The violent protests in Ferguson last August were driven by the indelible image of an unarmed black teenager, Michael Brown, lying in the street after a white police officer, Darren Wilson, shot him dead. But the outrage in Ferguson, and the national debate that accompanied it, were also about something harder to see: racism, and the allegation that Ferguson’s largely white cops were deeply, systematically and violently prejudiced against black residents. Now, as one of his last acts as U.S.