The president of Paris' Île-de-France region, Valérie Pécresse, has told reporters the fire at Notre-Dame Cathedral was an accident.
"This was an accident. It wasn’t intentional," she said, as reported by CNN.
Paris public prosecutor Remy Heitz said there was no evidence of arson at this stage of the investigation.
"We are favouring the theory of an accident," he said.
Queen Elizabeth has sent a message to French President Emmanuel Macron to say she was deeply saddened by the fire which engulfed the famous cathedral on Monday, and that her prayers were with all of France.
Elizabeth’s oldest son Prince Charles, the heir to the throne, said he too was “utterly heartbroken” to learn of the fire, Buckingham Palace said.
“I extend my sincere admiration to the emergency services who have risked their lives to try to save this important national monument," the Queen’s message said.
'Cause may never be determined': Australian fire expert
As the world comes to terms with the loss of an international treasure, Australian fire investigator James Munday is asking one question: 'what was different today from yesterday?' to engulf the Notre-Dame, and leave it gutted.
Australian senior fire investigator James Munday said there must be a reason why the fire started now and not before.
"I was surprised to hear of the fire itself but not too surprised to hear of the firefighting difficulties or that there was renovation work going on at the time," he said.
Investigators will now begin determining the cause of the blaze at the Notre-Dame Cathedral in Paris (AAP) Source: AAP
The cathedral's centuries-old wooden roof beams, stone exterior and soaring Gothic architecture made the blaze difficult to tackle.
The spire and most of the roof of the centuries-old cathedral have been destroyed, but the two bell towers and the main edifice were saved.
It took nine hours and about 400 firefighters to extinguish the fire.
Mr Munday from Fire Forensics said the fire took so long to contain simply because of the age of the building, praising the 400-odd firefighters who battled the blaze.
Australian fire expert James Munday says the cause of the Notre-Dame fire may never be determined. Source: LinkedIn
"Fire in heritage buildings are often difficult to fight due to the construction which is radically different from modern buildings," he said.
"There would have been large amounts of exposed, aged and dry timber with no or minimal fire resistant coverings, probably no sprinklers, and very likely difficult to access both externally and internally."
Mr Munday, who was involved in the investigation of the 1992 Windsor Castle fire in the UK, expects the investigation to be "quite protracted".
An aerial view of Windsor Castle showing the damage caused by fire in November 1992. Australian fire expert James Munday was part of the investigation squad. Source: Press Association
"It is possible that the exact origin and cause may never be determined," he said.
"On the other hand, there may be a witness who can pinpoint the start of the fire very precisely."
He said investigators will now look at the remains to try to determine the area of origin from physical evidence, such as burning patterns and differential heat indicators.
More than 400 firefighters on scene
A video released by the French Interior Ministry showed the scale of the response.
Authorities deployed about 400 firefighters, pumped water from the Seine and flew drones to survey the damage.
Pope's thoughts and prays
From the Vatican, Pope Francis said he was praying for Roman Catholics and Parisians alike after the devastating fire.
"The pope is close to France, he is praying for French Catholics and for the people of Paris in face of the terrible fire which has ravaged Notre-Dame cathedral," the head of the Vatican press office Alessandro Gisotti wrote on Twitter.
"He offers his prayers to those trying to face up to this dramatic situation," he added.
Additional reporting: AFP, Reuters
