A fire that tore through a bar at a Swiss ski resort killing at least 40 people likely started when “fountain candle” sparklers were held aloft too close to the ceiling, the region’s chief prosecutor said on Friday.

Axar.az informs, citing Reuters, Investigators are combing through the blackened ruins of the venue, examining videos on social media and interviewing survivors for clues on how the fire began in the early hours of New Year’s Day in the bar’s basement and spread so fast.

Witnesses have recounted seeing bar staff carrying sparkling fountain candles attached to bottles of champagne. The Valais canton’s chief prosecutor, Beatrice Pilloud, said the sparklers were a leading line of enquiry that looked to be firming up.

“Everything suggests that the fire started from the burning sparklers ... that had been attached to champagne bottles and were moved too close to the ceiling,” Pilloud told a news conference. “From there, a rapid, very rapid and widespread blaze ensued.”

However investigators are pursuing several hypotheses and no scenario has been ruled out, she added.