Towers Spire Transept The fire was first visible in this area of the cathedral in images taken Monday evening. Within an hour, the fire brought down the iconic spire and continued to spread along the roof. Later, the fire burned through most of the wooden roof.

In just over an hour, a fire spread through the wooden attic of the Notre-Dame Cathedral in Paris and toppled its 300-foot spire on Monday.

Around 6:30 p.m., Paris time, smoke began to pour out of the cathedral’s roof, near scaffolding that had been put up over the last few months to conduct renovations on the spire.

The fire started in the cathedral’s attic, said Jean-Claude Gallet, commander of the Paris firefighters.

The attic is an oddly shaped space, seldom visited, that lies above the soaring stone arches visible from the floor of old European cathedrals. Diagrams of Notre-Dame and official descriptions of the space indicate that it is crisscrossed by giant, timber trusses that add structural integrity to the cathedral.

Attic Wood Stone Attic Wood Stone Attic Wood Stone

“Once these massive timber structures start to burn, they almost never can be stopped,” said Jonathan Barnett, an international fire safety authority at Basic Expert in Australia. “We focus on their masonry walls and forget all the massive timber within.”

Each beam of the cathedral attic’s wooden frame is made from an individual tree — more than 1,300 in total — giving it the nickname “the forest.”

Notre-Dame De Paris/Maurice de Sully association

A dry space like the attic tends to accumulate dust and debris, making it even more flammable. “That’s where your risk really is,” said Andrew Tremlett, the Dean of Durham Cathedral in the United Kingdom, which has a similar wooden vaulted roof like Notre-Dame’s.

After taking over the attic, the blaze quickly spread across the roof and engulfed the spire, a wood-frame structure covered in lead.

Thibault Camus/Associated Press

Within minutes, the spire collapsed, toppling to the side before breaking off.

Ian Langsdon/EPA, via Shutterstock

Fire safety experts said houses of worship can pose special problems for firefighters. The cathedral’s wood roof, for instance, is made of a flammable material and is difficult to reach.

“These cathedrals and houses of worship are built to burn. If they weren’t houses of worship, they’d be condemned,” said Vincent Dunn, a fire consultant and former New York City fire chief.

Images taken on Tuesday showed the scale of the damage inside the cathedral.

Christophe Petit Tesson/Agence France-Presse — Getty Images

Notre-Dame is located directly in the center of the city on a small island called Île de la Cité, which may have been more difficult for firefighters and emergency workers to reach. As of 8:30 p.m., Paris time on Monday, all roads on the island were closed.

Louvre Pompidou Center Paris Seine Île de la Cité Pont d'Arcole Quai de l'Hôtel de ville Notre-Dame Cathedral Quai de Montebello Pont Saint-Louis Pont de l'Archevêché Luxembourg Palace PARIS Eiffel Tower Notre-Dame Cathedral Seine 3 miles Louvre Notre-Dame Cathedral Eiffel Tower PARIS Seine Quai de l'Hôtel de ville Île de la Cité Notre-Dame Cathedral Quai de Montebello Paris Notre-Dame Cathedral Île de la Cité PARIS Notre-Dame Cathedral Paris Seine Paris Île de la Cité Notre-Dame Cathedral PARIS Seine Notre-Dame Cathedral The New York Times | Sources: OpenStreetMap; aerial image by Google.

About five hundred firefighters were battling the blaze on Monday evening, according to France’s interior ministry.

Photographs from Monday showed Paris’s firefighters using compact trucks equipped with aerials to battle the blaze from up high. These nimble trucks most likely allowed the fire department to navigate the island’s narrow streets and position its equipment close to the cathedral, surrounding the building.

Benoit Moser/Paris Fire Brigade, via Associated Press (left); Stephane de Sakutin/AFP/Getty Images

At least one boat was deployed to pump large amounts of water from the river. Large-diameter hoses, about five inches wide, were connected to water-pumping trucks, which fed water to the fire trucks. The hoses effectively created water mains, supplying a large volume of water to fight the fire.

Where Firefighting Equipment Was Deployed Approximate locations of fire trucks Rue du Cloître-Notre-Dame Notre-Dame Cathedral A fire boat pulled water from the Seine. At least two trucks were on this side. A water-pumping truck supplied the fire trucks. Seine Where Firefighting Equipment Was Deployed Approximate locations of fire trucks Notre-Dame Cathedral A fire boat pulled water from the Seine. At least two trucks were on this side. A water-pumping truck supplied the fire trucks. Seine Where Firefighting Equipment Was Deployed Approximate locations of fire trucks Notre-Dame Cathedral Seine A fire boat pulled water from the Seine. At least two trucks were on this side. A water-pumping truck supplied the fire trucks. Note: Locations are based on analysis of photographs taken on Monday. · Source: OpenStreetMap

By 11 p.m. on Monday, Gen. Jean-Claude Gallet, the Paris fire chief, said that the structure, including the two magnificent towers soaring above the skyline, had been “saved and preserved as a whole,” but that two-thirds of the roof was destroyed. The cause remained unknown.