Based upon a study of the trial of Tarek Mehanna, a young American of Egyptian descent, who was convicted of terrorist conspiracy in April 2012 and sentenced to seventeen years and a half in prison, this article examines the role of experts and social scientists in the legal construction of the category of terrorism. The object of this paper is not to attempt to measure the influence of expert witnesses on law, but, rather, to understand how the political logic of preventing terrorism functions. This logic, I will argue, informs the practice of social scientists, judges and lawyers. This article also explores the debates about the interpretation of the First Amendment in the context of the War on Terror. It analyses the current struggles to establish distinctions between what counts as free speech and what is seen as constituting dangerous thought, or between free religious practice and practice that is deemed radical and suspect.