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.

Bottles with sparklers held in the air

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.

BBC Verify determined these images were taken after midnight on 1 January by confirming their authenticity against public photos of Le Constellation.

Questions about foam padding on ceiling

Another focus is on foam-like padding on the bar's ceiling and whether it was compliant with safety standards. Fire safety experts indicated that the visible materials appeared to show egg box foam, a type often used in venues.

However, untreated polyurethane can be highly flammable. Dr. Peter Wilkinson noted that once ignited, it can result in rapid flame spread and produce toxic smoke, significantly accelerating fire growth.

Exit routes from the bar

The authorities are also investigating exit routes at the bar, which has both ground and basement levels. Videos show people trying to escape through narrow stairways as the fire took hold. Officials confirmed that there were multiple exits but did not clarify if the emergency exit was accessible during the fire.

Sparklers used before

Investigators are reviewing earlier videos of the venue, showing that sparklers had been utilized before, raising new concerns about safety practices and protocols.