ukrainian fascist leader and suspected Nazi collaborator from prosecution after World War II and set him up in a New York office to wage covert war against the Soviet Union, a new report to Congress says.

Mykola Lebed led an underground movement to undermine the Kremlin and conduct guerrilla operations for the CIA during the Cold War, says the report, prepared by two scholars under the supervision of the National Archives. It was given to Congress on Thursday and posted online.

During World War II, the report says, Lebed helped lead a Ukrainian nationalist organization that collaborated with the Nazis in the destruction of the Jews of the western Ukraine and also killed thousands of Poles. The report details postwar efforts by U.S. intelligence officials to throw the federal government's Nazi hunters off Lebed's trail and to ignore or obscure his past.

"You can make the argument the CIA never should have gone near this guy because of his past," said Norman J.W. Goda of the University of Florida, who wrote the report with Richard Breitman of American University in Washington. But Goda said the CIA found the relationship to be so valuable for getting information into and out of the Soviet Union that "the relationship couldn't be sacrificed."

"This was somebody that was very, very useful and remained so for the entire Cold War," he added.

The report, titled "Hitler's Shadow: Nazi War Criminals, U.S. Intelligence, and the Cold War," draws from an unprecedented trove of records that the CIA was persuaded to declassify, and from more than a million digitized Army intelligence files that had long been inaccessible.

Among other things, the authors say, the files also show that U.S. intelligence officials used and protected ex-Nazis during the Cold War to a greater extent than previously known. They also show that no U.S. intelligence agency aided Nazi henchman Adolf Eichmann's escape from Europe after the war.

CIA spokesman George Little said Friday: "The CIA at no time had a policy or a program to protect Nazi war criminals or to help them escape justice for their actions during the war. The agency has cooperated for decades with the Justice Department's Office of Special Investigations." The OSI was Justice's Nazi-hunting unit.

The U.S. government relocated Lebed in 1949 to New York City, where he was safe from assassination, the report says. Through his CIA-funded organization, Prolog, he gathered intelligence on the Soviets into at least the late 1960s. In 1991, he was still considered a valuable asset to the agency, the report says.

Lebed was eventually identified by federal investigators as a possible war criminal but was never prosecuted. "Lebed remained one of the agency's oldest contacts until his death in 1998," according to another declassified CIA document.

The records that were used to write the report were made available under the Nazi War Crimes Disclosure Act of 1998, one of the most exhaustive efforts by the government to expose its own secrets. The papers include correspondence, legal documents, clippings and medical records. They illuminate the activities and postwar whereabouts of some of highest-profile alleged Nazi war criminals.

One of the report's chapters deals with how the Americans used Gestapo officers, including Rudolf Mildner, after the war. Mildner oversaw security in Denmark in 1943 when most of the country's 8,000 Jews were ordered arrested and deported to Auschwitz - though they were rescued after Danish resistance leaders were tipped off. The Army detained Mildner, and saved him from landing in the hands of war-crimes investigators, because his knowledge of communist subversion was considered useful.