As young partygoers rang in the New Year at a packed bar in the Swiss Alps, the celebration quickly turned into a nightmare when a devastating fire tore through the basement, killing at least 40 people and injuring 119 others.
The blaze at Le Constellation in Crans-Montana has been described by Switzerland's president as one of the country's "worst tragedies."
As families anxiously await word about missing loved ones and experts work to identify the victims, authorities are still piecing together how the fire started and why it spread so fast.
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Swiss authorities said Friday that the champagne sparklers were the likely cause of the fire, though the official inquiry remains ongoing.
Emerging videos and eyewitness accounts reveal a deadly combination of hazards: fire-prone ceiling panelling and a crowded bar packed with young patrons funnelling toward a narrow escape route.
A night of revelry
Le Constellation, located in the heart of Crans-Montana, a luxury ski resort in the French and German-speaking canton of Valais, is a popular nightspot among young locals and visitors.
Around 200 people were inside the venue on New Year's Eve, according to eyewitnesses, ready to celebrate 2026 with music, drinks and dancing.
Sparkles that turned to smoke
Imagery widely shared online shows staff in motorcycle helmets perched on others' shoulders, holding champagne bottles with sparklers amid the packed crowd.
One clip shows at least six bottles raised in the air as flames and smoke erupt from the ceiling.
The sparklers appear to have ignited what experts believe is acoustic panelling on the ceiling, a material designed to improve sound but that can also be highly flammable.
Independent fire consultant Stephen MacKenzie described it as "plastic petrol," adding: "That's why we're seeing reports of many of the young people with a first, second, third and unfortunately, fourth degree burns."
Swiss chief prosecutor Beatrice Pilloud told reporters on Friday that investigators are examining the installation of foam panelling at the bar, and whether it complied with regulations.
READ MORE: Witnesses blame 'sparklers in champagne bottles' after 40 killed in Swiss Alps blaze
A fast-spreading fire
Once the fire broke out, it spread quickly.
One video shows a young man attempting to put the fire out by hitting it with a cloth, while others record on their phones or continue dancing, seemingly unaware of the imminent danger.
"Once the ceiling was on fire, within about 10 seconds, the entire nightclub was on fire," one witness said.
Another eyewitness, Axel, who did not give his surname, told Reuters news that he and others in the bar didn't initially realise the danger was serious.
"We were yelling, 'Fire! Fire!' and then we thought it was a joke or that it wasn't necessarily serious.
And then suddenly, a huge cloud of black smoke, we couldn't breathe anymore," he said, describing how he eventually got upstairs and smashed a window to get out.
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Horrific escape
As smoke filled the lounge and the fire blazed, partygoers rushed toward a narrow staircase.
Video verified by CNN shows dozens trapped at the exit, with one person jumping through a window as thick, red smoke engulfed the building.
MacKenzie explained the fire's rapid spread through a process called a flashover, in which nearly everything in a room ignites almost simultaneously.
"Combustions pick up at ceiling level," causing the fire to "spread laterally," he said. This process is like a "stone drop in an ocean," with smoke rippling out sideways and starting to "preheat" everything in front of it.
When the fire door was opened, it could have created a "chimney effect" which accelerated the flow of smoke and combustible gases upward, MacKenzie said. "The smoke is actually on fire, a 'flashover,'" he added.
On Friday afternoon, Pilloud, the Swiss prosecutor, said all signs supported that theory.
"As things stand, everything points to the fire starting from sparklers or flares placed in champagne bottles that came too close to the ceiling, which very rapidly led to a flashover fire."
Laetitia Place, a 17-year-old partygoer from Lausanne, recounted her harrowing escape from the fire, saying that there was a crush at the narrow exit that made it difficult for others to get through.
"The first stairs are pretty easy to get through since they're wide and all that. But after that, there's the small door where everyone was pushing, and so we all fell."
"We were piled on top of each other, some people were burning, and some were dead next to us," the teenager told Reuters.
Video obtained by CNN shows multiple people lying motionless outside, while bystanders try to help.
Resident Samuel Rapp, 21, witnessed that aftermath.
"There were people screaming, and then people lying on the ground, probably dead."
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Emergency services arrive
Swiss emergency services launched a response within minutes of the fire, transporting the injured to hospitals across Switzerland and abroad.
Around 50 patients have been or will be transferred to hospitals in other European countries for specialist treatment, authorities told journalists on Friday.
Edmund Coquette told CNN-affiliate RTL Germany that he saw "bodies on the streets," and young people "totally burned in the face," who were missing fingers.
A man living near Le Constellation, who ran to assist people fleeing the burning building, described searching for an emergency exit behind the bar and finding a jammed door with people trying to get out.
The man, Paolo Campolo, told French media outlet "20 Minutes" that he asked the fire captain for help but soon decided to intervene himself.
"We didn't wait. With someone who happened to be passing by, we broke the door open and got the people out," Campolo said from a hospital bed.
Dr Robert Larribau, head of emergency care at Geneva University Hospital, told CNN that most patients the hospital received were 15 to 30 years old, many with "extremely serious injuries" caused by flashover and possible backdraft.
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Flashover typically causes severe burns, particularly on the face, back and upper limbs, often combined with critical inhalation injuries from radiant heat and superheated gases.
Backdraft, an oxygen-driven explosion, can cause instantaneous fatal burns and toxic inhalation.
An investigation has been opened into the circumstances surrounding the fire and how it spread so quickly, as Swiss officials said Friday that the two French owners of the bar have been interviewed by police to gather information.
CNN has reached out to the owners of the bar through their businesses but is yet to receive a response.
Meanwhile, as of Friday evening, many loved ones of the victims and injured were still waiting for answers, and authorities continue the process of identifying the dead.
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