Agreed, let's shelve this until we know. However unless it's just some lunatic (escaped from an asylum, coming out of nowhere, working alone etc; so no, i'm not calling all terrorists lunatics although, in some ways at least, of course they are) I am willing to bet, ISIS or another group/person, it was religion based.
Which was why I added the possibility of a lunatic. But I was indeed thinking of that. As I just left the Netherlands when that happened and people were quite shook up.