Third arson attack since June leaves historic mill in ruins
The woman whose family built Herdman's Mill in Sion Mills said she was immensely saddened as she watched flames from the third major blaze at the site this year.
Firefighters battled for almost four hours to extinguish the inferno shortly after 9pm on Sunday.
It is being treated as arson, and follows two previous arson attacks in June.
Celia Ferguson (nee Herdman) from the Sion Mills Buildings Preservation Trust said while many historically significant documents had been removed for safekeeping, a magnificent wooden staircase in the building was burned to cinders.
She said: "I actually couldn't believe my ears when I got the call on Sunday night about this fire - but I had to believe my eyes when I arrived at the Mill.
"It is incredible that once again so soon after June someone set the mill on fire.
"Sadly, this time the wonderful cedarwood staircase that escaped the fire last time has been completely destroyed, but the archive material that was being stored there has been removed."
The Fire Service said the blaze had left the building unsafe to enter.