LEBANON, Ore. -- Firefighters said Monday that it will be extremely difficult to figure out what caused a huge fire at a mill in Lebanon.

The flames destroyed the Old Champion Mill near Santiam Highway and Market Street early Monday morning filling the sky with plumes of black smoke.

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Firefighters said they won't even be able to start their investigation until Tuesday because the scene was just too hot.

The building that went up in flames was about 200,000 square feet and the sheer size of the fire created extreme heat and made it tough for crews to put out.

Firefighters said no one was at the mill complex at the time and the building that was destroyed was only being used as office space for a couple of employees.

Five different departments helped fight the fire and stopped the flames from spreading to the main mill building.

The property is owned by a company in Lebanon and according to the Albany Democrat Herald newspaper it hasn't been used for production for several years.

Riverview Elementary School had to be closed because of the smoke from the fire but school officials said it will be open again Tuesday.