Officers investigating a fire at a derelict leisure centre in Thorpe St Andrew over the weekend believe it was started by someone inside the building.
The blaze at the Oasis Sport and Leisure Club in Pound Lane was attended by 10 fire engines, which were called at around 6.30pm on Saturday, April 9.
Crews remained at the scene on Sunday to continue investigation work and to make the scene safe.
A police spokeswoman said: "The fire caused significant damage to the derelict building and fire investigators believe the blaze was started by someone inside, although it is unknown how.
"Officers will be carrying out enquiries and speaking to witnesses to establish the circumstances surrounding the fire and what caused it."
Locals living next to the leisure centre have said that youngsters have been spotted breaking into the building "every night" and that they had been seen climbing on the roof.
Anyone with information should contact PC Allan Jennings at Sprowston Police Station on 101 or call Crimestoppers anonymously on 0800 555 111.
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