A BLAZE at the derelict Herdman's Mill in Sion Mills was arson, the Fire Service has said.
The fire broke out at the former linen mill in Co Tyrone shortly after 9pm on Sunday.
A total of 22 firefighters from Strabane, Derry, Castlederg and Newtownstewart tackled the blaze, and the operation finished at midnight.
Northern Ireland Fire and Rescue Service western commander Mark Deeney said the building had been "significantly damaged by the fire".
The site has been targeted by several arson attacks in recent years, including two fires in the space of two days in June.
Dating back to 1835, the former linen mill was owned by the Herdman family for more than 170 years.
It once employed more than 1,000 people but it closed as a working mill in 2004.
A decade later the derelict site was bought by lottery winner Margaret Loughrey from Strabane.
It was reported at the time that she paid £1m for the site just months after winning almost £27m in the Euromillions jackpot in December 2013.
