Around 35 firefighters tackled a blaze at a derelict hospital building at Gransha Park in Derry on Sunday evening.
The fire was reported at about 7.30 pm.
It was brought under control in the early hours of Monday morning.
NI Fire and Rescue Service group commander Alastair McConville said operations were “hampered” by hazardous materials onsite.
At the time, police advised people nearby to keep their doors and windows shut.
Four fire engines were at the scene along with an aerial appliance, a water tanker and a command support unit.
