To make their case, prosecutors have turned to a group long known for spreading anti-Muslim conspiracy theories to provide them with an expert witness to define what antifa is supposed to be. That organization, the Center for Security Policy, has been designated by the Southern Poverty Law Center as a hate group. Its founder and former leader, Frank Gaffney, spent decades claiming that “creeping Sharia” was infecting American life. (Gaffney didn’t return an emailed request for comment.) In recent years, the Center for Security Policy has added countering the supposed antifa threat to its repertoire.