rifle was stolen overnight while agent 's car was parked at a salt lake city hotel across the street from the state 's fbi office .
gun was ` secured properly ' in a case and truck safe with padlocks and chains .
police believe thief tied a rope around the case and used another car to break the handle off by ripping the case through the car 's window .
agent 's stolen backpack and gear bags were recovered at a nearby hotel , but the gun has not been found .