Progress dealing with smoldering areas of debris has been hampered by large sections of collapsed roof and rubble.

The facility at Black Rock Quarry was at the fit-out stage of construction and forms part of a £127m PFI deal to build a new operational base in Bridgewater, two police stations and the tri-force firearms training centre.

Firefighters and police forensic teams hope to be able to start their investigation early this week once the building has been made completely safe.

An anarchist group calling its self the Angry Foxes Cell claimed on a website last week that its members were responsible for torching the site.

More than half the building has been completely destroyed after extensive sections of the roof collapsed.

A spokesman for Avon and Somerset fire brigade said: “There are still a number of hotspots burning under the area where the roof has collapsed, although these are fewer than yesterday.

“Smoke levels in the area have been significantly reduced and although there is still some smoke, it is far less than in previous days.

“Work to gain access to the area under the collapsed roof using cutting equipment will continue today.

“This is an ongoing incident and the fire investigation will not begin until the fire is completely extinguished and it is deemed safe to enter the building.”