A previously unheard-of group, The Secret Organization of Al-Qaeda in Europe, is claiming responsibility.
I recommend following the BBC or Washington Post on this; they seem to be on top of it so far.
My initial reactions:
No warning - no intelligence "chatter" beforehand. That's significant - I'm just not sure how yet.
If these guys are really a European-based group, what can the Europeans do about it?
Find those responsible, and destroy them without mercy. They target civilians. They operate outside the rules of war - and therefore are ineligible for any rights under those rules.
Updating at The Wasatch Front.