A woman in her 70s who worked at a design school has died as the result of an explosion in central Paris a month ago that destroyed a historic building and ignited a huge fire in the Left Bank neighbourhood, bringing the death toll to three.

One body was found in the rubble six days after the June 21 blast, and a 59-year-old insurance agency worker later died in the hospital. A third critically injured person died on Thursday, the Paris prosecutor’s office said.

The third victim was a woman born in 1946, the prosecutor’s office said. She had worked at the Paris American Academy, a school in the collapsed building that specialises in design and arts, according to Florence Berthout, mayor of the city’s 5th district.

The blast, not far from the Luxembourg Gardens, left six people critically injured and more than 50 with lighter injuries or in psychological shock, according to the Paris prosecutor.

A manslaughter investigation is under way, with a gas leak among the possible causes being examined. Prosecutors are looking into whether the explosion was caused by intentional violation of safety rules.