Authorities have revealed the cause of a fire on a Harlem movie set that killed a firefighter in March. Fire Marshals determined that heat from a boiler ventilation pipe ignited nearby combustible materials and caused the fire in the basement of a building on St. Nicholas Avenue. Firefighter Michael Davidson was trapped and killed. He was posthumously promoted to Lieutenant. Davidson, who was assigned to the nozzle with the responsibility of operating a hoseline to suppress the fire, became separated from his fellow firefighters as conditions rapidly were deteriorating. Officials said after a desperate search to locate Davidson, he was found unconcious.