At least 13 people have died and six people were injured after a “totally accidental” fire ravaged and destroyed a bar in Rouen, the capital of Normandy, French officials said. Most of the victims were young people attending a birthday party.

Incendie dans un bar à #Rouen 13 morts 7 blessés dont un grave @RTLFrancepic.twitter.com/GQJnZbCos3 — frederic veille (@fredveille) August 6, 2016

The fire started after midnight at a bar where young people had been gathering to celebrate a private birthday party in the northern city of Rouen, the Paris-Normandie newspaper reports. Firefighters responded to the emergency, and have declared 13 people dead so far.

Mayor of Rouen, Yvon Robert, who arrived at the scene as soon as he heard of the fire called the tragedy "the greatest catastrophe".

The fire was "totally accidental," Robert told Europe 1 radio, adding that the blaze was very “brief.”

The birthday party was “a moment of joy for those involved that ... ended tragically,” he said.

#Rouen : Explosion dans un premier temps et incendie dans le bar "Le Cuba Libre". Au moins 13 morts. pic.twitter.com/vWVk04pcao — Betty De Beketch (@betty_debeketch) August 6, 2016

French Interior Minister Bernard Cazeneuve confirmed the initial count of 13 dead and six injured, saying that more than 50 firefighters were battling the blaze in the center of the city, AP reported.

[Communiqué] Incendie dans un bar du centre-ville de Rouen. pic.twitter.com/G4fVBXpyd2 — Bernard Cazeneuve (@BCazeneuve) August 6, 2016

"Judicial inquiry is underway to determine the cause of the fire," Cazeneuve added.

A birthday cake with candles and sparklers fell to the floor, which set the carpet ablaze, Reuters reported, citing police.

"There was no explosion, candles used for a birthday party, ignited the ceiling made of polystyrene, releasing gas that poisoned the victims," police said as cited by Paris-Normandie.

#Rouen#France - Multiple fatality - multiple casualty incident - Explosion & fire in a nightclub - https://t.co/yaeYg3KN8P — Hal Newman (@BigMedicine) August 6, 2016

Police and fire emergency units were called in at 12:50am local time when the blaze broke out in the basement of the Cuba Libre bar.

Dans un bar de #Rouen la soirée d'anniversaire tourne au drame 13 morts et 1 blessés dans l'incendie d'un bar pic.twitter.com/uNR05x4Jkt — frederic veille (@fredveille) August 6, 2016

It took firefighters some 30 minutes to bring the flames under control. First responders managed to pull out six injured people, all aged 18 to 25. By 3:00am, the fire and rescue operation was complete.

Firefighters discovered thirteen bodies inside. The bodies of the victims were transported to the University Hospital of Rouen, where a psychological team gathered to helpfamilies deal with the crisis.

Explosión accidental de bar en Rouen, Francia deja al menos 13 personas muertas y 6 heridos pic.twitter.com/Fe9J5xGjNa — @AereoMeteo (@AereoMeteo) August 6, 2016

Eleven witnesses were taken to Rouen Police Hall to be interviewed to determine further details about the incident.

Local deputy prosecutor, Lawrence Labadie, emphasized that there had been no explosion that triggered the fire. Local residents who reported an explosion heard glass shattering as a result of the heat caused by the fire.

13 killed , 8 injured after a fire broke out at a bar in #Rouen, of #Francepic.twitter.com/q7ZvuZaMaZ — Rahul Singh (@rahulreports) August 6, 2016

The publication also said that investigators are trying to determine why the basement ceiling was so quick to combust. Insulation material used to soundproof the room could have been a problem.