Around 50 people were moved from their homes in Newport on Sunday night while firefighters tackled a blaze at a disused swimming pool.
The fire, at the old Maindee swimming pool on the Victoria Avenue, took more than two hours to bring under control.
A Gwent Police spokesperson said the blaze was being treated as suspicious.
South Wales Fire Service said it was called to the pool twice on Sunday, with the second blaze being fought “in dangerous and arduous conditions”.
The first call was received at 1513 BST and involved rubbish on fire on the first floor, which was put out at 1722 BST.
The second call came in at 2101 BST and station manager Jeff Scrivens said: “Around forty firefighters worked to extinguish the blaze at the Maindee Swimming Pool in dangerous and arduous conditions.
“They gained entry via various methods, such as ladders and staircases and worked through the thick black smoke to deal with the situation.
“Ten firefighters in breathing apparatus entered the derelict building, and found plastic seating, rubbish and debris alight.”
People living nearby were moved to Newport leisure centre during the fire, which was put out by 2319 BST.