Complex sentence: A first hand account of the arrest from CIA documents clearly states that the Americans who arrested Kaltenbrunner were actively searching for Kaltenbrunner and were highly confident they had the right man when Kaltenbrunner was arrested, even before his mistress removed any lingering doubt.
Split into simpler sentences: A first hand account of the arrest from CIA documents clearly states that the Americans who arrested Kaltenbrunner were actively searching for Kaltenbrunner. The americans were highly confident they had the right man when Kaltenbrunner was arrested, even before his mistress removed any lingering doubt.