From 1945 to 1979, the Immigration and Naturalization Service was responsible for identifying and prosecuting Nazi collaborators and potential war criminals in the United States. It failed in this task for a number of reasons. The first of these was that the agency was severely disorganized and mismanaged. Reliance on interagency cooperation, lack of manpower and resources, and lack of institutional support for “Nazi hunters” posed further problems. Morale crises among employees and the legal difficulties of actually prosecuting Nazi collaborators also hampered the agency’s effectiveness. Most importantly, the agency was overwhelmingly focused on policing the southern border and preventing the entry of unauthorized Mexican migrants. This policy focus prevented resources from being devoted to other initiatives, including investigating the presence of Nazi collaborators in the United States. In this paper I analyze the existing historiography on this topic and discuss its shortcomings. These include a focus on the small number of cases prosecuted by the INS, from which historians have tended to make inapplicable generalizations, and a focus on the Cold War and anticommunism as explanations for the INS’s failure. I have also surveyed historical works on denazification in Germany, which I argue provide a better template for historians working on the collaborator presence in the United States.