"In the famous photograph of the assassination of Martin Luther King Jr. on the balcony of Memphis's Lorraine Motel, one man kneeled down beside King, trying to staunch the blood from his fatal head wound with a borrowed towel. This kneeling man was a member of the Invaders, an activist group that was in talks with King in the days leading up to the murder. But he also had another identity: an undercover Memphis police officer reporting on the activities...