What we know about the Notre Dame fire so far

Paris(CNN) A massive blaze at Notre Dame Cathedral in Paris devastated large parts of the 850-year-old church on Monday evening. Here's what we know so far.

How did the blaze unfold?

• At 6:20 p.m. local time (12 p.m. ET) on Monday, a fire alarm rang out, interrupting mass. Security guards started to evacuate the cathedral even though they did not see any sign of a fire, a spokesman for the Paris fire brigade told CNN.
• François-Xavier Lochet, a 70-year-old worshiper, said the congregation had just begun the Universal Prayer when the siren sounded. He said visitors were ushered out but those gathered for mass remained in place. Lochet 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, at 6:43 p.m. (12:43 p.m. ET), a second alarm blared and the fire was visible at that point, the fire department said.
• Around 400 firefighters were deployed to the scene but were delayed slightly by rush hour traffic.
• Police in Paris confirmed a fire had taken hold at Notre Dame and asked the public to avoid the area in a tweet sent at 7:20 p.m. (1:20 p.m. ET). A minute later, Paris Mayor Anne Hidalgo posted a tweet saying "a terrible fire is underway" with photographs taken outside the church: one of smoke billowing from near the spire and over the towers, and a second showing the fire department responders working at the scene.

• Shortly before 8 p.m. (2 p.m. ET), the cathedral's famed spire burned to a blackened shell before finally toppling, as thousands of Parisians who had gathered in the streets watched in horror. 
• Around 11 p.m. (5 p.m. ET), French President Emmanuel Macron announced: "The worst has been avoided. The façade and the two main towers did not collapse."

• At 9:30 a.m. (3:30 a.m. ET) on Tuesday, French firefighters announced on Twitter that after nine hours of intense battle, the flames had finally been extinguished. Two policemen and a firefighter were injured, the tweet added.

What's been damaged?

• An investigation into the fire was opened on Monday night by Paris prosecutors.
• Prosecutor Rémy Heitz told CNN on Tuesday that the fire was "likely accidental" and that "nothing shows that it's an intentional act."
• While the main structure has been saved, firefighters were unable to save the central spire, which had been added during a restoration project in the 19th century.
• The majority of the 13th-century oak roof, called "the forest" because it required a forest of trees to build it, was also largely destroyed.
• The wooden latticework will be difficult to replace given that there are no trees in the country large enough to replace the ancient beechwood beams consumed by the fire, Bertrand de Feydeau, vice president of the French Heritage Foundation, told CNN.

What will survive?

• The cathedral of Notre Dame in Paris is home to scores of priceless artifacts, artwork and relics collected over the centuries.
• The cathedral's iconic bell towers -- immortalized in Victor Hugo's tale "The Hunchback of Notre Dame" -- survived, along with the cathedral's elaborate stonework façade.

• The Crown of Thorns, believed to be a relic of the passion of Christ, and the Tunic of Saint Louis were among the venerated artifacts saved, according to Hidalgo.
• The church's irreplaceable rose windows and organ are in good condition, a city official said Tuesday.

• After being rescued from the flames, some of the artifacts have been taken to the Louvre museum for safekeeping.

What happens next?

• Teams of experts are continuing to survey the structural integrity of the building.
• Workers will attempt to preserve the infrastructure of Notre Dame over the next 48 hours, French Interior Minister Christophe Castaner told reporters outside the cathedral on Tuesday.
• There is still structural risk to the building, which is being surveyed, he said.
• Castaner added that it will take an enormous amount of time to reconstruct the building, saying that the process would take "days (and) months."
• Macron has pledged to rebuild the cathedral and said he would launch an international fundraising campaign to help finance those efforts.
• Donors have already pledged hundreds of millions. The Île-de-France region will unlock an emergency fund of €10 million ($11.3 million), and Paris City Hall said it would give €50 million ($56.45 million) towards rebuilding efforts.

This report has been updated to reflect developments released by French officials on Tuesday.