The U.S. Chemical Safety & Hazard Investigation Board (CSB) has released a factual update and video animation of an April 2018 explosion and fire at the Husky Superior refinery in Superior, Wis. The incident occurred in the refinery’s fluid catalytic cracking unit at the start of a maintenance shutdown. A mixture of hydrocarbons and oxygen was released and ignited, leading to an explosion that sent debris 61 meters. Some of the debris punctured a large storage tank containing about 8,000 m3 of asphalt, spilling the hot liquid into the refinery. The asphalt caught fire and burned for nine hours. The fire generated potentially toxic smoke and risked compromising other process equipment containing hazardous chemicals, including hydrofluoric acid, used in the alkylation unit. An evacuation notice was issued covering a 5-km radius around the refinery plus a rectangle extending 16 km south. Overall, 36 people sought medical attention, including 11 refinery and contract workers. In its update, CSB offers several scenarios for what might have caused the incident. The agency expects to issue its final causal determination within 18 months.