TULSA, Okla. (AP) - Authorities say last week's fire that destroyed the Tulsa School of Arts and Sciences has been ruled accidental.

Investigators announced Friday that the fire and subsequent explosions were caused by a construction project when workers installed an exhaust vent into the school's laboratory area.

Investigators say fire was smoldering in the crawl space between the chemistry lab's ceiling and the floor of the room and hallway above. Explosions broke out after firefighters entered the lab.

Authorities say the increased oxygen level caused the blasts, which sent eight firefighters to the hospital.

The fire broke out early in the morning and no employees or students were in the building at the time. Students are now attending classes at Sequoyah Elementary School, which had been vacant.