Workers questioned over Notre-Dame inferno as donations pour in

As firefighters put out the last smouldering embers, a host of French billionaires and companies stepped forward with offers of cash worth around 600 million euros (USD 680 million) to remake the iconic structure.

Most of the roof has been destroyed, the steeple has collapsed and an unknown number of artifacts and paintings have been lost. The main organ, which had close to 8,000 pipes, has also suffered damage.

But the walls, bell towers and the most famous circular stained-glass windows at France's most visited tourist attraction remain intact, leading the vicar general Philippe Marsset to call it "more than miraculous".

"We're all just dumbfounded. It's more than miraculous, it's heroic," Marsset said, who paid tribute to the more than 400 firefighters who toiled through the night.

Junior interior minister Laurent Nunez told reporters at the scene that work to secure the structure would be take place into Thursday which would enable firefighters to go inside to remove any remaining artifacts and artworks.

Though "some weaknesses" in the 850-year-old structure had been identified, overall it is "holding up OK", he added.

The Paris fire service announced that the last remnants of the blaze were extinguished Tuesday 15 hours after the fire broke out.

Ongoing renovation work on the steeple, where workers were replacing its lead covering, is widely suspected to have caused the inferno after the blaze broke out in an area under scaffolding.

Investigators interviewed witnesses overnight and began speaking to the employees of five different construction companies which were working on the monument, said public prosecutor Remy Heitz.

"Nothing indicates this was a deliberate act," Heitz told reporters, adding that 50 investigators had been assigned to what he expected to be a "long and complex" case.

The architect in charge of the renovation project slated to last until 2022 said that no workers were on the site when the flames first appeared shortly before 7 pm (1700 GMT) on Monday.

French President Emmanuel Macron struck a defiant tone on Monday night as he visited the scene with his wife Brigitte, telling reporters: "We will rebuild Notre-Dame because it is what the French expect." He described the cathedral as the "epicentre of our life".

A public appeal for funds drew immediate support from French billionaires and other private donors as well countries including Germany, Italy and Russia which offered their expertise.

French billionaire Bernard Arnault and his LVMH luxury conglomerate, rival high-end designer goods group Kering, Total oil company and cosmetics giant L'Oreal each pledged 100 million euros or more.

The privately run French Heritage Foundation launched a call for donations on its website  -- www.fondation-patrimoine.org -- and several pages were also set up on the Leetchi fundraising portal.

Specialised craftsmen and rare materials are expected to be needed to restore Notre-Dame, which is seen as an emblem of France and, more broadly, a symbol of Western civilisation which has survived revolutions and war.

"We have everything to be able to rebuild it in exactly the same way," Culture Minister Franck Riester told France Inter, adding that the government was looking at ways to encourage donations.

Thousands of Parisians and tourists watched in horror from nearby streets on Monday as flames engulfed the building and rescuers tried to save as much as they could of the cathedral's treasures built up over centuries.

Many more travelled Tuesday to the banks of the river Seine to gaze up at where the roof and steeple once stood.

A firefighter suffered injuries during the blaze, which at one point threatened to bring down one of the two monumental towers on the western facade of the cathedral that is visited by around 13 million tourists each year.

The Holy Crown of Thorns, believed to have been worn by Jesus at his crucifixion, was saved by firefighters, as was a sacred tunic worn by 13th-century French king Louis IX.

Rescuers formed a human chain at the site of the disaster on Monday night to bring out as many artifacts as possible which have been stocked temporarily at the Paris town hall.

Some damaged paintings will be sent to the Louvre museum, a world centre for restoration.

The sense of shock at the damage was palpable in France and stirred reactions from governments across the world.

In a rare statement, Britain's Queen Elizabeth II said she was "deeply saddened", while Pope Francis sent his hopes that Notre-Dame "may once again become, thanks to reconstruction work and the mobilisation of all, a jewel in the heart of the city."

The cathedral has figured as a central character through the ups and downs of French history since construction began in mid-12th century -- and has featured in countless books and movies.

Bells at cathedrals across France were set to ring out on Wednesday night at 6:50 pm (1650 GMT) in tribute.