At 17:46hrs units were dispatched to the reported apartment complex office building on fire in the 1000 block of Dulles Ave. Upon arrival smoke was evident from all sides of a 1-1/2 story office type building. Truck 21 arriving first in stretched a 2″ attack line thru the A Side door and began fire attack as Engine 22 secured a water supply and Ladder 31 reported inside to open up as Engine 23 formed initial RIT and then deployed second line. Shortly after locating the seat of the fire in the attic space a report of a collapse was given and all companies withdrawn as Truck 21’s aerial was positioned for ladder pipe ops and command requested 2-additional companies. With all companies outside and accounted for crews made a good hit on the fire and achieved a knockdown without use of the ladder pipe. With the fire knocked down and searches negative companies hit hot spots and assisted FBCFMO investigators. Following completion of assignments all units returned to service. Additional companies on scene were Engine 3 as RIT, B21 as Ops Section, B31 as C Division, Engine 33 and Ladder 2.