A strong explosion hit a building in Paris’ Left Bank on Wednesday, leaving four people injured and igniting a fire that sent smoke soaring over the domed Pantheon monument and prompted the evacuation of buildings, police said. The cause of the blast was not immediately known.

The facade of a building in the 5th arrondissement, or district, collapsed and emergency services were working to determine if anyone was still inside, a Paris police official said.

Florence Berthout, mayor of the arrondissement, said four people were in “absolute emergency” condition.

“The explosion was extremely violent,” she said, describing pieces of glass still falling from buildings.

The neighborhood was cordoned off and scores of emergency workers filled the area.

Paris police spokeswoman Loubna Atta said it was too early to determine the source of the fire and could not confirm reports it was a caused by a gas explosion.