The Innocent by David Baldacci

Description from Goodreads: "It begins with a hit gone wrong. Robie is dispatched to eliminate a target unusually close to home in Washington, D.C. But something about this mission doesn't seem right to Robie, and he does the unthinkable. He refuses to kill. Now, Robie becomes a target himself and must escape from his own people. Fleeing the scene, Robie crosses paths with a wayward teenage girl, a fourteen-year-old runaway from a foster home. But she isn't an ordinary runaway-her parents were murdered, and her own life is in danger. Against all of his professional habits, Robie rescues her and finds he can't walk away."

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