WASHINGTON — In the decades after World War II, the C.I.A. and other United States agencies employed at least a thousand Nazis as Cold War spies and informants and, as recently as the 1990s, concealed the government’s ties to some still living in America, newly disclosed records and interviews show.

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 as secret, anti-Soviet “assets,” declassified records show. 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.”

And 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, according to a government official.