The #1 thing you can do is to name the party responsible (once it is known for certain), and to name the ideology that motivates these repeated, ever more brazen attacks. If this is what everyone thinks it is (but is cautiously tip-toeing around), "extremism" is not specific enough.
Didn't you just tip-toe around it?
Now that the Islamic State has officially claimed responsibility, I can say that this ideology is fundamentalist Islam.
I highly recommend this podcast by Sam Harris on this matter: https://soundcloud.com/samharrisorg/still-sleepwalking-01