The Northern Ireland Fire and Rescue Service have confirmed they are treating the fire at the Old Herdman’s Mill in Sion Mills as deliberate.
The 'significant fire' started in the central ground floor area of the disused factory and quickly spread to involve the entire ground floor of the building.
63 fire fighters and 9 appliances tackled the blaze, working for over 7 hours to contain the fire and prevented the fire from spreading to other parts of the building.
The fire was brought under control at approximately 8 o’clock on Sunday evening however fire fighters remained at the scene to dampen down hot spots.
The incident concluded at 21.41 and the scene was handed over to PSNI in Strabane.
They are treating the incident as deliberate and an investigation is underway.
Anyone who witnessed anything in the area at the time is asked to contact police in Strabane on the non-emergency number 101.