Firefighters were called to tackle a massive fire which gutted a derelict mill in Clayton-le-Moors.
The fire service is treating the blaze, which destroyed the inside of one of the mill buildings and caused the roof to collapse, as arson.
The two-storey building has been derelict for several years and is part of the planned Clayton Triangle redevelopment.
Crews from Hyndburn, Great Harwood and Blackburn attended the former Gazelle Contract Furnishings building at around 11pm on Friday, August 26.
John Taylor, of Lancashire Fire and Rescue Service, said the incident is being treated as an arson attack.
"It was a derelict mill and we are pretty sure at this stage that it was an arson and started deliberately," he said.
"Firefighters used an aerial ladder platform to jet water down on the fire and used breathing apparatus sets to get into close proximity because of the swirling and toxic smoke.
"An inspection is taking place to determine how structurally sound the building is before an investigation can begin. We will then look for evidence to inform the police to try to find the culprits." Witness Diane Oldale, 66, of Spring Brook House sheltered accommodation, said: "I was sitting near the window at 11 o’clock and I could smell something burning.
"I pulled the curtains back but I couldn’t see the houses opposite because of the thick black smoke and I contacted the fire brigade.
"I heard six loud explosions which sounded like fireworks but bigger. I think it was something in the building.
"After the plume of smoke changed direction we could see part of the roof glowing red and then it got bigger. Then the whole thing went ‘whoosh’.
"We did wonder if we were OK because the fire and the smoke was still blowing and we didn’t know if anything else would catch fire."
A police spokesman said: "We received a call from the fire service after they found a derelict mill on fire. We spoke with a number of people in the area but they hadn’t seen anything suspicious. We are continuing to investigate."