The headlines are among the most stark documenting gang violence. A Latino gang member, without saying a word, guns down a 14-year-old black girl standing on a sidewalk. A black gang member shoots a Latino toddler point-blank in the chest. For the most part, though, the role racial animosity has played in gang crime has gone unexamined, largely undocumented in crime statistics and often tamped down by politicians and law enforcement officials anxious about inflaming tensions. That changed this month when Los Angeles Mayor Antonio Villaraigosa, Police Chief William J. Bratton and L.A. County Sheriff Lee Baca all spoke with...