NOW: French official says "both towers of the cathedral are safe" and the fire is cooling
From CNN's Antoine Crouin
Both towers of the Notre Dame Cathedral “are safe,” said Laurent Nunez, secretary to the interior minister, hours after a massive fire toppled its spire, burned most of its roof and threatened the centuries-old art and architecture inside.
“The fire is now weaker. We are now in a time of cooling but both towers of the cathedral are safe. We're still working to save the cathedral's work of arts," Nunez told reporters at the scene.
One firefighter has been seriously injured, said Paris Fire Brigade commander general Jean-Claude Gallet. Gallet said that two-thirds of the roof has been destroyed.
5:26 p.m. ET, April 15, 2019
Here's why Notre Dame Cathedral is so important to Catholics worldwide
From CNN's Daniel Burke
This week is Holy Week, when millions of Western Christians mark the death and resurrection of Jesus. Under normal circumstances, Notre Dame Cathedral in Paris would have been preparing to display its holy relics to the faithful on Good Friday.
But as fire engulfed the sacred site on Monday, Catholics across the world reacted in horror and disbelief, particularly when the cathedral's iconic spire toppled amid the flames.
For generations, Notre Dame has been a place of pilgrimage and prayer, and, even as religion in France has declined for decades, it remained the beating heart of French Catholicism, open every day for Mass.
About the cathedral: Notre Dame is not a parish church, meaning that it does not have a regular body of worshippers who "belong" to the church. But it is still the home church of Paris Archbishop Michel Aupetit, and draws Catholics for vespers (evening prayers), Masses and the Sacrament of Reconciliation, also known as Penance.
And every year during Holy Week, Notre Dame unveils some of the most coveted relics in Christendom. Among them is the Holy Crown, believed by many to be from the crown of thorns placed on the head of Jesus. Catholics have prayed with the Holy Crown for more than 16 centuries, according to the cathedral.
Notre Dame counts among its treasures two other relics connected to Holy Week: a fragment of the Wood of the Cross, believed by many to be a part of the "true cross" on which Jesus was crucified; and one of the nails that the Romans used to crucify Jesus.
Obama: We mourn when history is lost -- but we also rebuild
Former US President Barack Obama just tweeted about the destructive fire at Notre Dame Cathedral in Paris.
"Notre Dame is one of the world’s great treasures, and we’re thinking of the people of France in your time of grief. It’s in our nature to mourn when we see history lost -- but it’s also in our nature to rebuild for tomorrow, as strong as we can," he said.
4:25 p.m. ET, April 15, 2019
UN chief: "Horrified by the pictures coming from Paris"
UN Secretary-General António Guterres said he is "horrified" to see the images coming out of Paris.
"Horrified by the pictures coming from Paris with the fire engulfing Notre Dame Cathedral — a unique example of world heritage that has stood tall since the 14th century," Guterres said in a tweet. "My thoughts are with the people and government of France."
5:27 p.m. ET, April 15, 2019
Next hour and a half will be crucial to saving Notre Dame, firefighter says
From CNN's Martin Goillandeau
The next hour and a half will be crucial to efforts to save what remains of the Notre Dame Cathedral, said Jean-Claude Gallet, commander general of the Paris Fire Brigade.
“There’s a risk that the great bells fall. If the bells fall, it’s the tower that collapses. There are firefighters inside and outside. The next hour and a half will be crucial,” he told reporters on the scene.
“We need to win this battle and block the spreading of the flames. The most efficient action is from the inside. We are not sure if we will be able to stop the spreading of the flames to the North Tower," he said.
He said the initial call to emergency services notified authorities of a fire in the attic of the cathedral, although the cause of the blaze is unknown.
“We are evacuating the most precious artwork that is being sheltered,” Gallet said.
4:07 p.m. ET, April 15, 2019
What we know about Notre Dame and the fire that downed its iconic spire
From CNN's Eric Levenson and Melissa Bell
The spire atop the iconic Notre Dame cathedral in Paris collapsed Monday evening during a massive blaze.
The flaming spire, which was surrounded by scaffolding, leaned to one side and then collapsed onto the burning roof. The fire rapidly spread and took over the iconic cathedral.
The collapse drew gasps from a crowd watching nearby. They were joined by somber people who had come to recognize the landmark as a symbol of Paris.
Here's what we know about Notre Dame:
The cathedral: Notre Dame's foundation stone was laid in 1163 by Pope Alexander III, and the cathedral was completed in the 13th century. Today, with its towers, spire, flying buttresses and stained glass, Notre Dame is considered a feat of architecture, as well as a major religious and cultural symbol of France.
The central spire: It was built in the 19th century amid a broad restoration effort, partly buoyed by the success of Victor Hugo's novel "The Hunchback of Notre-Dame" in 1831.
The cathedral draws millions of visitors: Located in Île de la Cité, a small island in the middle of the city, the cathedral is one of Paris' most popular attractions, drawing an estimated 13 million visitors a year.
3:56 p.m. ET, April 15, 2019
The Vatican expresses "shock and sadness"
From CNN's Livia Borghese
The Vatican has released the following statement in response to the Notre Dame fire:
“The Holy See has learned with shock and sadness the news of the terrible fire that has devastated the Cathedral of Notre Dame de Paris, symbol of Christianity, in France and in the world. We express our closeness to the French Catholic and to the people of Paris. We pray for the fire fighters and for all those who are doing everything possible to face this dramatic situation.”
5:27 p.m. ET, April 15, 2019
Theresa May: "My thoughts are with the people of France"
UK Prime Minister Theresa May just tweeted about the fire at the Notre Dame Cathedral:
3:52 p.m. ET, April 15, 2019
French government agency responds to Trump's suggestion of using "flying water tankers": That could collapse the "entire structure"
From CNN's Martin Goillandeau
The French civil security agency, Sécurité Civile, has responded to US President Donald Trump’s suggestion that “flying water tankers” should be used to fight the Notre Dame fire.
The agency tweeted, “The drop of water by air on this type of building could indeed result in the collapse of the entire structure.”
“The weight of the water and the intensity of the drop at low altitude could indeed weaken the structure of Notre-Dame and result in collateral damage to the buildings in the vicinity,” it said.
The civil security agency is part of the Interior Ministry.
Earlier today, Trump had said on Twitter, “Perhaps flying water tankers could be used to put it out. Must act quickly!”