A fire of undetermined origin broke out late this morning. Engine 9 arrived on the scene with a report of smoke showing from the second and third floors. A report of an infant trapped on the third floor took firefighters in the building for an agressive search. The fire initially proved difficult to locate as crews encountered heavy smoke conditions but no visible fire. After a few minutes it was determined that the building was not occupied. Shortly there after firefighters located the fire in the ceiling voids between divisions 1 and 2. Two firefighters were injured when they fell through the floor from the 2nd to the 1st floor. THe first firefighter fell and landed on division 1. A second firefighter landed on top of him after he too fell through the hole. Both firefighters were transportred to York Hospital as trauma patients. The last report recieved one was to be relased and the other admitted for observation. His injusties were not reported as life threatening.