PARIS — Experts are assessing the blackened shell of Paris’ iconic Notre Dame cathedral to establish next steps to save what remains after a devastating fire destroyed much of the almost 900-year-old building.

With the fire that broke out Monday night and quickly consumed the cathedral now under control, attention is turning to ensuring the structural integrity of the remaining building. The fire destroyed the cathedral roof and collapsed the spire.

France’s culture minister says Notre Dame’s “most precious treasures” have been saved after the devastating fire, including the crown of thorns Catholic relic and the tunic of Saint Louis.

Officials consider the fire an accident, possibly as a result of restoration work taking place at the global architectural treasure.

Some 50 investigators are working on the probe, interviewing workers from five companies that had been hired to work on renovations to the cathedral’s roof, which was being repaired before the fire and which is where the flames first broke out.

A representative of one of the five companies which had been hired to work on renovations to the Notre Dame cathedral’s roof says “we want more than anyone for light to be shed on the origin of this drama.”

Julien le Bras’ company has 12 workers involved in the refurbishment, though none were on site at the time of the fire.

Le Bras insisted that “all the security measures were respected,” and “workers are participating in the investigation with no hesitation.”

Funding for the cathedral’s reconstruction is piling up at a spectacular rate, with two of France’s richest families together quickly pledging 300 million euros.

Businessman Francois-Henri Pinault and his billionaire father Francois Pinault said they were immediately giving 100 million euros from their company, Artemis.

A statement from Francois-Henri Pinault said: “This tragedy impacts all French people” and “everyone wants to restore life as quickly as possible to this jewel of our heritage.”

That donation was then trumped by French tycoon Bernard Arnault and his luxury goods group LVMH, which pledged 200 million euros.

LVMH called the cathedral a “symbol of France, its heritage and its unity.”

French companies Total and L’Oreal are pledging to each donate 100 million euros ($113 million) to support the reconstruction of Notre Dame cathedral.

The chief architect of Cologne cathedral says it could take decades to repair the damage caused to the Notre Dame cathedral by a massive fire.

Peter Fuessenich, who oversees all construction work for the Gothic cathedral in the German city, told broadcaster RTL on Tuesday that “it will certainly take years, perhaps even decades, until the last damage caused by this terrible fire will be completely repaired.”

Cologne cathedral was heavily damaged during World War II and work to repair it is still ongoing more than 70 years later.

The director of UNESCO, Audrey Azoulay, says expert work must be carried out immediately to protect the cathedral’s remaining structure.

She said “the first 24, 48 hours” are crucial to protecting the stone and wood structure from water damage and assessing next steps. She warned that parts of the cathedral remain “extremely fragile,” notably hundreds of tons of scaffolding set up around the cathedral spire that collapsed.

She said Notre Dame has “a particular place in the world’s collective imagination.” Notre Dame is part of a UNESCO heritage site that includes the surrounding quais and islands, and UNESCO has offered its expertise to help rebuild.

Meanwhile the French Bishops’ Conference says that the bells of all cathedrals across the country will ring on Wednesday at 6:50 p.m. local time (1850 GMT; 12:50 p.m. EST), the time when the fire started.

The Bishops’ Conference said Tuesday in a statement that this will show the solidarity of all dioceses toward Paris and that the fire at Notre Dame “is a shock that affects far beyond just the Catholics of our country.”

France has some 103 Catholic cathedrals.