An investigation into the cause of a blaze which seriously damaged a police firearms training centre is not likely to start for several days.
The fire at the £16m centre at Black Rock, Portishead, was spotted early on Tuesday by a passing police officer.
Avon Fire and Rescue said until the fire was completely out and the building had been made safe no-one would be allowed inside.
Firefighters are still finding hotspots, a spokesperson said.
On Thursday, specialist equipment was brought in to cut through thick walls to allow access to areas underneath the collapsed roof of the building which is still under construction.
"Where the roof has fallen this has provided insulation and allowed the heat and fire to fester underneath.
Angry Foxes Cell
"Once the fire is completely out then building assessors and structural engineers will have to make the building safe before we can let our investigators in," a spokesperson added.
On Wednesday, an anarchist group claimed responsibility for the fire.
A group called "Angry Foxes Cell" claimed in a post on the Bristol Indymedia website that its members started the fire on Monday night.
The group said it "puts smiles on our faces to realise how easy it was" to target the building.
Police said it was too early to decide "cause or responsibility".
The training centre is to be used by the Avon and Somerset, Gloucestershire and Wiltshire police forces.
The facility, which will contain two indoor firing ranges as well as classrooms, had been due to open in January.
It is part of a Private Finance Initiative (PFI) deal, worth more than £100m, which will also involve police stations and other operational facilities being built in Patchway, Keynsham and Bridgwater.