Having fled Germany in 1938, the late Frederick (né Friedrich) Mayer enlisted in the U.S. army immediately after the bombing of Pearl Harbor. He quickly found himself working for the Office of Strategic Services (OSS), the precursor to the CIA. Emily Langer writes:
By late 1944, Mayer had arrived in Europe, where, dissatisfied by what he considered the slow pace of his work, he talked his way into the Secret Intelligence division of the OSS. He became the leader of Operation Greenup, a mission to gather intelligence in the area of Innsbruck, where the Allies suspected the Germans might mount a final stand in the war.
The night of February 25, 1945, Mayer flew from his base in Italy to the Austrian mountains, parachuting onto a frozen lake in the treacherous terrain. . . .
Working alongside Mayer were Hans Wynberg, a Dutch-born Jew whose family had been deported to Auschwitz, and Franz Weber, an Austrian officer whose patriotism had led him to defect from the German army. The three fashioned a pair of skis into a sled and made their way down a mountain, at times navigating snow as deep as their shoulders. . . .
[Mayer] was credited with gathering intelligence and building a network of informants that helped determine the location and dimensions of Hitler’s Führerbunker in Berlin, the condition of Nazi war plants, and the movement of enemy freight and troops, particularly through the Brenner Pass.
Read more on Washington Post: https://www.washingtonpost.com/national/frederick-mayer-german-jew-who-returned-to-nazi-europe-as-us-spy-dies-at-94/2016/04/26/e5360cac-0af2-11e6-bfa1-4efa856caf2a_story.html