On March 23, 2012, President Barack Obama gave a press conference in the Rose Garden. A little less than a month before, George Zimmerman, a self-appointed neighborhood watchman, had shot 17-year-old Trayvon Martin after a violent encounter prosecutors said was instigated by Zimmerman in Sanford, Florida. Local police had released Zimmerman after five hours of questioning, saying there was not enough evidence to refute his claim of self-defense. It was an obviously incomplete investigation, and the Martin family was demanding justice. In the weeks that had passed, journalists, especially Trymaine Lee then at The Huffington Post and Ta-Nehisi Coates at...