Very sad news. May the victims rest in peace. I wish their families patience and solace.
I don't think this attack has anything to do with "Islamic extremism." Not once in my whole life have I read or heard any fatwas agreed on by the majority of mainstream Islamic scholars implying that driving a truck into a crowd of innocent people, Muslim or not, is permissible at all. Actually, I decided to make extra sure just now, so I googled fatwas in Arabic about whether or not it is permissible to kill peaceful non-combatant non-Muslims, and even the fatwa I found on a fundamentalist Saudi website says that no, it isn't.
At this point I feel like some people would still blame "Islamic extremism" if an insane Muslim decided to try to blow up the Kaaba. There are many solid grounds on which to criticize Islam. I don't think harping on repeatedly refuted misinformation is one of them.