Investigations are under way into blazes at two derelict buildings in South Yorkshire on the same night.
Firefighters spent more than three hours at the first blaze, which was reported to emergency services after flames were spotted at a disused office block in Millhouse Green, Barnsley.
The alarm was raised at 9.50pm, and when fire crews arrived at Manchester Road they found a disused former office block well alight, causing the roof to collapse.
An industrial unit next door was also damaged in the blaze.
Crews from Penistone, Barnsley and Tankersley dealt with the blaze and remained at the scene until around 1pm.
It is not yet known what started the fire.
On the same night, firefighters were also called to a derelict building found burning on Sylvester Street in Sheffield city centre.
Crews were alerted at 2.20am and spent about an hour at the scene.
Rubbish and debris inside the building had been deliberately set on fire.
n Anyone with information about those responsible should call South Yorkshire Police on 101 or Crimestoppers on 0800 555111.