Investigators are trying to determine how and why the deadly fire that broke out on New Year’s Eve in Crans-Montana spread so rapidly.

Swiss authorities said on Friday that sparklers attached to champagne bottles were the cause of the fire. However, how the blaze spread with such intensity—killing at least 40 people and injuring 119, many of them seriously—has now become a key focus for officials, along with the bar’s safety record.

The BBC spoke with experts who examined videos and photos taken by survivors in order to identify the mistakes that led to the tragedy.

Champagne bottles with sparklers held aloft

Two striking images shared online show people holding champagne bottles with lit sparklers above their heads, surrounded by a crowd.

One image shows flames beginning to gather on the ceiling above people holding five of these bottles high.

The second image is a close-up and shows a person wearing a helmet and holding a bottle with a lit sparkler, sitting on the shoulders of another person wearing a mask.

The sparks from this bottle appear to be closer to the ceiling.

The BBC confirmed that these images were taken after midnight on January 1, verifying that there were no earlier versions, and compared them with publicly available photos of Le Constellation, using details such as the bar’s design and distinctive piping.

In other videos from the night of the fire, some people in the bar can be seen filming the flames while loud club music plays in the background. In one video, people begin rushing toward an exit staircase while shouting.

On Friday, Béatrice Pilloud, chief prosecutor of the Valais region, said all evidence points investigators to the conclusion that the fire started from sparklers attached to champagne bottles that were “moved very close to the ceiling.”

Questions about the foam ceiling lining

Another factor is the foam-like lining on the bar’s ceiling and whether it complied with safety standards.

Two fire safety experts told the BBC that the materials visible in photos and videos of Le Constellation appeared to show “egg-crate foam,” a type of sound-absorbing material made from polyurethane.

In the image with the bottles held high, flames are visible on part of the ceiling covered with foam-like material.

🚨 🇨🇭 INCROYABLE : « Wallah, il brûle », des jeunes essaient d’éteindre le feu dès le TOUT DÉBUT de l’incendie à Crans-Montana, en Suisse.

Terrifiant… 😞 pic.twitter.com/3U22VsH81f

— Wolf 🐺 (@PsyGuy007) January 1, 2026

This foam is often treated with fire retardant before being installed as a noise absorber in factories and entertainment venues, but if untreated, it can be extremely flammable.

“Once ignited, polyurethane acoustic foam can exhibit rapid flame spread across its high surface-area profile and produce dense, toxic smoke, significantly accelerating fire development and reducing available escape time,” said Dr. Peter Wilkinson.

Professor Ed Galea of the University of Greenwich said the effectiveness of fire-retardant treatment in PU foam can degrade over time.

Swiss authorities say they cannot confirm what type of foam was used in the bar or whether it complied with safety standards.

At Friday’s press conference, officials spoke about a “flashover” that occurred in the bar. Professor Galea explained that this happens when hot gases rise to the ceiling, reach a critical temperature, and then ignite the entire room almost instantaneously.

According to Michael Klippel, a fire safety expert at ETH Zurich, “survival after a flashover is very unlikely.”

The authority responsible for overseeing fire safety inspections in Crans-Montana is the Valais Cantonal Fire Control Office (OCF). Inspections are carried out by local officials.

Swiss authorities said at the press conference that inspections of a building the size of Le Constellation should be conducted annually.

The bar’s exits

Authorities say they will also focus on the exits of the bar, which spans two levels—ground floor and basement. The fire is believed to have started in the basement, where the two photos mentioned above were taken.

Videos recorded at the moment the fire broke out show people trying to extinguish the flames before attempting to escape from the basement by climbing a narrow staircase.

Professor Galea said stairway exits can become deadly bottlenecks, with people tripping and being trampled.

He added that even if other emergency exits existed, panicked people in unfamiliar surroundings were more likely to try to leave the way they entered.

Officials also confirmed that there was more than one exit from the building, but added that they are “not currently in a position to say” whether the emergency exit was open or closed at the time.

Valais state councillor Stéphane Ganzer said: “There is not just one door, although at the time of the fire it appears that most people left through the main entrance. But this building is a public venue. Obviously, it was equipped with an emergency exit.”

Pilloud told reporters that the two French managers of the bar had been questioned, as well as people who escaped the fire.

One of the bar’s owners reportedly told local media that the establishment had been inspected three times over the past ten years and that everything had been done in accordance with regulations.

Sparklers used repeatedly

Investigators say they have also analyzed other videos from the venue.

One video available online shows sparklers attached to bottles being used inside the bar as early as 2024.

It shows women wearing distinctive helmets carrying bottles and fireworks to customers, before detaching them and serving drinks.