PARIS >> A strong explosion hit a building in Paris’ Left Bank today, leaving 24 injured and igniting a fire that sent smoke soaring over city monuments and prompted the evacuation of surrounding 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. The explosion happened near the historic Val de Grace military hospital.

Paris police chief Laurent Nunez said the building where the explosion occurred was a private school, the Paris American Academy, which was founded in 1965 and offers teaching in fashion design, interior design, fine arts and creative writing.

The fire was contained but not yet extinguished. Some 270 firefighters were involved in putting out the flames and 70 emergency vehicles were on the scene.

A Paris police official told The Associated Press that 24 people were injured, including four in critical condition and 20 with less severe injuries. The official says the injuries were sustained mainly when people were blown off their feet by the blast.

Officials from the 5th arrondissement attributed the blast and blaze to a gas leak.

District Mayor Florence Berthout said, “The explosion was extremely violent,” describing pieces of glass still falling from buildings.

The Paris prosecutor said an investigation was opened into aggravated involuntary injury and the probe would examine whether the explosion stemmed from a suspected violation of safety rules.

Paris Prosecutor Laure Beccuau said investigators would seek to “determine whether or not there was failure to respect a rule or individual imprudence that led to the explosion.”

Nunez, the Paris police chief, said firefighters prevented the fire from igniting two neighboring buildings that were “seriously destabilized” by the explosion and evacuated. The explosion blew out several windows in the area, witnesses and the police chief said.

Smoke was no longer visibly rising from the building by this evening. Sirens still wailed as ambulances passed through the neighborhood, but residents were starting to move freely again on the street, rue Saint-Jacques, which was cordoned off earlier.

A student at the private school said he was in a building about 100 meters (yards) from the explosion.

“I was sitting on the windowsill, and we moved 2 meters away from the window, carried by a small blast (from the explosion) and huge fear,” Achille, whose last name was not given, told BFM television.

“We came down (from the building) and saw the flames,” he said. “The police gave us great support and we evacuated quickly.”

With more than 2 million people densely packed within the city limits and historic, sometimes aging, infrastructure, Paris is not a stranger to gas explosions. A January 2019 blast in the 9th district killed four people and left dozens injured.