MADILL, OK -- The Oklahoma state fire marshal's office says they've determined the cause of the deadly explosion at a Marshall County plant.
The office investigates for criminal activity so their investigation ended when the explosion was ruled an accident.
The fire marshal's office says the fire erupted in a large furnace that melts metal when liquid got into the molten metal.
Oklahoma State Fire Marshal's office Operations Chief Sam Schafnitt said, "Whatever the fluid was expanded and once inside that vapor atmpsphere expanded faster than that vessel could contain it and that's when it ruptured."
OSHA is still doing their own investigation on the explosion to see if any of their standards were violated.
The explosion killed two men and burned another.