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PARIS — A fast-moving fire swept through a birthday party in the basement of a bar in northwest France overnight Friday, killing at least 13 people and injuring six others, authorities said.

Firefighters leave after inspecting the Cuba Libre bar in Rouen. MATTHIEU ALEXANDRE / AFP - Getty Images

The fire in Rouen was "totally accidental," Mayor Yvon Robert told Europe 1 radio, mentioning a possible gas explosion "from the heat in a basement room doubtless too small for the number of people present."

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More than 80 firefighters had battled the early Saturday blaze at the downtown bar in the city in Normandy, Robert said.

It was not known how many people were partying at the Cuba Libre club. The area has been blocked off and Interior Minister Bernard Cazeneuve said a judicial investigation has been opened to discover the cause of the blaze.

He gave an initial count of 13 dead and six injured in the fire, which started after midnight Friday. The prefecture, which runs the region, said one of the six injured was in a life-threatening condition.

Images from outside the bar on French television showed a large ground-floor window broken open, burned red bar chairs and a tattered awning in front.

The mayor said the fire was "very brief."

The birthday party was "a moment of joy for those involved that ... ended tragically," Robert said.

The local Paris-Normandie newspaper said the fire ignited in the basement of the bar, which was rented for the evening for the birthday celebration. The paper, without citing sources, said the ceiling, which was covered in sound-insulating material, quickly ignited when candles were lit.

The paper quoted the deputy prosecutor at the site as denying initial reports of an explosion. Laurent Labadie was quoted as saying that the noise reported by neighbors was the sound of windows exploding from the heat.

It's the second tragedy in Normandy in the a little more than a week. An elderly priest was murdered by ISIS-linked attackers who took several nuns and worshippers hostage in Saint-Etienne-du-Rouvray on July 26. Both attackers were later shot dead by police.