Around 40 people were killed and at least 115 others injured early on New Year’s Day when a devastating fire tore through a popular bar in the Alpine resort town of Crans-Montana, Swiss authorities said.

The blaze broke out at about 1:30 a.m. local time at the Le Constellation, according to police in the Canton of Valais. Officials said the cause of the fire has not yet been determined, and investigators are working to establish how it started.

Switzerland’s rotating president, Guy Parmelin, described the incident as “one of the worst tragedies our country has experienced,” saying what should have been a moment of celebration had turned into national mourning. Flags across the country will be flown at half-mast for five days in memory of the victims.

Authorities confirmed that a criminal investigation has been opened by the Valais public prosecutor’s office, though they stressed there is no indication the fire was linked to terrorism.

Emergency services mounted what officials called a “major” response, deploying 42 ambulances, 13 helicopters and three disaster-response vehicles to the scene. Many of the injured remain in critical condition, according to regional officials.

As Switzerland mourns the victims, the tragedy has sent shockwaves through the Alpine nation and beyond, casting a sombre shadow over the start of the new year.