Crews arrived to find a large volume of fire both under the structure in the parking area as well as fire on the interior of two apartments. Once the fire on the ground level was knocked down, the three-person Engine company entered the apartment above to conduct a primary search. They encountered fire so they began extinguishment while searching. A backup line was brought up by a second three-person Engine company. Within 20 seconds of the second crew entering the apartment, the floor collapsed dropping all six firefighters to the ground level.All were rescued immediately and minor injures were reported by three of the six firefighters.