Jiro, keep in mind, any activity that involves interstate, federal government usually takes over, mean states can't do anything. The reason behind this is no one can be charged twice for same crime. Because mail usually crosses state lines, two states can not charge one for same crime. This will result federal prosecution.
Same principle, if one decided to kidnap somebody from Massachusetts and brought to New York, then that scumbag got busted, who is going to take over? Its Federal, not state because two states already involved. If crimes commit in same state, and not on federal property, then state is the one who deals with this scumbag.