Switzerland is reeling from one of its worst tragedies after a fire ripped through an Alpine bar during a New Year’s Eve celebration, leaving more than 40 people dead and dozens of others with serious burn injuries.

The prosecutor in charge of the case said on Friday that initial investigations suggested the most likely cause of the fire was sparkling flares being carried too close to the ceiling.

The undated photo below shows the bar with what could be soundproofing or insulation covering the ceiling.

The blaze at Le Constellation appears to have started at about 1.30am (00.30 GMT) as revellers were ringing in the new year.

Video, photos and reports from witnesses suggest the flames spread quickly across the packed club, and that people had trouble escaping.

The basement venue served many local people, and some tourists. With no entrance fee, it attracted a young crowd and many of the victims are teenagers. In Switzerland, 16-year-olds are allowed to buy wine and beer but not spirits.

One image posted online showed the low ceiling alight as sparklers attached to champagne bottles are held by partygoers below. This image may have been subjected to some digital enhancement and subsequently the Guardian has blurred some identifying personal details.

Another image shows a woman in what appears to be the same room holding a sparkler while sitting on someone’s shoulders. These two photos have been widely reported to have shown the moment the ceiling caught fire.

According to officials, the fire may have also triggered the release of combustible gases that could then ignite violently, explaining the rapid spread.

The speed of the flames is evident in this video, which shows a young man attempting to put out a small fire that licks at the roof. Music still plays in the bar.

Witnesses described a crowd surge as people frantically tried to escape from the basement nightclub up a narrow flight of stairs and through a small doorway.

This video shows people rushing up the stairs.

Le Constellation had a maximum capacity of about 300 people.

People broke windows to escape the conflagration, and ambulances arrived at the scene after reports of smoke. Firefighters smashed windows at a nearby apartment block to enter.

Panicked parents rushed to the scene in cars to see whether their children were trapped inside. By daylight on New Year’s Day, red-and-white caution tape, flowers and candles were in the street where the disaster happened. Police shielded the site with white screens as forensics teams got to work.

Many of the victims could not be identified immediately because of the severity of their burns. The intensive care unit and operating room at the regional hospital quickly hit full capacity.

Authorities later said about 50 people had been sent or would be transferred to other European countries for treatment in specialised burn units.

Switzerland ordered the national flag to be flown at half-mast for five days as a sign of mourning.

Citizens of nearby France and Italy were among the 113 people identified as injured, authorities said.