At least 13 people were killed and six injured when a fire swept through a bar in the northern French city of Rouen early Saturday, police said.
The blaze, described as "totally accidental" by the mayor, broke out around 1 am (2300 GMT Friday) in the basement of the Cuba Libre bar, where many young people had gathered to celebrate a birthday.
The cause of the fire has yet to be determined.
France Bleu radio reported that investigators believe candles on the birthday cake ignited the basement's ceiling.
But in an interview with Europe 1 radio, Mayor Yvon Robert said the room might have burst into flames due to a gas explosion.
Images from the scene show the bar's front window shattered, its outdoor awning charred and tables overturned.
The victims, aged between 18 and 25, were not immediately identified, the Paris-Normandie newspaper reported.
Robert said "the basement was probably too small for the number of people" present and that investigators will look into whether the business had met all building codes.
Around 50 firemen were dispatched to the scene and extinguished the blaze within 30 minutes, the French Interior Ministry said.