Early Friday morning, an explosion rocked the Plum Creek Timber Co. fiberboard plant in Columbia Falls, but no one was hurt.

The cause of the 3:30 a.m. explosion in a fiberboard production line is still under investigation, said Hank Ricklefs, the company's vice president for manufactured products in Columbia Falls.

Workers were at the plant, which operates 24 hours a day, but none were near the explosion. An unknown ignition source ignited fine dust particles in the fiber feeding and mat-forming section of the line, Ricklefs said.

"There was no major damage to the key equipment and electrical control systems. It was more building damage," Ricklefs said.

Most of the structural damage resulted from a fire that followed the explosion.

"There are vents that absorb that type of impact from a dust explosion," Ricklefs said.

With Columbia Falls, Creston, Whitefish and Badrock Fire Departments responding, the fire was extinguished in about two hours. Columbia Falls ambulance and the Office of Emergency Services also went.

Automatic spark detection and fire suppression systems that are integrated with fiberboard production lines probably helped contain the fire's spread, Ricklefs said.

Data collected by those systems will be analyzed to identify the source of ignition, he added.

A second production line, which became operational in 2002, was shut down as standard procedure. That line was expected to restart by Friday afternoon.

The original production line, where the explosion occurred, will probably be closed until early next week, Riklefs said.