Firefighters have been tackling a fire overnight at the former Nestle factory on the outskirts of Omagh.
It is thought the fire, in which a large quantity of wood chip pellets were burned, may have been started deliberately.
At its height almost 40 firefighters battled against the blaze on the Beltany Road, which was reported at about 1700 BST on Monday.
Crews from as far away as Portadown were in attendance.
"Initially we thought it was rubbish on fire, but crews discovered that it was wood chippings which are stored in the factory," said the Fire and Rescue Service's Aidan McGuinness.
"There are now signs of structural collapse in the building."
An investigation into what happened is under way.
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