Internal wear on a valve is what caused the initial explosion that sparked a massive leak and fire at the Husky Refinery in Superior in April.
The finding was released as part of a factual update by the U.S. Chemical Safety Board on Thursday.
The CSB says the explosion stemmed in the refinery's Fluid Catalytic Cracking Unit as it was being shut down for planned maintenance at 10 a.m. on Apr. 26.
The CSB says the fault lay with one of the valves that controls the flow of flammable hydrocarbons in the reactor to a regenerator containing air.
The valve had worn internally, and could have allowed air to enter the reactor where the hydrocarbons are held, which in turn flowed into equipment downstream of the reactor that also contained hydrocarbons.
When the explosion happened, it sent debris 200 feet into the air, and punctured an above ground storage tank that spilled 15,000 barrels of hot asphalt, which caught fire around 12:15 p.m., causing the massive fire.
The CSB released an animated video explaining how it happened.
Thirty-six people were injured in the explosion and subsequent fire, though it could have been much worse had many workers not been on a break at the time.
It also prompted the evacuation of the city of Superior and rural areas to the south of the refinery.
Husky Energy has not yet resumed normal operations at the facility, saying this might not happen until 2020. KSTP reports it's incurred $53 million in costs as a result of the fire, while $27 million of damage was caused to the facility itself.
The investigation into the gigantic blaze, that sent smoke billowing miles into the air, will continue