A listed former hospital building in Coleraine has been extensively damaged in a suspected arson attack.
It took firefighters several hours to bring the blaze on Mountsandel Road under control after it started at about 20:45 GMT on Sunday.
The roof of the building has been completely destroyed.
Group Commander Max Joyce said those responsible for starting the fire had endangered the lives of the fire-fighters involved.
"We have had several fires in this building in the past few weeks and months," he said.
"By their reckless actions, they could lead to the injury of firefighters and much worse."
DUP councillor Trevor Clarke said he was saddened at the damage of one of the town's historic buildings.
"What started out in the 1840s as the entrance block at the Union Workhouse building later became synonymous with Coleraine's Bannview hospital, and is now reduced to ruins," he said.
"Many people in the local community will have memories of significant events in their lives there."
The police have appealed for anyone with information about the fire to come forward.