One worker was treated for injuries after the Airgas facility in Waukesha exploded into flames Thursday afternoon.
Company officials say the explosion occurred in the plant's hydrogen storage area. All workers have been accounted for, Airgas says.
Investigators later found that a number of small cylinders had been leaking, and a small fire was sparked in that area, FOX6 reports.
Most of the flames had gone out before fire crews arrived. Firefighters extinguished the rest in the hydrogen storage area.