Contractors at the Husky Energy refinery in Superior were told to return to work April 26 after hearing a "strange knocking noise," which caused them to fear for their safety and temporarily leave the work area.

But, within 30-40 minutes of returning to work, the explosion occurred, resulting in numerous onsite injuries and the evacuation of most of Superior, according to a lawsuit filed by seven contractors.

In a lawsuit filed Aug. 17 in Douglas County Circuit Court, contractors with Evergreen North America and Jamar Contractors claim they were "instructed" to return to work after "plaintiffs and other employees rushed out of the refinery, fearing that the refinery was unsafe."

Refinery manager Kollin Schade and safety and security manager John O'Brien, defendants in the lawsuit, were overseeing the contractor's work on the refinery's turnaround, according to the lawsuit.

In addition to Schade and O'Brien, the lawsuit also lists current refinery owner Husky Energy, former refinery owner Calumet and Superior Refining Company as defendants.

"Plaintiffs suffered substantial injuries and burns as a result of the incident. The intense and terrifying nature of plaintiffs' injuries have also resulted in severe mental pain and anguish. As a result of their injuries, plaintiffs have been required, and will continue to be required, to undergo substantial medical treatment," according to the lawsuit.

A Husky spokesperson declined to comment on ongoing litigation when contacted by the Duluth News Tribune on Wednesday, Sept. 5, but added: "We are cooperating fully with those agencies investigating the incident."

According to the U.S. Chemical Safety and Hazard Investigation Board, 36 people were injured in the April 26 explosion and fire. While the specifics and severity of the injuries are unknown, the Duluth News Tribune reported that only one patient "in good condition" remained in the hospital April 27.

Husky brought the contractors on to work on a turnaround, when all or parts of the refinery were shut down for maintenance.

According to the U.S. Chemical Safety and Hazard Investigation Board's Aug. 2 factual update, the explosion occurred during that shutdown. The board found a worn-out valve may have allowed air to mix with hydrocarbons within the fluid catalytic cracking unit before coming into contact with iron sulfide deposits, which can spontaneously ignite if in contact with air.

A class action complaint against Husky Energy regarding the explosion, fire and subsequent evacuation was also filed last month.