At the height of the Cold War in the 1950s, law enforcement and intelligence leaders like J. Edgar Hoover at the F.B.I. and Allen Dulles at the C.I.A. aggressively recruited onetime Nazis of all ranks. They believed the ex-Nazis' intelligence value against the Russians outweighed what one official called moral lapses" in their service to the Third Reich. The agency hired one former SS officer as a spy in the 1950s, for instance, even after concluding he was probably guilty of "minor war crimes." In 1994, a lawyer with the C.I.A. pressured prosecutors to drop an investigation into an ex-spy outside Boston implicated in the Nazis' massacre of tens of thousands of Jews in Lithuania. In 1980, F.B.I. officials refused to tell even the Justice Department's own Nazi hunters what they knew about 16 suspected Nazis living in the United States. The bureau balked at a request from prosecutors for internal records on the Nazi suspects, memos show, because the 16 men had all worked as F.B.I. informants, providing leads on Communist "sympathizers." Five of the men were still active informants. In all, the American military, the C.I.A., the F.B.I. and other agencies used at least 1,000 ex-Nazis and collaborators as spies and informants after the war, according to Richard Breitman, a Holocaust scholar at American University. The full tally ... is probably much higher. But many records remain classified even today, making a complete count impossible.