PARIS (AP) — French President Emmanuel Macron pledged Tuesday to rebuild Paris’ beloved Notre Dame Cathedral “even more beautifully” after a raging fire destroyed its spire and its roof but spared most of the structure, including the church’s twin medieval bell towers.

Macron said he wanted to see the renovation of the beloved Roman Catholic architectural landmark completed within five years.

“We have so much to rebuild,” Macron said in a televised address to the nation. “We will rebuild Notre Dame Cathedral even more beautifully. We can do it, and once again, we will mobilize (to do so).”

Authorities consider the fire an accident, possibly as a result of restoration work at the global architectural treasure that survived almost 900 years of tumultuous French history but was devastated in the blaze on the second day of Holy Week.

Paris prosecutor Remy Heitz said the inquiry into the fire would be “long and complex.” Fifty investigators were working on it and would interview workers from five companies hired for the renovations to the cathedral’s roof, where the flames first broke out.

Heitz said an initial fire alert was sounded at 6:20 p.m. Monday but no fire was found. The second alert was sounded at 6:43 p.m., and the blaze was discovered on the roof.

Investigators have already questioned nearly 30 people, said a Paris judicial police official, speaking on condition of anonymity in order to comment on an ongoing probe.

“Notre Dame has survived the revolutionary history of France, and this happened during building works,” said influential former Culture Minister Jack Lang.

News that the fire was probably accidental has done nothing to ease the national mourning for the symbol of national pride. Hundreds of people gathered for a vigil prayer across the Seine from the cathedral, singing and listening to music by a string quartet.

Interior Minister Christophe Castaner said there were still some risks to the structure and that it was “under permanent surveillance because it can still budge.”

A plan to safeguard the masterpieces and relics was quickly put into action after the fire broke out.

The Crown of Thorns, regarded as Notre Dame’s most sacred relic, was among the treasures quickly transported after the fire broke out, said Deputy Mayor Emmanuel Gregoire. Brought to Paris by King Louis IX in the 13th century, it is purported to have been pressed onto Christ’s head during the crucifixion. Also saved was the tunic of St. Louis, a long, shirt-like garment from the 13th century, said Culture Minister Franck Riester.

The cathedral’s famous 18th century organ that boasts more than 8,000 pipes also survived.

“The works of art, the most precious treasures were secured last night,” Riester told reporters, thanking teams from city hall, the culture ministry, firefighters and the bishopric who worked to save the items.

Some of the works were being transferred from City Hall to the Louvre, where they will be dehumidified, protected and eventually restored. The minister said the cathedral’s greatest paintings would be removed starting Friday. “We assume they have not been damaged by the fire but there will eventually be damage from the smoke,” he added.

The 3-meter-tall copper statues that looked over Paris from Notre Dame’s 96-meter peak already had been removed from the roof days ago and sent to southwestern France as part of a 6 million-euro ($6.8 million) renovation on the spire and its 250 tons of lead.