A fire which broke out in the roof of a Bristol University building started accidentally, investigators have found.
Around 50 firefighters tackled the blaze in the Grade II listed Fry Building on Saturday.
A number of study areas were evacuated, including the Wills Memorial Library, although exams planned for Monday went ahead as normal.
Avon Fire and Rescue said the fire was a result of ongoing works.
Group manager Steve Quinton said: "We know that refurbishment works were being carried out in the roof on Saturday afternoon which we believe has led to the accidental fire starting.
"The extent of the damage at the Fry Building means we will never be able to identify exactly where the fire started but we are satisfied it started accidentally as a result of the works taking place that day."
He said the blaze caused extensive damage to the fifth floor and roof but as the fire service was alerted quickly it was able to prevent the flames from spreading to other historic buildings nearby.
Mr Quinton said: "I would also like to recognise the assistance we received on the night from the police and ambulance services, Bristol City Council, the University of Bristol and the many members of the public who sent us messages of support."
The Fry Building is currently undergoing a £33m redevelopment to become the university's new maths department.
The fire could delay its scheduled opening.