Investigators are racing to establish how and why the deadly New Year’s Eve fire at a bar in a Swiss ski resort spread so rapidly.

Authorities on Friday said in a press conference that sparklers attached to champagne bottles that were held “too close to the ceiling” appear to have started the blaze in the basement of Le Constellation bar in Crans-Montana.

But how the fire took hold with such ferocity, killing at least 40 people and leaving 119 injured, many seriously, is now a key focus for officials – as is the bar’s safety record.

​​BBC Verify has been examining videos taken by survivors and onlookers and speaking to fire safety experts to find clues about what went wrong.

Two striking images shared widely online show people carrying champagne bottles with lit sparklers above their heads, with a crowd around them.

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

The second image is a closer-up angle, showing a person wearing a crash helmet and holding a bottle with a lit sparkler, sitting on the shoulders of another person wearing a Guy Fawkes mask.

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

​​BBC Verify determined these images were taken after midnight on 1 January by confirming there were not earlier versions and matched them against public photos of Le Constellation – using details including the bar design and distinctive pipework.

And there was no evidence the images had been ​​manipulated using artificial intelligence (AI).

In other videos we verified from the night of the fire, some people in the bar can be seen filming the flames as loud club music thumps in the background. In one video, some people start to hurry for an exit stairwell while shouting.

On Friday, Béatrice Pilloud, the attorney-general of the Valais region, said everything led investigators to believe the fire had started from sparklers attached to bottles of champagne that were “moved too close to the ceiling”.

Another focus is on foam-like padding on the bar’s ceiling and whether it was compliant with safety standards.

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

In the photo of the bottles being held up, flames are visible on a part of the ceiling with a foam-like covering.

PU foam is often treated with fire-retardant before being installed as a noise dampener in factories and entertainment venues.

But untreated, it can be highly 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 growth and reducing available escape time,” said Dr Peter Wilkinson of Loughborough University.

Professor Edwin Galea, from the University of Greenwich, said the effectiveness of retardant treatment on PU foam can wear off over time.

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

In Friday’s press conference, officials talked about a “flashover” happening in the bar.

Professor Galea explained this is what happens when hot gases rise to the ceiling, reach a critical temperature and then ignite the room near instantaneously.

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

The authority responsible for overseeing fire safety inspections in Crans-Montana is the Office Cantonal du Feu (OCF) of the Canton of Valais. The inspections are carried out by local officials.

Swiss authorities said in a press conference that inspections on a building the size of Le Constellation should have been carried out each year.

BBC Verify has contacted the OCF to request access to previous inspection documents.

The authorities say they will also focus on exit routes at the bar, which sits across two levels – a ground floor and a basement. The fire is thought to have started in the basement, where the two images referred to above were taken.

Videos filmed as the fire took hold show people trying to extinguish the flames before trying to get out of the basement up a narrow set of stairs.

Prof Galea said staircase exits can be fatal bottlenecks with people tripping and getting trampled.

He said even if there were other fire exits, panicked people in unfamiliar spaces were more likely to go out the way they came in.

Officials also confirmed there was more than one exit from the building, but added they were “not currently able 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, even though at the time of the fire, it seems that most people left through the main entrance. But this building is a public place. It was obviously equipped with an emergency exit.”

Pilloud told journalists that the two French managers of the bar had been interviewed as well as people who escaped the fire.

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

The investigators say they have also been analysing other videos of the venue.

One video we found shows sparklers attached to bottles being used inside the bar as far back as 2024.

It shows women dressed in distinctive crash helmets carrying the bottles and pyrotechnics to customers, before detaching them and pouring drinks.

The footage was uploaded to YouTube in May 2024 by the account @ConstellationCransMontana, though we can’t be certain when it was filmed.