ISIS again, probably
Everything else was pretty well-coordinated so far, why not this too?
After all, France was an easy sitting target, thinking that the Charlie Hebdo and Jewish deli attacks were simply isolated incidents, and that Islam was no real menace ...
France (and Europe) ignored & downplayed all the warning signs, and this is the result
The west, despite all the political bluster, is low hanging fruit for terrorist warfare. Just as guerrilla warfare is difficult for massed armies to combat, terrorism by its very nature is always in the shadows and can pop out at any time. Eternal vigilance is the price we pay for freedom both at home and away.