An investigation is underway to determine the cause of a fire that occurred around 8 p.m. Wednesday night at the Weyerhaeuser medium-density fiberboard plant in Columbia Falls, according to plant officials.

Four Weyerhaeuser employees were transported to a local hospital and treated for smoke inhalation, and have since been released, said Tom Ray with Weyerhaeuser.

“Our mechanical fire systems functioned as designed and with the help of our employees and local first responders, the fire was quickly extinguished,” Ray said.

Columbia Falls residents had reported hearing an “explosion” around 8 p.m. then seeing smoke coming from the plant shortly after. The impact reportedly was felt by residents miles away from the plant. However, Ray said it was not an explosion, and what residents heard were the duct gates opening quickly to prevent an explosion as part of the plant’s mechanical fire-suppression system.

“It is very loud, but not an explosion,” Ray said.

Very minor damage is reported to the plant itself and Ray said officials expect the plant to be operating again soon.

“The response from our team at MDF and the first responders from the local fire departments was outstanding, and their efforts are greatly appreciated,” Ray said.

The incident happened on production line two about 8 p.m., according to Columbia Falls Fire Chief Rick Hagen. He said the initial fire was knocked down by the automatic deluge fire suppression system on the line and Weyerhauser employees.

Firefighters from Columbia Falls, Bad Rock, Whitefish and medical crews from Three Rivers Ambulance all responded. It took them most of the night to go through the line, battling the smoke to find any hot spots and to water down wood material to make sure the fire was dead out, Hagen said. Firefighters were working on the third floor of the massive building.

They didn’t get off the scene until about 1 a.m. Thursday, he said. He said the fire-suppression systems in the plant appeared to have worked properly.

A total of 22 firefighters from Columbia Falls responded, along with 11 from Bad Rock and three from Whitefish. Three Rivers provided rehabilitation for firefighters who were in the plant for extended periods of time.

A failed bearing was suspected as the cause of the blaze, according to a preliminary inspection by Weyerhaeuser employees, who then reported that information to firefighters.

Fires over the years have not been uncommon at the plant, but since an explosion and fire a few years ago, the company has bolstered its fire-suppression system.

Reporter Kianna Gardner can be reached at 758-4439 or kgardner@dailyinterlake.com. Hungry Horse News Editor Chris Peterson contributed to this report.