Longview, Washington, USA 16 February 2017 -- (From The Daily News) -- KapStone has reportedly resumed running a furnace where a major fire burned earlier this week, Longview fire officials said Wednesday. There still may be some hot spots in the furnace itself, but any visible signs of smoldering had dissipated by Wednesday, according to Cowlitz 2 Fire & Rescue. KapStone employees and crews will continue to monitor the situation.

The unusually difficult-to-fight fire inside of a furnace kept crews from Longview Fire and Cowlitz Fire 2 on scene at the paper mill for more than 24 hours after starting at midday on Monday.

"This was an extremely challenging industrial fire," said Joe Tone, lieutenant at Cowlitz 2 who was duty chief Monday. "We pulled all of the rabbits out of the hat on this one to see what we could do."

The exact cause of the fire isn't known, and because it occurred nearly 100 feet above ground in a hard-to-reach location near the chimney of the furnace, there's no way to get fire investigators to confirm the cause, said Phil Jurmu, Longview fire chief.

However, it's believed that the fire was started as a result of construction work being performed on the building holding the furnace, fire officials said. Construction crews had already removed some of the metal siding on the building as they prepared to conduct maintenance upgrades on the furnace.

Tone said the fire occurred within the insulation in between the metal piping of the chimney and underneath a hard black casing.

Contractors hired by Kapstone used a high-pressure water hose to try to cut spaces out to allow for crews to shoot water into the chimney, Tone said.

Eventually, Longview fire used its truck with its tallest ladder (85 feet) to reach a space in between the side of the building and the roof, where water could be jetted into the furnace, Jurmu said. The roof ranged from 85 feet tall in some places to more than 100 feet tall in others, he added.