Paris CNN  — 

Investigators sifting through the fire-ravaged Notre Dame Cathedral in Paris warned of potential weaknesses in the remains of the building Tuesday, as experts said it could take months just to identify the scale of the reconstruction task.

While the building was assessed to be structurally sound overall, pictures showed gaping holes in the roof where the ancient vaulted ceiling had collapsed into the nave.

French Junior Interior Minister Laurent Nunez said experts had identified “some vulnerabilities” in the building, particularly in the roof, much of which has been destroyed, and part of the north transept.

Some nearby residential properties were evacuated as authorities assess the scale of the damage from the blaze that engulfed the medieval landmark Monday night. Firefighters said it took more than nine hours before the flames were brought under control.

It was not immediately known what caused the fire, which began on Monday evening in the attic, according to the Paris Fire Brigade. The cathedral, and its central spire – an area where the fire was first visible to onlookers – had been surrounded by scaffolding amid construction work.

Scores of priceless artifacts were rescued from the flames, and will be taken to the Louvre museum for safekeeping.

Paris prosecutors have opened an investigation, but on Tuesday Prosecutor Rémy Heitz said “nothing shows that it’s an intentional act,” and that the start of the fire was “likely accidental.” Heitz also said that the investigation, which is in its early stages, is expected to be a long process.

Lt. Col.Gabriel Plus, a spokesman for the fire brigade, said internal security officers first heard the fire alarm at 6:20 p.m. (12:20 p.m. ET) and began to evacuate the cathedral, even though they didn’t see any sign of fire.

François-Xavier Lochet, a 70-year-old worshiper, said the alarm interrupted a Mass. Lochet said he noticed visitors began to get ushered out but those who had gathered for the service remained and the priest resumed. He said Mass continued until a police officer went up and told the priest, “This is no joke. You’ve got to get out.”

Twenty-three minutes later, the fire alarm rang again, Plus said. That’s when the cathedral’s security officers noticed the flames.

Firefighters were unable to save the cathedral’s 19th century spire, which burned to a blackened shell before toppling as thousands of Parisians who had gathered in the streets watched in horror. Two police officers and a firefighter suffered minor injuries, the brigade said.

A day after flames engulfed the cathedral, hundreds of people marched on Paris’ streets Tuesday night, praying and singing hymns together.

People attend a vigil near the Notre Dame cathedral in Paris on Tuesday.

Pledge to rebuild

French President Emmanuel Macron said he wants the cathedral to be rebuilt in five years.

“We will rebuild Notre Dame even more beautiful and I want that to be done in the next five years,” Macron said in a televised address to the nation on Tuesday night.

“Throughout our history, we have built towns, ports, churches. Many have been burnt due to revolutions, wars, due to mankind’s mistakes. Each time we have rebuilt them.

“The fire of Notre Dame reminds us that our story never ends. And that we will always have challenges to overcome. What we believe to be indestructible can also be touched,” Macron added.

At the scene of the fire on Monday night, Macron had vowed to reconstruct “because that is what the French expect, that is what our history merits, and this is our deep destiny.”

But Frédéric Létoffé, the co-president of a group of French companies specializing in restoring historic buildings, cautioned that full restoration of the Gothic cathedral could take significantly longer – around “10 to 15 years.”