Fifteen firefighters were called to put out a suspicious blaze at a house in Burnley.
Crews from Burnley and Nelson initially believed someone was in the burning property, in Burns Street, but a search revealed no one was inside.
Firefighters said flames were coming out of the front downstairs window while thick black smoke was billowing from upstairs when they arrived around 6-30pm on Saturday.
They used breathing apparatus and hose reels to put out the blaze and thermal imaging cameras to help them search the house.
It took crews 10 minutes to extinguish the flames, though they spent two hours at the scene to make sure the fire was completely out.
Watch manager Steve Harrison, from Burnley fire station, said the house, particularly the living room, was severely damaged.
“The cause of the fire is not yet clear, but it is being treated as suspicious and a joint investigation involving the police and the fire service is under way.
“There were no working smoke alarms in this property, and although the property was unoccupied at the time of the fire, smoke alarms may have alerted passers by or neighbours before the fire had taken hold, allowing for an earlier fire service attendance,” he said.