North Wales fire service says it is "90% certain" vandals or squatters started a fire at a hall described as one of the most important buildings in Wales.
The fire began just after midnight (picture by Robert Jackson)
The blaze broke out just after midnight on Thursday in the Grade 1 listed Hafodunos Hall in north Wales.
Developers who wanted to turn it into an hotel have revealed that they could not get insurance because of the risk.
The fire service will now pass a report to police.
Earlier on Thursday, Welsh Culture Minister and local AM Alun Pugh had asked North Wales Police to hold a "full forensic investigation".
Mr Pugh, the Labour AM for Clwyd West, said he had formally asked the chief constable, Richard Brunstrom, to establish the cause of "this dreadful fire."
The fire took until Thursday afternoon to bring under control, with eight fire appliances there at the peak of the blaze at the three-storey hall in Llangernyw, near Abergele. Nobody was injured.
Plans to turn the 19th Century hall into a hotel had previously been unveiled by developers SFJ Ltd.
SFJ managing director John Rochelle said: "I am totally devastated.
"The whole application hinges on the restoration of the hall, and now it looks like the hall may be beyond economic repair.
"This could mean the loss of one of the region's finest buildings, which would be a tragedy. I have no information yet on how the fire started but we are talking to the fire service about the precise cause of the blaze.
Before the fire: the hall has been used as a school and college
"Hafodunos Hall was not insured - we could not get insurance because the risk was too great.
"Obviously we will have to review our planning application. The application has been going on now for three years and I've repeatedly expressed my concerns to the council about the possibility of a fire."
Hafodunos Hall was designed by architect Sir George Gilbert Scott in the 1860s.
He was commissioned by the Sandbach family to build it on the site of a manor house which dated back to the reign of Charles I.
The architect's other works include London's St Pancras Hotel, and the Albert Memorial, in Kensington Gardens.
Aerial shots of damage at Hafodunos Hall
Mr Rochelle has previously called the hall "one of the most important buildings in Wales" and praised its "stunning design and craftsmanship" as superb.
He had also predicted that while restoration would cost millions of pounds, it would bring the building "back to life" and create about 30 jobs.
The grounds of the hall were designed by Kew Gardens director William Hooker.
The hall has had recent spells as a boarding school and an accountancy college, but fell into decline over the past decade.
The police are appealing for any witnesses to the fire to come forward.