PARIS — Inside the cavernous cathedral of Notre-Dame in Paris, the last Mass of the day was underway on Monday of Holy Week when the first fire alarm went off. It was 6:20 p.m., 25 minutes before the heavy wooden doors were scheduled to close to visitors for the day.
Worshipers, sightseers and staff were ushered out, and someone went up to check the most vulnerable part of the medieval structure — the attic, a lattice of ancient wooden beams known as “the forest” — but no fire was found, Rémy Heitz, the Paris prosecutor, said on Tuesday.
At 6:43 p.m., another alarm rang. It was just 23 minutes later, but when they returned to the attic, it was clear they had a major problem: It was on fire. Soon much of the roof and the delicate spire rising high above it were also engulfed in flames, fanned by a strong breeze.
Exactly how the fire broke out is now the subject of an intensive investigation by the French authorities, who are so far treating the disaster as an accident.
Much remains to be learned. But already it is emerging that Notre-Dame, irreplaceable as it is to France’s heritage, lacked the fundamental fire-prevention safeguards that are required in more modern structures and have been grafted onto other ancient cathedrals elsewhere in Europe.
Some of those elements, like firewalls or a sprinkler system, were absent by choice — so as not to alter the landmark’s design or to introduce electrical wiring deemed a greater risk amid the timbers that supported Notre-Dame’s ornate lead roof.
“There had been a systematic refusal to install anything electrical” within “the forest” because of the risk, said Pierre Housieaux, president of the Paris Historical Association. “Everyone knew that the attic was the most fragile part.”
Inevitably, some of those decisions are being called into question in the aftermath of a calamity that scarred a jewel of Gothic architecture precious to all the world, and left a gaping wound in the heart of Paris.
“The fire-detection system existed, not the fire compartments,” said Jacques Chanut, president of the French Building Federation, referring to the structures commonly used elsewhere to contain blazes. “That’s the typical example of something we are going to have to think about tomorrow.”
However it began, the fire galloped unimpeded across the attic and roof, and up the wood structure inside the spire. The flaming spire stood out over the city like a Roman candle until it toppled over, crashing through the ceiling and into the cathedral.
As flaming pieces of the upper structure fell to the cathedral floor, some of the interior furnishings also caught fire.
Firefighters deployed a robot equipped with tank-type treads and a camera to pull hoses into the cathedral and aim water at the flames. Firefighters also used aerial drones to get a view, including thermal imaging, into the inferno.
The firefighters brought out irreplaceable artifacts, including candelabras, statues, furnishings and religious relics like a linen fabric associated with Saint Louis, and what tradition holds is the crown of thorns worn by Jesus. Mayor Anne Hidalgo described seeing them passing treasures hand to hand in a human chain.
But in the absence of fire-prevention measures at the cathedral, there was only so much the firefighters could do.
“The lack of fire security allowed the fire to spread quickly,” said Jean-Michel Leniaud, the former director of the École Nationale des Chartes a French university institute that specializes in the sciences supporting historical work. “If there were sprinklers everywhere it might have been different, but there weren’t.”
Mr. Leniaud, who visited the interior of Notre-Dame on Tuesday, said the state, which owns and maintains the cathedral, had fire safety regulations for all buildings, but that “sometimes they can be hard to apply.”
Paradoxically, that can be especially so for some of its most cherished structures. “There’s always been a hesitation to disfigure the monument in question,” Mr. Leniaud said.
One reason the fire swept through the open space beneath the roof is that there were no barriers — sometimes called firewalls — to compartmentalize the blaze until firefighters could arrive, said Jim Lygate, a visiting professor of fire investigation at University of Edinburgh. For that reason, he said, such barriers are legally required in similar structures in Britain.
That is not to say that Paris’s firefighters were unprepared for potential calamity. Scores of them drilled regularly for just such an emergency at Notre-Dame. That proved critical to saving many of its treasures.
“We don’t act without planning,” said Gabriel Plus, a spokesman for the Paris fire brigade. “We know the cathedral. So we know what to do when something like that happens, we know, for instance, that we need to deploy boats on the Seine really quickly to pump large amounts of water.”
About 500 firefighters responded, some of them unfurling hoses and training them on the blaze. About 100 focused on saving its religious and cultural treasures, Mr. Plus said.
“Once we realized that the roof would be partially lost, we aimed to stop the fire at the two towers, in order to limit the damage,” he said.
Laurent Nuñez, France’s junior interior minister, said about 20 firefighters had risked their lives by entering the towers to battle the fire, “which enabled the building to be saved.”
“For 15 minutes, half an hour, it could have gone either way,” he said.
The danger of a disastrous fire among the attic timbers was well known.
“At the cathedral, we have fire monitors,” Msgr. Patrick Chauvet, rector of the cathedral, told the radio station France Inter on Tuesday. “Three times a day they go up, under the wooden roof, to make an assessment.”
Notre-Dame had an on-site firefighter, posted daily at a command post on the floor of the structure, and a security agent, said André Finot, a spokesman for the cathedral. In case of an alarm, the firefighter would dispatch the security agent to the area where it rang.
Paris firefighters held two training exercises at Notre-Dame last year, focused on saving the relics and works of art, Mr. Plus said.
Lt.-Col. José Vaz de Matos, an official who works on inspecting France’s national monuments, said, “a good number of priceless collections were saved and brought to safety.” But large items, “some of which have been affected by the fire,” remained inside, he said.
“At this stage, we are not able to send our teams to recover them,” he said.
By late Monday night, most of the damage had been done. But it was not until Tuesday morning that the fire brigade declared the blaze extinguished, and firefighters spent the day watching for hot spots and continuing to pull valuables from the building.
Franck Riester, the culture minister, said on Tuesday that the cathedral’s famous rose windows did not appear to have been damaged. But there were three major holes in the ceiling, one of them caused by the collapse of the spire.
Before the blaze, restoration work had begun and much of the building was sheathed in scaffolding, which was still being erected. Julien Le Bras, the chief executive of Le Bras Frères, the company that handles the cathedral’s scaffolding, told reporters that 12 employees worked on the site, but that none were there when the fire started.
Experts say that restoration, which often involves combustible chemicals and electrical tools, always presents a fire danger, as does electrical wiring.
Mr. Housieaux, the Paris Historical Association president, noted that in the last decade, fires linked to restoration work had destroyed the town hall of La Rochelle, and the Hotel Lambert on the Île Saint-Louis, one of the small islands in the Seine in Paris.
The Notre-Dame restoration project was to focus on reinforcing the spire and repairing some of the attic beams, said Olivier de Chalus, a construction engineer and volunteer guide at the cathedral. He described the structure under the roof as “the cathedral’s jewel, the true piece of art that wasn’t accessible to many.”
But experts say the beams, many of them dating to the cathedral’s construction in the 12th and 13th centuries, became tinder-dry as they aged.
Experts will analyze the building to gauge the risk of collapse, officials said. Stabilizing it is likely to involve removing the damaged scaffolds, erecting new ones, and taking steps to protect the mortar between stones, which may have been weakened by the fire, from eroding when it rains.
Mr. Nuñez, the junior interior minister, said that while “overall the structure is holding,” inspectors had identified “vulnerabilities” in the arched ceiling and in a gable of the northern transept. He said that five apartment buildings on Rue du Cloître, which runs along the cathedral’s northern edge, had been evacuated for 48 hours as a precaution.
Mr. Heitz said that nearly 50 investigators were working on finding the cause of the fire, but he warned that the inquiry will be long and complex. So far, he said, the hypothesis is that it was an accident.
“Nothing at this stage suggests a voluntary act,” he said.