I think it's safe to say that regardless of who did this, this was a horrific act and there is no doubt some suspicions behind who might be behind the attack.
What I find annoying is that there seems to be a 'tug of war' game between two groups of people on this thread, as if far-rights extremism and Islamic extremism can somehow deflect blame on either ideologies.
Both ideologies are not acceptable. Whatever motives these attackers have will not alter the harmful nature of these ideologies. As a continent we are still not facing these arising problems seriously enough. We are more ready to shift blame than addressing both ideologies head on. At the end nothing is done but mere child's play.
A mixed solution is obviously required, I would like governments to start consulting and enforce prevention policies. Uncontrolled acceptance of refugees cannot remain in place. Countries must do more to encourage existing ethnic minorities to be included in society, not by positive discrimination but to ensure equal distribution of resources. Potential sources of radicalisation must be closely monitored. Governments must consider regulating faith schools and make sure that faith serves as an enrichment, not a curriculum.
However tough or 'politically incorrect', governments must start acting with meaningful purpose and direction. You cannot expect these ideologies to simply go away if you do nothing, they will simply grow.