Bloodline: A Gabe Wager Mystery (Walker Mystery)

Like many major cities, Denver, Colorado, is beset by gang violence: gunfights to decide who will have the right to sell drugs in a neighborhood, drive-by shootings for slights real or imagined, murders because someone is wearing the wrong color ... or refuses to wear any color at all.
Cops learn early on that they can't take these things personally, that they must not allow events to get to them. Having learned the lesson, Gabe Wager faces the moment when the choice is taken from him. It comes when his cousin Julio is found in an alley, murdered, after denying to Gabe that he was involved in anything. Wager has to find the secret the boy died to keep ... and why.