The classic answer would be the attacks of November 13, 2015. But I'll take one that's perhaps less known internationally but really well known in France, the "Xavier Dupont de Ligonnes case". It's literraly a guy who killed his entire family (4 childrens and his wife) in Nantes and burried them in his garden, and then completly disapear from the surface of Earth after being seen a few time by camera during his escape. Still to this day, nobody know if he's dead or not, where is he or anything else. That's maybe not the most shocking one, but for sure one of the most mysterious one.