A fire service manager said a blaze at a former leisure centre near Norwich could have been started deliberately.
Crews will remain on scene for the majority of Sunday following a large fire at the former Oasis Sports and Leisure Club.
The blaze in Pound Lane in Thorpe St Andrew was attended by 10 fire engines, which were called at around 6.30pm on Saturday, April 9.
Overnight, a single crew remained on scene to monitor the building.
Simon Mason, group manager with Norfolk Fire and Rescue Service, said: "There's no indication of what started the fire, but we've had previous arson incidents at the site.
"There's no gas and electric so, at this moment in time, we're treating it as a deliberate fire. But whether we get into a full fire investigation will become apparent later on today.
"On arrival, we had flames through the centre of the roof and the fire seemed contained in the roof space initially.
"Our plan from the offset was to get our aerial appliance in and over the top and taking the tiles off to make sure we had good access and were able to handle the blaze.
"We're going to put the aerial appliance back over the top of the building today. If everything looks cold and extinguished, that will be the end of our operation. If not, we'll keep monitoring it over the course of today."
He explained that the large presence - which included aerial ladder platform, control unit armed with a drone and a water carrier equipment - was due to the size of the building so enough resource was on hand to contain the fire.
Ten lengths of 20 metre hose was used to get water from a nearby hydrant to the Oasis building.
Melanie Hambidge lived in the house next to the leisure centre until 2019 before it was demolished.
Ms Hambidge, who now lives in Dussindale, said: "Kids have been breaking in there every night.
"My friend has been calling the police to say there have been kids on the roof. I reckon it was deliberate, it was a massive blaze.
"I'm not surprised that this has happened.
"I'm worried about those who live nearby. It needed to be kept secure. It's a real shame."
A nearby neighbour, whose Gunner Close home backs onto the property, said:
"I was in the shower when it started last night. I could smell smoke and came out when I heard all the commotion."
"It's a real worry and cause for concern.
"Looking at all this debris here, all someone's got to do is light a match and it'll go up."
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