Firefighters spent six hours overnight at the scene of a blaze which badly damaged a building at the former Ferodo car parts factory in Gwynedd.
A special decontamination shower was brought in for firefighters to use due to the danger of asbestos on the site near Caernarfon.
An investigation is due to be carried out on Friday to determine the cause.
Controversial plans to build a prison on the site were withdrawn in September last year.
A spokesperson for the North Wales Fire and Rescue Service said firefighters were alerted to the incident at the disused factory on Bangor Road, Caernarfon at 2157 BST on Thursday.
Five appliances from Caernarfon, Bangor, Menai Bridge and Llanberis together with and incident command unit from Rhyl were called to the scene.
The fire was isolated to a section of the unoccupied two-storey office block, which was well alight.