Fire crews from half a dozen stations across Northern Ireland were called in to deal with a blaze at an empty building on the north coast.
Fifty-two firefighters were involved in tackling the blaze in the unoccupied end-of-terrace property in Portrush.
At about 9pm on Sunday night the fire brigade received a report of a fire at Bath Street in the town.
It is understood that the building which caught fire was number 15, and that it stood close to a care home.
The fire was described as well-developed by the time fire crews arrived.
Teams from Portrush were supported by others coming from Coleraine, Portstewart, Ballymoney, Ballycastle, Ballymena and Strabane.
The fire was said to have been brought under control after midnight.
However, the emergency services said fire crews looked set to remain on scene for several hours after that to ensure that the blaze was fully extinguished.
On Monday morning, police said the blaze was being treated as suspicious by the fire brigade.
Detective Sergeant Wallace, from Reactive and Organised Crime, said: “The building will be entered and examined by PSNI crime scene investigators once building control officials have determined it is safe to do so.”
He added: “I would appeal to anyone with information about this crime to please contact police by calling 101 quoting reference number 1029 16/04/17.”