Detectives today began interviewing specialist restorers directly linked to the fire that destroyed large parts of Notre Dame Cathedral in Paris.
The blaze was finally extinguished in the early hours of Tuesday morning after eight hours, but a criminal enquiry by prosecutors in the French capital began in earnest.
Its focus is currently on Le Bras Freres (Le Bras Brothers), a company based in Jarny, in the Meurthe-et-Moselle department of north-eastern France.
Some of its 200 specialist employees were working on the wooden and lead spire of Notre Dame Cathedral that collapsed on Monday evening after the fire broke out on the roof just before 7pm.
“Many of these restorers were interviewed over night,” said a source close to the investigation that has been opened into “accidental destruction by fire”.
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“The fear is that a small fire began in the rood where they were working, and quickly spread across the building.
“The irony is that the restorers had just begun working on the spire which collapsed along with much of the roof.”
Le Bras Freres is one of the most respected companies of its kind, and last year finished a widespread restoration programme of Reims Cathedral in eastern France.
It had won the contract to restore the Notre Dame spire that was designed by the architect Eugene Viollet-le-Duc and erected in 1859.
They were due to be on site for up to four years along with Europe Scaffolding, another company which had just put 250 tons of scaffolding around Notre Dame, along with a lift that could move up and down the 300ft fire.
On Tuesday morning Julie Le Bras, the CEO of the Bras Brothers, said he had no initial comment on the enquiry.
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It will focus on the equipment being used by the restorers in a medieval cathedral where light and power sources were limited.
In such circumstances, cables and wires have to be attached to dozens of generators, and hoisted high up on to building.
Paris prosecutors have opened a preliminary investigation, and have not yet ruled out any theories for the start of the fire, which blazed from 6.50pm on Monday until 3am on Tuesday.
An unidentified fire fighter was the only human casualty of the blaze, and he was today in hospital with ‘non life threatening injuries,’ said a spokesman for the Paris emergency services.
He confirmed that when the fire broke out, the cathedral’s alarms started and the visitors inside Notre Dame were quickly evacuated.
Andre Finot, communication manager of the Cathedral: “At the moment, we don’t know why the cathedral caught fire.
“Normally, there should be no more workers on the site because they are supposed to stop between 5 pm and 5:30 pm at the latest. But we are not sure there was no one left on the site”.
Mr Finot confirmed that a 100m crane had just been deployed to take 16 copper statues depicting Apostles and Evangelists off the roof, and all had been saved.
Olivier De Chalus, the head of volunteer guides at the Cathedral, said: “The fire left the attic, and then spread extremely fast to a large part of the roof.
“The flames devoured the wooden structure that we call the Forest because of the large number of beams that are all made from single trees.”
It came as French billionaire Francois-Henri Pinault, who is married to Hollywood actress Salma Hayek, pledged 100 million euros (£86.2 million) towards the rebuilding of the Cathedral.
French President Emmanuel Macron has also vowed to rebuild Notre Dame with help from the international community.
Mr Macron said a national fundraising campaign to restore the historic building would be launched on Tuesday, and he called on the world’s “greatest talents” to help.
The French leader credited the courage and “great professionalism” of firefighters with saving Notre Dame’s spectacular Gothic facade and two landmark towers from being destroyed, saying ‘the worst has been avoided’.
‘We have been dealt a knockout blow,’ a grief-stricken Paris Archbishop Michel Aupetit said at the scene.
The blaze broke out just before 7pm local time in a roof area undergoing around £6m of renovations.
More than 400 firefighters battled the flames, which quickly spread along the roof, causing burning timbers to collapse onto the ceiling of the vault below.
At around 3am local time, Paris fire brigade chief Jean-Claude Gallet said: “We can consider that the main structure of Notre-Dame has been saved and preserved as well as the two towers.”
And a brigade spokesman added: “We will continue to watch over any residual pockets of fire and cool down the areas that are still red-hot, like the wooden beam framework.”
Paris mayor Anne Hidalgo said: “Thank you to the Paris fire brigade, policemen and the municipal agents who have made this evening a tremendous human chain to save the works of Notre Dame. The Crown of Thorns, the tunic of Saint Louis and several other major works are now in a safe place.”
Priceless works that couldn’t be moved fared less well. Firefighters told how the building’s stained-glass rose windows, high on the north, west and south faces of the cathedral, “exploded” in the heat of the inferno.
“They exploded because of the heat of the blaze,” said one, referring to the Rosette West, which was created in 1225, the Rosette North and the Rossette South, both from 1250.
The Vatican expressed its “incredulity” and “sadness” over the fire and offered prayers for firefighters tackling the blaze and solidarity with the French people.
In Washington, Donald Trump tweeted: “So horrible to watch the massive fire at Notre Dame Cathedral in Paris.”
The Archbishop of Canterbury Justin Welby sent the best wishes of the Anglican church to people at the scene.
“Tonight we pray for the firefighters tackling the tragic Notre Dame fire – and for everyone in France and beyond who watches and weeps for this beautiful, sacred place where millions have met with Jesus Christ,” he said.
And British Prime Minister Theresa May added: “My thoughts are with the people of France tonight and with the emergency services who are fighting the terrible blaze.'”