“A safety emergency that unfolded in plain sight” — and how those ignored warnings led to a fatal explosion of a clearly corroded pressure vessel — is the focus of a safety video from the U.S. Chemical Safety Board.

The fatal explosion, which occurred on April 3, 2017, at the Loy-Lange Box Co. in St Louis, occurred when a severely corroded pressure vessel catastrophically failed, causing an explosion that launched the pressure vessel into a neighboring building. One worker and three members of the public were fatally injured.

“The incident at Loy-Lange resulted from a tragic series of circumstances: ineffective corrosion management, inadequate pressure vessel repair, a lack of inspections and the absence of sound safety management systems,” says Steve Owens, chairman of the Chemical Safety Board, in the video.

The safety video, “Ignored Warnings: Explosion in St. Louis,” includes an animation of the sequence of events leading to the incident. At the time of the explosion, the water in the leaking pressure vessel was suddenly exposed to atmospheric pressure and flashed into steam, expanding in volume. CSB says the result was a boiling liquid expanding vapor explosion, or BLEVE for short. The force of the explosion launched the pressure vessel from the Loy-Lange building. It traveled 520' before crashing through the roof of a nearby business, killing three employees there. The operator of the steam generator system was also killed.

Key among the findings was that ineffective corrosion management led to the catastrophic failure. “We found that all of the corrosion failures occurred in components of Loy-Lange’s steam system that — when operated under normal design conditions — should have experienced minimal or no corrosion at all,” says Drew Sahli, CSB lead investigator, in the video. Sahli added that this indicates that Loy-Lange’s operating practices led to oxygen not being properly managed or controlled within the steam system.

Watch the full video here, or read the CSB final report on its investigation of the incident.