So has mine. Long Beach and Los Angeles have carried AR-15's since the North Hollywood Shootout.
As for the previous comments about the Ruger Mini-14. It doesn't have the accuracy at distances that the police need. I've shot one and my groupings started to really faulter after 100 yards, especially since i was using run of the mill ammo and not hand loads.
Part of the reason for that has to do with the gas operation system the Mini 14 employs. It has a floating piston attached to a recoil arm that seats in a fixed gas cylinder inside the forend at the front of the stock. Gas is bleed off the barrel into the fixed cylinder and forces the floating piston out of the cylinder. When the hot gas vents into the forend it heats the underside of the barrel diferentially from the top of the barrel. That causes the barrel to "warp" slightly from the heat.
At close ranges, it means nothing. Out past 100 yards the warping causes the point of aim to shift as the weapon heats from use. This is bad for accuacy. The advantage of the system is that it works and it works all the time. After the weapon heats totally, the point of aim changes again. It can be annoying.