MINNEAPOLIS (WCCO) – Officials with the U.S. Chemical Safety Board continue to investigate what caused an explosion at a Superior, Wis., refinery that injured 16 workers and caused evacuations in both Superior and Duluth.

The incident happened at about 10 a.m. on April 26 at the Husky Energy Oil Refinery. Officials say the investigation shows the initial explosion happened in the refinery’s “Fluid Catalytic Cracking” unit, which mainly produces gas.

What actually caused the explosion is still under investigation. CSB officials have talked with about 50 refinery employees and contractors who were working at the time of the explosion. The refinery was preparing for a five-week shut down for maintenance at the time. During that time, equipment is shut off so it can be opened, inspected and repaired.

The explosion, in addition to causing property damage, caused a loss of containment of an asphalt tank. The contents spilled out and the material exploded. Investigators are still collecting debris from the explosion and storing it for future analysis.