The pilot stated while on patrol 'at about 500 feet above ground level, the helicopter experienced major engine failure'. The pilot attempted to autorotate the helicopter to a vacant parking lot. Upon impact, the tail rotor snapped off and the helicopter turned 180 degrees to the left. Examination of the engine found the number two cylinder had separated from the engine block and the rod cap bolt had been sheared. Maintenance records show the removal and reinstallation of the number one cylinder at 646.2 hours since overhaul which was 66.6 hours prior to the accident.