At approx. 1200hrs TK12 was dispatched as the second due truck to the 8500 block of Greenbelt Rd for the apartment fire. Chief 814B arrived on scene and reported a three story garden style apartment with heavy smoke showing from the eves. Truck 12 arrived on scene and immediately went to work to open up the walls of apartments within the building to expose the fire in the walls. The exterior crew from TK12 also worked to ladder the Charlie side of the building and vented the roof. Chief 12 operated as the Division 1 supervisor. Due to the majority of the fire being inside the walls, there was a large amount of extension which required significant opening up that had to be completed by the truck company. Units operated in scene for two hours before returning to service.