What images and videos tell us about why Swiss bar fire spread so quickly

How the blaze took hold with such ferocity, killing at least 40 people and leaving many more critically injured, is now a key focus for officials.

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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.