The security employee monitoring the smoke alarm panel at Notre Dame Cathedral was just three days on the job when the red warning light flashed on the evening of 15 April: “Feu.” Fire.

It was 6:18pm on a Monday, the week before Easter. The employee radioed a church guard who was standing just a few feet from the altar.

Go check for fire, the guard was told. He did and found nothing.

It took nearly 30 minutes before they realised their mistake: the guard had gone to the wrong building.

The fire was in the attic of the cathedral, the famed latticework of ancient timbers known as “the forest”. The guard had gone to the attic of a small adjacent building, the sacristy.

Notre Dame fire: What was lost and what was saved Show all 13 1 /13 Notre Dame fire: What was lost and what was saved Notre Dame fire: What was lost and what was saved Statue of Saint Denis Notre Dame https://commons.wikimedia.org/wiki/File:Paris_-_Cath%C3%A9drale_Notre-Dame_-_Portail_de_la_Vierge_-_PA00086250_-_003.jpg Thesupermat/CC-BY-SA Thesupermat/CC-BY-SA Thesupermat/CC-BY-SA Notre Dame fire: What was lost and what was saved "Close up of Virgin Mary and Baby Jesus statues at Notre Dame Cathedral in Paris, France. Other images of the Notre Dame Cathedral:" gregobagel Getty Images/iStockphoto Notre Dame fire: What was lost and what was saved Safe: Rose windows Among the most famous architectural features of the Gothic masterpiece, the stained glass rose windows are treasured artworks. The three rose windows, which date back to the 13th century, adorn the north, south and west facades. There were hopes the windows had escaped being destroyed by the fire after firefighters stopped its spread. Photos the following morning suggested the circular window of the nave had remained intact. AFP/Getty Notre Dame fire: What was lost and what was saved Destroyed: Irreplaceable wooden roof Crowds of tourists and Parisians watched in horror as the spire and roof of Notre Dame cathedral came crashing down to the ground. EPA Notre Dame fire: What was lost and what was saved Safe: Descent from the Cross The cross and statue on Notre Dame's high altar - sculpted by Nicolas Coustou in 1723 - were believed to have survived in tact despite being surrounded by smoke and debris. Reuters Notre Dame fire: What was lost and what was saved Safe: Great Organ With nearly 8,000 pipes, some dating back to the 1700s, Notre Dame's master organ is one of the largest in the world. The monumental instrument, the largest in France, was fully restored in 2013 with each pipe cleaned. Paris' deputy mayor, Emmanuel Gregoire, said the instrument remained intact following the fire. AFP/Getty Notre Dame fire: What was lost and what was saved Safe: Bells Housed in the two western towers, Notre Dame's bells have rung out at key moments in France's history. Emmanuel, the largest bell, was lifted into the south tower in 1685 and weighs over 23 tonnes. The fire was prevented from spreading to the bell towers. AFP/Getty Notre Dame fire: What was lost and what was saved Safe: Crown of Thorns Believed to be a relic of the wreath of thorns placed on the head of Jesus Christ at his crucifixion, the object was stored in the cathedral's treasury. French King Louis IX brought the relic, which is contained in an elaborate gold case, to Paris in 1238. The Crown of Thorns was saved and has been taken into safekeeping. AP Notre Dame fire: What was lost and what was saved Safe: Tunic of Saint Louis The tunic allegedly worn by Saint Louis IX as he brought the Crown of Thorns to Paris was kept inside the cathedral. It has been saved. AFP/Getty Notre Dame fire: What was lost and what was saved Safe: Statues on the roof The week before the fire, religious statues set atop the cathedral were removed for the first time in over 100 years as part of the restoration project. A 100-metre-high crane lowered the copper statues representing the 12 apostles and four evangelists onto a lorry. AFP Notre Dame fire: What was lost and what was saved Destroyed: Spire Crowds of tourists and Parisians watched in horror as the spire and roof of Notre Dame cathedral came crashing down to the ground. EPA/I Notre Dame fire: What was lost and what was saved Unknown: Choir screen It is not known what happened to the detailed choir screen at Notre Dame cathedral. AFP/Getty Notre Dame fire: What was lost and what was saved Some of the artworks from Notre-Dame sheltered as the fire devastated the cathedral AFP/Getty

By the time the guard climbed 300 narrow steps to the attic, the fire was burning out of control — putting firefighters in a near impossible position when they were finally called, half an hour after the alert.

The miscommunication, uncovered in interviews with church officials and managers of the fire security company, Elytis, has set off a bitter round of finger-pointing over who was responsible for allowing the fire to rage unchecked for so long.

Who is to blame and how the fire started are at the heart of an investigation by French authorities that will continue for months.

But the damage is done. What happened that night changed Paris.

The New York Times conducted scores of interviews and reviewed hundreds of documents to reconstruct the missteps — and the battle that saved Notre Dame in the first four critical hours after the blaze began.

What became clear is just how close the cathedral came to collapsing.

The first hour was defined by that initial, critical mistake: the failure to identify the location of the fire and by the delay that followed.

The second hour was dominated by a sense of helplessness. As people raced to the building, waves of shock and mourning for one of the world’s most beloved and recognisable buildings rippled in real time across the globe.

That Notre Dame still stands is due solely to the enormous risks taken by firefighters in those third and fourth hours.

“There was a feeling that there was something bigger than life at stake,” said Ariel Weil, mayor of the city’s 4th Arrondissement, home to the cathedral, “and that Notre Dame could be lost.”

To many Parisians, the sight of Notre Dame in flames was unendurable.

“For Parisians, Notre Dame is Notre Dame,” said its rector, Monsignor Patrick Chauvet. “They couldn’t think for one second that this could happen.”

Remarkable footage shows firefighters battling Notre Dame flames

The fire warning system at Notre Dame took dozens of experts six years to put together and in the end involved thousands of pages of diagrams, maps, spreadsheets and contracts, according to archival documents found in a suburban Paris library by The New York Times.

The result was a system so arcane that when it was called upon to do the one thing that mattered — warn “fire!” and say where — it produced instead a nearly indecipherable message.

“The only thing that surprised me is that this disaster didn’t happen sooner,” said Albert Simeoni, an expert born and trained in France, who is now head of fire protection engineering at Worcester Polytechnic Institute in Massachusetts.

The ponderous response plan, for example, underestimated the speed at which a fire would spread in Notre Dame’s attic, where, to preserve the architecture, no sprinklers or fire walls had been added.

The plan’s flaws may have been compounded by the inexperience of the security employee.

All the sensitive technology at the heart of the system had been undone by a cascade of oversights and erroneous assumptions built into the overall design, said Glenn Corbett, an associate professor of fire science at John Jay College of Criminal Justice in New York City.

Notre Dame fire: Global tributes Show all 5 1 /5 Notre Dame fire: Global tributes Notre Dame fire: Global tributes Fenice theatre, Venice The Fenice theatre in Venice illuminated with the colors of the French flag EPA Notre Dame fire: Global tributes The dome of the Chapel of the Shroud, Turin The dome of the Chapel of the Shroud in Turin illuminated with the colors of the French flag EPA Notre Dame fire: Global tributes One World Trade Centre, New York City One World Trade Centre is lit in the colors of the French flag in solidarity with the people of France REUTERS Notre Dame fire: Global tributes Stari Most bridge, Mostar The Stari Most bridge in the Bosnia and Herzegovina city of Mostar lit in colours of the French flag AFP/Getty Images Notre Dame fire: Global tributes Empire State Building, New York City Empire State Building is lit in the colors of the French flag in solidarity with the people of France REUTERS

“You have a system that is known for its ability to detect very small quantities of smoke,” Corbett said. “Yet the whole outcome of it is this clumsy human response. You can spend a lot to detect a fire, but it all goes down the drain when you don’t move on it.”

If it took more than half an hour to call the fire department, it took just minutes once smoke appeared for the images to begin circulating around the world on social media.

Mr Chauvet, the rector, had been chatting just a couple of hundred yards from the cathedral with shopkeepers, when suddenly one of them pointed up and exclaimed: “Look, there is smoke coming out!”

He pulled out his mobile phone and warned his staff. They said the fire department had been called but had yet to arrive.

“I was incapable of doing anything,” Mr Chauvet said. “I couldn’t say anything. I watched the cathedral burn.”

Mr Weil, the mayor of the 4th Arrondissement, was just leaving a meeting at the nearby Hôtel de Ville, the city hall, when he saw the smoke and ran towards Notre Dame.

He called the mayor of Paris, Anne Hidalgo, and she rushed to meet him.

By the time Master Corporal Myriam Chudzinski arrived, a few minutes before 7pm, Notre Dame was surrounded by hundreds of horrified bystanders. The fire already glowed through the roof.

Ms Chudzinski’s team was one of the first to arrive and headed to the attic.

Once at the top, Ms Chudzinski and her team stopped on a cornice outside the attic, and she took the lead dousing the flames, about 15 feet away.

The fire was starting to surround them, threatening to trap them outside, in the middle of the inferno. They retreated inside, towards the attic.

In the attic, the flames advanced as an unstoppable wall.

About 7:50, almost an hour into the fight, a deafening blast engulfed her.

The cathedral’s 750-ton spire, wrought of heavy oak and lead, had collapsed.

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The generals overseeing the operation called everyone back. They battled the fire from the ground, drawing water from the Seine.

But it wasn’t working.

Before the blast, Ms Chudzinski and her colleagues had made a critical observation: The flames were endangering the northern tower. That realisation would change the course of the fight.

Inside that tower, eight giant bells hung precariously on wooden beams that were threatening to burn. If the beams collapsed, firefighters feared, the falling bells could act like wrecking balls and destroy the tower.

And if the northern tower fell, they believed, it could bring down the south tower — and the cathedral with it.

Near 8:30pm, President Emmanuel Macron arrived to survey the damage, along with Prime Minister Édouard Phillippe and other officials.

A group of about 20 officials convened across the plaza at police headquarters for a briefing by General Jean-Claude Gallet, head of the Paris fire brigade.

Mr Gallet, 54, had served in Afghanistan and specialised in crisis management. He entered the conference room and gave them the bad news.

The attic could no longer be saved; he had decided to let it go. He would have his brigades throw all their energy into saving the towers, focusing on the northern one, already on fire.

Mr Macron remained silent but appeared to give tacit approval to General Gallet’s decision.

Out on the square, a temporary command post had been set up. There, General Gallet’s deputy, General Jean-Marie Gontier, was managing the firefighters on the front lines.

At the command post, Master Sergeant Rémi Lemaire, 39, had an idea.

What if they went up the stairs in the southern tower, where he had been earlier in the fight? That way, they could carry up two additional hoses plugged directly into a fire truck.

And then from there, the firefighters could enter the blazing northern tower.

Designs from People’s Notre Dame Roof competition Show all 10 1 /10 Designs from People’s Notre Dame Roof competition Designs from People’s Notre Dame Roof competition This most ambitious of designs, known as the Floating Fish, would see an artificial waterway crafted to link the River Seine with a pool in place of the roof of Notre Dame cathedral Bay Huynh Architects Designs from People’s Notre Dame Roof competition Architects Bay Huynh are yet to reveal how the water would be pumped up the waterway to the roof Bay Huynh Architects Designs from People’s Notre Dame Roof competition This design from MA2 architects incorporates a variety of Catholic symbols. The ribbed roof is intended to represent the crown of thorns, the thin spire: the nails of the crucifixion and the spire will lead to a large cross MA2 Designs from People’s Notre Dame Roof competition In this design from Slovakian architect Vizum Atelier, the new spire would shoot a beam of light up towards the sky in an attempt to "connect heaven with earth" Vizum Atelier Designs from People’s Notre Dame Roof competition This design is called: 11 spires, 11 voices. The idea is that these gold-plated spires, modeled after organ pipes, would serve to project the sounds, songs and sermons from inside the cathedral across Paris Chicago Underground Practice Designs from People’s Notre Dame Roof competition The architects of Fran Canos Studio believe that the roof should be restored exactly as before save for the introduction of a stone frame as a "symbol of protection" from fire Fran Canos Studio Designs from People’s Notre Dame Roof competition Inspired by Noah's Ark, this design would see the original spire restored and a greenhouse installed in place of the previous roof Won C Kim Designs from People’s Notre Dame Roof competition Inspired by Noah's Ark, this design would see the original spire restored and a greenhouse installed in place of the previous roof Won C Kim Designs from People’s Notre Dame Roof competition In light of news that planning and reconstruction of the roof will take over five years, RCDF studio have designed a temporary roof for the meanwhile. The roof would be made of fibreglass but the spire will appear only as a hologram RCDF Studio Designs from People’s Notre Dame Roof competition Frank Liu suggests a glass roof with a graded opacity to lead the eyes to the top of the spire Frank Liu

It was a high-risk strategy. But General Gontier agreed.

Mr Lemaire had already seen the perils that the northern tower held earlier that evening. In the time it took to decide on the new plan, things had only gotten worse.

A group of firefighters from a neighbouring suburb refused to go in, but another team said it would do it.

They broke a gate, and Mr Lemaire led the firefighters up the southern tower to the height of the bells.

The next 15 minutes proved decisive. By 9:45, the flames were tamed.

Around 11:30, Mr Macron addressed the nation from in front of the cathedral. “The worst was avoided, even though the battle is not yet over,” he said.

Then he made a pledge: “We will rebuild this cathedral together.”

Over the past three months, investigators have conducted some 100 interviews and sifted the rubble, looking for clues to what started the fire.

Emmanuel Macron promises Notre Dame rebuild following devastating fire

They have focused on the possibility of a short-circuit in the electrified bells of the spire or in the elevators that had been set up on the scaffolding to help workers carry out renovations.

They are also considering cigarette butts, which were found on the scaffolding, apparently left by workers.

“We’re not ruling out any scenario; we just know it wasn’t criminal,” said a Paris police official.

The miscommunication that allowed the fire to rage unchecked for so long is now the source of a dispute over who is responsible.

Church officials say the employee for Elytis, the fire security company, never mentioned the framework of the cathedral’s roof.

“Several of them had a walkie-talkie and all heard ‘Attic Nave Sacristy’,” said André Finot, a spokesman for Notre Dame. “That’s all.”

Arnaud Demaret, chief executive officer of Elytis, said his employee was still in shock. The company received two death threats in the days after the fire, he said.

But he insisted his employee had communicated the fire’s location properly.

Miraculously, no one was killed.

Three days later, Ms Chudzinski and Mr Lemaire were among the hundreds of firefighters and police officers honoured by Mr Macron at the Élysée Palace.

“These people were heroes,” Mr Weil said.

The sense of the cathedral as a living, wounded entity has only intensified since the fire.

“First off, this is all about our fragility,” Mr Chauvet, the rector, said. “We are as nothing. The fragility of man, in respect to God. We are nothing but creatures.”