(Polk County, IA) -- Investigators are trying to figure out what sparked a deadly explosion in Polk County. Fifty-five-year-old Tad Costello of Johnston was killed in the explosion and fire at Metro Motors, 2529 NE 46th Avenue in Delaware Township yesterday (Wednesday) afternoon. Deputies and Fire Department personnel arrived and found the rear portion of the building on fire. Fire crews worked to extinguish the fire and found Costello. Costello suffered burns and was taken to the hospital in critical condition, where he died this morning. Costello was the only person in the building when the explosion happened.