The prevalence of sparklers at a Swiss ski resort bar where dozens of people died in a fire this week has been branded "irresponsible", as authorities probe potential compliance issues there.

At least 40 people were killed and 119 injured when Le Constellation bar, in the alpine town of Crans-Montana, burst into flames around 1:30am, local time, on January 1.

A multi-agency investigation is examining whether revellers were crushed trying to flee the burning building via a narrow staircase, whether the emergency exit was blocked, whether foam cladding on the ceiling was legal, and if too many people were inside the venue at the time the fire broke out.

Authorities on Friday revealed they believed so-called fountain sparklers were responsible for starting the blaze.

"Even if they're small, using them in such a venue seems pretty irresponsible to me," said Italian Foreign Minister Antonio Tajani, who travelled to the town after the fire because dozens of his country's citizens are injured or missing.

This image, taken from a video inside the venue, appears to show flames on the ceiling. (Supplied)

The sparklers, sometimes known as bangers, are readily available across Europe and elsewhere, but not in Australia.

Beatrice Pilloud, the local region's attorney-general, said investigators had already reviewed videos filmed in the venue shortly before the blaze began.

The clips show wild New Year's Eve celebrations, including people sitting on each other's shoulders and waving champagne bottles with fountain sparklers attached to them close to the ceiling.

"From that, the blaze began very quickly," Ms Pilloud said, adding the two bar's two French owners had already been interviewed.

"The next steps of the investigation will focus on the [building] work carried out within the bar, the materials used, the operating permits, the safety measures ... the number of people who were present that evening and the number of people the bar is authorised to hold."

Ms Pilloud said compliance issues relating to the foam cladding attached to the bar's roof were being scrutinised and that criminal charges were possible, pending the investigation's findings.

Le Constellation bar in Crans-Montana has become a crime scene. (ABC News: Daniel Pannett)