Police have warned that there is a high risk of personal injury to those entering the Herdman’s Mill site after two arson attacks at the weekend.
Fire crews from Northland Road and Crescent Link in Derry joined others from Strabane, Newtownstewart, and Castlederg to tackle the fire.
Specialist Aerial and Control Unit Appliances had to be used.
Detectives have launched a public appeal for information has been launched following the incidents at Sion Mills outside Strabane.
Part of the derelict factory at the historic Mill Lane site was set alight on Friday evening.
PSNI Sergeant Allen said that the blaze cause substantial damage.
“A second fire started around 8.30pm on Saturday, June 11 further damage,” he said, adding:
“We are treating the fires as arson attack and I would advise anyone with information to contact police on 101.
“I would also like to advise members of the public this building has been condemned under no circumstances should anyone enter the building due to health and safety concerns and a high risk of serious injury.”
There have been a number of previous fires at the Mill over the past decade.