Investigators are rushing to find out why a deadly fire at Le Constellation bar in Crans-Montana, Switzerland spread so quickly on New Year's Eve. Officials said that sparklers attached to champagne bottles held too close to the ceiling appear to have started the fire in the bar's basement. The blaze killed at least 40 people and injured 119, many seriously. Videos show people holding bottles with lit sparklers raised high near the ceiling. One image captures flames gathering above the crowd. BBC Verify confirmed these images were genuine and taken just after midnight on January 1. Attention is now on the foam-like padding on the bar's ceiling. Experts say the material looks like polyurethane (PU) foam, commonly used for soundproofing. Untreated PU foam is highly flammable. Professor Peter Wilkinson said, "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." Professor Edwin Galea explained the fire caused a "flashover," where hot gases ignite the room almost instantly. This rapid spread made survival very unlikely. Authorities are checking if the foam met safety standards but haven’t confirmed this yet. Fire safety inspections in Crans-Montana are carried out yearly by local officials. Another focus is on the bar's exit routes. The fire started in the basement, where videos show people rushing up a narrow stairwell trying to escape. Experts warn staircases can become deadly bottlenecks during fires. Officials said there were multiple exits but couldn’t confirm if emergency exits were open during the fire. Bar managers have been interviewed. One owner claimed the venue was inspected three times in ten years and followed all regulations. Investigators also found older videos showing sparklers on bottles used inside the bar in 2024. This practice may have been common, raising questions about safety measures. Swiss officials continue to probe to understand exactly how the tragic fire spread so fast and to prevent future disasters.