The Elusive Smoking Gun (Short Stories, #3‪)‬

Within the confines of a highly secured area, a sniper kills the state senator.  The police apprehend the suspect in under three minutes.  Successful in his devious plan to hide the rifle; the killer waited on a bench, and showed no sign of fleeing.  An extended search by the FBI failed to locate the murder weapon.  With no evidence to hold him, the sniper walked.  The case remained unsolved for years.