PARIS (AP) — Notre Dame Cathedral would have burned to the ground in a “chain-reaction collapse” had firefighters not moved as rapidly as they did to battle the blaze racing through the beloved landmark building, a French government official said Wednesday.

The firefighters acted aggressively to protect wooden supports in the twin medieval bell towers from the flames, averting a bigger catastrophe, said José Vaz de Matos, a fire expert with France’s Culture Ministry.

“If the fire reached this wooden structure, the bell tower would have been lost,” de Matos said at a news conference. “From the moment we lose the war of the bell towers, we lose the cathedral, because it’s a chain-reaction collapse.”

Monday’s fire destroyed most of the lead roof of the 850-year-old architectural treasure and caused its spire, which was added in the 19th century, to collapse.

An initial fire alert was sounded at 6:20 p.m., as a Mass was underway in the cathedral, but no fire was found. A second alarm was sounded at 6:43 p.m., and the blaze was discovered on the roof. No one was killed in the fire, after firefighters and church officials speedily evacuated those inside.

Firefighters acted bravely and as fast as they could to save the cathedral, said senior fire official Philippe Demay, denying that there was any delay in their response.

Despite extensive damage, many of the cathedral’s treasures were saved, including Notre Dame’s famous rose windows, although they are not out of danger.

Paris Firefighters’ spokesman Lt.-Col. Gabriel Plus said that even though they are in good condition, a “threat” continues to the gables, or support walls, because of the heavy stone statues perched on top of them.

“The roof no longer holds (the gables) up. They are holding up all by themselves,” he said, adding that some statues must be removed to lessen the weight on the gables.

Scaffolding that had been erected for a renovation of the spire and roof must also be properly removed because of its weight and because it is now “crucially deformed,” he added.

The cathedral is still being monitored closely by firefighters and experts to determine how much damage the structure suffered and what needs to be dismantled to avoid collapse.

“The experts are scrutinizing the whole of the cathedral, part by part, to identify what is weakened, what will need to be dismantled or consolidated,” Plus said.

Nearly $1 billion has been pledged for the cathedral’s restoration, and French President Emmanuel Macron has vowed to finish it in five years, in time for the 2024 Olympics in Paris. “We will rebuild the cathedral to be even more beautiful, and I want it to be finished within five years,” Macron said Tuesday.

Experts, however, have questioned that ambitious timeline for such a massive operation. Even Prime Minister Edouard Philippe acknowledged Wednesday that it would be difficult.

“This is obviously an immense challenge, a historic responsibility,” Philippe said in an address.

Prominent French conservation architect Pierluigi Pericolo told Inrocks magazine it could take triple that time.

“No less than 15 years … it’s a colossal task,” said Pericolo, who worked on the restoration of the 19th-century St.-Donatien Basilica, which was badly damaged in a 2015 blaze in the French city of Nantes. He said it could take between two to five years just to check the stability of the cathedral that dominates the Paris skyline.

“It’s a fundamental step, and very complex, because it’s difficult to send workers into a monument whose vaulted ceilings are swollen with water,” Pericolo told France-Info. “The end of the fire doesn’t mean the edifice is totally saved. The stone can deteriorate when it is exposed to high temperatures and change its mineral composition and fracture inside.”

Notre Dame’s rector said he would close the cathedral for up to “five to six years,” acknowledging that “a segment” of the structure may be gravely weakened.

Pledges for the restoration have come from ordinary worshippers and wealthy magnates, including those who own L’Oreal, Chanel and Dior. Presidential cultural heritage envoy Stephane Bern told broadcaster France-Info that 880 million euros ($995 million) has been raised since the fire.

The government was gathering donations and setting up a special office to deal with big-ticket offers.

Criticism already has surfaced in France from those who say the money could be better spent elsewhere, on smaller, struggling churches or on workers.