A blast ripped through a street in the busy Latin Quarter of central Paris on Wednesday, causing the facade of a design school popular with foreign students to collapse, blowing out windows and starting a huge blaze.
At least 16 people were injured, including seven who are in a critical condition.
The Paris prosecutor's office said it was too early to say what caused the blast.
But the local deputy mayor, Edouard Civel, referred to a gas explosion in a Twitter post and witnesses told BFM TV there had been a strong smell of gas moments before the blast.
Television images showed rubble from the Paris American Academy strewn across the Rue Saint-Jacques and smoke rising from at least two nearby buildings that were ablaze.
More than 200 firefighters were involved in the emergency response. Paris police chief Laurent Nunez said later that the blaze had been brought under control.
Rue Saint-Jacques in the 5th arrondissement of central Paris leads from the Notre-Dame de Paris Cathedral to the Sorbonne University and the Val de Grace military hospital and is a few blocks from the popular Jardin du Luxembourg.
The area is usually packed with tourists and foreign students in the early summer.
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