Shortly before 1800 hours, Station 99 (York City) dispatched to 545 W. Princess Street for a structure fire. Engines 99-1, 99-2, Truck 99, and the on-duty Battalion Chief responded to the initial alarm. Engine 99-1 on location with heavy fire in the rear of an occupied, 2.5 story, semi-attached, dwelling. Second and third alarms were quickly struck bringing the remaining city companies and county mutual aid stations to the scene. Several interior hand lines were stretched to attack the heavily involved second floor rear. An aggressive attack brought the flames under control in about 45 minutes. At least one firefighter was injured battling the fire. Units remained on the scene for several hours mopping up. At least 16 people were displaced in the fire of unknown origin.