The Tulsa Fire Department released the following statement:
Investigators from the Tulsa Fire Department and The Bureau of Alcohol, Tobacco, Firearms and Explosions finished their investigation of the Barnard School Fire. The fire and resulting explosions have been determined to be “accidental” resulting from construction related to the installation of an exhaust vent in the lab area.
The investigation also revealed what caused the explosions which resulted in injuries to eight Tulsa Firefighters. The fire had been smoldering in the void between the chemistry lab ceiling and the floor of the room and hallway above. The fire migrated north under the hallway floor into the classroom and the crawl space below where it vented from the classroom window. The resulting smoke explosion or “backdraft” occurred when oxygen was introduced into the area by the firefighters entering the room to extinguish the fire. The larger explosive event observed approximately four seconds later was a sympathetic explosion of the hot gas layer that had accumulated between the drop ceiling and the roof. This event was essentially caused by the turbulent and sudden change in the environment as a result of the first explosion.
Traditionally backdrafts occur in a confined space or room, but in this case the fire had been slowly burning in the floor and the crawl space for hours charging both areas with smoke and gases. The fire venting out of the window had not allowed enough oxygen to the back of the room to reduce the fuel load. The Tulsa Fire Department will use this information to determine how to recognize this situation and if its protocols need to be refined to prevent these events in the future.
The Barnard school site was equipped with three monitored smoke detectors but the nature of this fire and its location were not sufficient to activate those alarms.