The federal agency investigating the 2018 explosion and fire at the Husky Energy refinery in Superior that injured 36 workers and led to the evacuation of much of Superior released an updated animated video Thursday of the incident.

The U.S. Chemical Safety and Hazard Investigation Board's 5 1/2 minute YouTube video details both the cause of the incident and the emergency crew's response. The board has released several updates in the 20 months since the explosion fire, but its investigation is ongoing.

In a factual update last year, the U.S. Chemical Safety and Hazard Investigation Board said that while the refinery had “process hazard analyses” considering what would happen when an open valve would allow hydrocarbons and air to mix within the fluid catalytic cracking unit — where the explosion occurred — the safeguard did not consider what would happen when the valve was closed but unable to hold a seal. In an August update, the board said a worn — but closed — valve within the fluid catalytic cracking unit allowed air to mix with hydrocarbons, creating an explosive mixture that then ignited.

Thursday's animated video of the incident is the second video of the incident shared by the Chemical Safety Board. The agency produced a video for its August 2018 update.

The agency also produced a video last year with interviews from refinery firefighters and members of the Superior Fire Department detailing the response.