What is a flashover? How deadly Swiss bar fire may have spread so quickly

The fatal fire that engulfed a Swiss bar on New Year's Eve would likely have killed even a firefighter in full protective gear — let alone the hundreds of young people freely celebrating inside.

At around 1:30 a.m. local time on Jan. 1, a fire broke out inside the Le Constellation bar in Crans-Montana, Switzerland, killing around 40 and injuring about 115 others, officials said.

Beatrice Pilloud, attorney general for the Valais region, said at a news conference on Jan. 2 that officials believe the fire was caused by sparkling candles atop champagne bottles.

"Everything suggests that the fire started from the burning candles or 'Bengal lights' that had been attached to champagne bottles. These went too close to the ceiling. From there, a rapid, very rapid and widespread conflagration ensued," Pilloud said, per Reuters.

While officials are continuing to investigate the incident and are working to identify the victims, Pilloud said the fire appears to have spread rapidly through what's known as a flashover.

What is a flashover?

A flashover is the "sudden, simultaneous ignition of everything in a room," according to the National Fire Protection Association.

In a flashover, temperatures can reach up to 1,000 degrees Fahrenheit in a matter of seconds — likely not survivable for even a firefighter in full protective gear, according to NFPA.

A small fire can become a flashover within three to five minutes after the blaze first begins, leaving little time for those inside a structure to escape, according to the U.S. Fire Administration.

Two French women identified as Emma and Albane told France's BFM TV that they saw the fire start in the basement section of the club after a bottle with candles was held up closely to the wooden ceiling.

"The fire spread across the ceiling super quickly," one of the women told the outlet. They said they were able to climb a narrow staircase to the ground floor and escape the building minutes before the fire reached the ground floor.

How does a flashover fire spread?

When a fire begins in a room without ventilation, the heat radiates downward, eventually engulfing all combustible items in its path, causing a flashover, per NFPA.

According to NFPA, there are signs that a fire will become a flashover, including dense black smoke with tightly packed curls and smoke that has accumulated as low as a doorknob. Firefighters are trained to recognize these signs.

Videos from inside Le Constellation verified by Reuters appeared to show wooden ceiling panels on fire as crowds frantically tried to escape.

Pilloud said investigators will check whether the ceiling's insulation foam was to blame for the rapid spread of the fire.

Melina Khan is a national trending reporter for USA TODAY. She can be reached at [email protected].