Investigators have released surveillance video images recorded Saturday night of a parking lot near Shank's sports bar on the north side of the city.
The officer's patrol rifle and two ammunition magazines, all inside a locked case, were swiped from his personal vehicle between 10:45 p.m. and 11 p.m.
Sgt. Jason Walker says the video shows a suspect vehicle temporarily blocking another car that is trying to leave the parking lot.
Officers are still trying to determine the make and models of the vehicles.
Police say the off-duty officer had taken the rifle home for cleaning and left it in his vehicle when he went into a restaurant. When he returned, he realized the vehicle had been broken into and the gun and ammunition were gone.
He has since been suspended with pay while the force looks into a possible code of conduct breach.
Walker said he and other officers with the service's Guns and Gangs Unit have been tasked with getting the gun back.
He hopes people who saw the suspect vehicle will call investigators.
"These folks might have seen something that is very important to us but at the time thought nothing of it, other than 'I want to get out of this parking lot. There's a car in my way and some guys doing stuff. What the heck?'"
He says investigators are still looking into whether other vehicles in the area were also broken into. So far, no one has reported any.
"Sometime people's vehicles get broken into and they don't think it's worth reporting — it is."