What happened

During a scheduled shutdown of the fluid catalytic cracking unit for maintenance, a hydrocarbon-oxygen mixture ignited, sending debris roughly 61 meters and puncturing a large asphalt storage tank; the spilled asphalt burned for about nine hours. More than 2,500 Superior residents were evacuated and Duluth, Minnesota issued a shelter-in-place order over fears that debris could instead have struck a nearby hydrofluoric-acid tank. The CSB's 2022 final report found the accident could have been prevented. This was an accidental industrial incident, not an attack.

When cited sources give different casualty, damage or spill figures for this event, they’re reported side by side above rather than merged into one number. See how confidence levels work.