It was, at first, just a weird feeling. "An abstraction," Marc Cohn called it. "There was a sense of danger. Something was unstable where we were." Cohn, the Grammy-award winning musician, was heading back to the Monaco Hotel on 16th Street after he and his band had performed at the Denver Botanic Gardens on Aug. 7. It wasn't even midnight; far too early for things to feel so creepy. Then Cohn heard the footsteps, sprinting, coming on fast. Almost as soon as he'd registered the guy, Cohn saw the gun in his hand, and nothing was abstract anymore. Not the...