When fire crews arrived, flames had taken hold of the fourth and fifth floors of the building.

It took appliances from Strabane, Londonderry, Newtownstewart and Castlederg three hours to bring the blaze under control.

Once firefighters left the scene the fire reignited.

Celia Ferguson is a trustee of the Sion Mills Building Preservation Trust.

For her it’s a personal tragedy.

She told UTV: “I’m watching the most important industrial heritage building in Ireland burning down. It’s just tragic.”

“My great, great grand-father built the building with his two brothers. It was designed by William Lynn who was probably the most important architect in Ireland at one time.”

“It’s a fire-proof building but it has cast iron columns. They can melt in great heat. But at the moment it’s the roof … there are big explosions in there at the moment and there are huge flames coming out of the roof.”

The cause of the blaze is now under investigation.

© UTV News