The fire that destroyed the Tulsa School of Arts and Sciences has been ruled accidental, the Bureau of Alcohol, Tobacco, Firearms and Explosions, along with the Tulsa Fire Department, announced Friday.
The historic Barnard School Building went up in flames about 5 a.m. on September 5. When firefighters arrived, the building exploded, rocking midtown Tulsa and sending eight firefighters to the hospital.
After a week-long investigation, officials said the fire and explosion resulted because of "construction related to the installation of an exhaust vent in the lab area," according to a news release.
The explosion occurred because the fire had been smoldering in the void between the chemistry lab ceiling and the floor of the room and hallway above, the release said.
Investigators said 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.
A larger explosion happened about 4 seconds later and was a "sympathetic explosion" of the hot gas layer between the drop ceiling and the roof and was caused by the sudden change in environment because of the first explosion, investigators said. Investigators called it a "sympathetic explosion" of the hot gas layer that had accumulated between the drop ceiling and the roof, and was 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 report said.
In the world of firefighting - once a fire burns through to the outside, it's relatively safe - so firefighters going inside were surprised by the explosions that left eight of them injured - but could have killed them all.
"Everybody that was involved ended up 30 - 40 feet further back into the building than where they started," said Tulsa Fire Department Public Information Officer Stan May.
"It, that second explosion knocked them off their feet, blew a couple of guys out of the entryway out onto the sidewalk. We were very fortunate."
Investigators looked very closely at video shot by News on 6 after realizing it was a rare double backdraft.
Two major explosions - four seconds apart - came from unburned gas trapped below in a crawlspace and then above firefighters in a suspended ceiling.
"You very seldom see that in a vented building where it's coming out already ,and I don't think we've ever had film like that to look at," May said.
The school had three monitored smoke detectors, investigators said, but the nature of the fire and its location were not sufficient to activate those alarms.
Superintendent of Tulsa Public Schools Dr. Keith Ballard expressed gratitude to the fire department and investigators.
"We are so appreciative of the hard work of the Tulsa Fire Department and the ATF in bringing this investigation to a conclusion," Ballard said on Friday.
"We remain concerned about the safety and welfare of the eight firefighters who were injured in the blaze, and our hearts go out to them and their families. Without a doubt, we are privileged in Tulsa to be served by such fine men and women as those serving with the Tulsa Fire Department."
TSAS students are currently meeting at Sequoyah Elementary School while officials decide on a permanent home.
"The district continues to explore its options with the Barnard property and we are still in information-gathering mode, working in conjunction with TSAS and our insurance companies," Ballard said.
"In the coming weeks, the TPS board will make a decision as to how we will proceed with the property."