A student at Ecole des Mines on Boulevard Saint-Michel told Le Parisien: “I was in front of the Val de Grâce, I heard a huge boom and I saw a ball of fire 20 or 30m high. And the building collapsed with a huge noise. I smelled gas, but took several minutes to come to my senses.”
Is there another source reporting that?
Witnesses reported smelling gas before it happened. BBC news report stated.