PARIS, France (AFP) — French investigators probing the devastating blaze at Notre-Dame Cathedral questioned workers who were renovating the monument on Tuesday as hundreds of millions of euros were pledged to restore the historic masterpiece.

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 (US$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 7:00 pm (1700 GMT) on Monday.