The Memorial Day explosion and fire at Verso Paper in Sartell was caused by overheating in one of the mill's air compressors, according to a report by investigators. The explosion killed one employee and injured four others.
Deputy State Fire Marshal John Steinbach wrote in a report released Monday that not enough cool water was reaching the air compressor, which failed to shut down when it overheated.
The report states that heat ignited oil vapors that caused a receiver tank to explode, blowing out concrete walls and igniting rolls of paper.
About an hour before the explosion, two air compressors shut off due to overheating. But a third compressor kept running, past protection devices that should have shut it down, "possibly caused by a mechanical failure and/or maintenance issue," Steinbach wrote.
Earlier on the day of the fire, one of two water cooling systems was turned off to repair a leak.
"In my opinion, severe reduction of the water flow to compressors for cooling the morning of the incident is a contributing factor to compressor #3 overheating, which resulted in a fire and explosion occurring at the mill air receiver tank," Steinbach's report states.
Verso employee Dennis Schneider told investigators that he and Jon Maus, who died in the explosion, saw smoke coming from the compressor room. Schneider saw flames shoot out of the top of the air compressor followed by an explosion that injured and knocked him to the ground. Maus was killed by blunt force trauma.
David Popp was working in the mill the day of the explosion. He told Sartell police in a statement he saw the fire in a room with roughly eight compressors, near the bottom of one of them.
"I seen like a red flame, it was a kind of a red flame. It wasn't a campfire flame, it was a more red," he said to police.
Popp said he saw Maus holding the fire extinguisher.
"I don't know if he tried to spray it," Popp told police, "but I seen the fire increase a little bit higher, maybe a foot higher, you know... I seen John's body reactions, it was like wait, I better wait, you know it was like, I don't think I'm doing this right."
According to Popp's statement, Maus turned and started walking away from the fire "and that's when that blew."
The State Fire Marshal's report notes the sprinkler system designed to put out fires was partly damaged in the explosion.
Firefighters were on the scene of the blaze for more than a week. Fire departments have billed the company for $350,000 in costs.
Despite offers of help from Gov. Mark Dayton and others, Verso decided to close the facility after the fire, which cost 259 workers their jobs.
Dayton and other officials are assembling a task force to examine other uses for the site.