Actually the number of Europeans (European nationals) that have joined ISIS does not exceed 2500 from what I recall and a very large percentage of those are actually native converts.
That happened (the thing you describde in your last sentence) in Oslo in a area that is well-known for being inhabited by Somalian people. It's basically a ghetto.
The most well-known proponent of ISIS in Norway is actually a Norwegian Islamist (Arslan Maroof Hussain) of Pakistani descent;
SV-tiltak mot ekstreme islamister - nyheter - Dagbladet.no
The problem is that a relative small (if not a tiny) minority among Muslims are causing trouble and mostly those are children of the lowest social classes. Meaning either people that wanted to escape their countries of origin due to their economic situation (often used political persecution as an excuse) or war refugees. Very few highly educated persons of Muslim background have emigrated to Europe (compared to those immigrants of a lower social class) and they never cause any trouble outside the 1-2 odd cases.
Basically those people and their descendants would have caused (or had the potential at least) similar problems in their native lands.
From my experience in France and Denmark and elsewhere in Europe the average Muslim is doing all-right and many of the descendants of Muslim settlers are actually outperforming the locals academically. This is evident in universities in various countries were people of Muslim heritage are overrepresented compared to their actual percentage of the total population (which is at most 10% as in France but usually less than 5%). Especially Muslim females.
Besides my impression is also that the second and third generations of Muslim immigrants are becoming more and more indistinguishable from the local natives and less religious for each year contrary to popular belief.
It is perfectly normal to see 10-20% of night club guests etc. at a night out in Paris or Copenhagen being of a obvious Muslim background. Seen that countless of times.
Anyway we never hear about those average Muslim Joe's in the media or the successful ones as they are not interesting. Only the few retards get all the publicity and many people base their sole knowledge about Islam and Muslims and impressions on that tiny minority.
That's just how it is.
Oh, regarding this news of this thread then it was not really a mosque but rather a room in a basement that is part of a larger housing complex. Basically there are no "real" mosques in Scandinavia aside from 10-15 mosques if not less. On top of my head that is.
This "mosque" in question belonged to a Somali community center that was based in a suburb of Western Stockholm.
Having ties to both world's and actually having lived in both "Christian" and "Muslim" countries gives me a good background to make such conclusions.
Lastly on the European right (politically wise) it is very common to blame Muslims for all the problems regardless of it being total bogus or partially correct. So that's also why you have the publicity that you currently have.
I am not sure how it is in the US as I have only studied in the US for 1 year (California) and I did not meet many Muslims aside from Arab students and other Muslim students here and there.
If you want more information then just ask.
P.S: No Western European countries accept non-European immigration (they are becoming very harsh toward Eastern Europeans that are even a part of EU) to the extend that they once did. The crisis in Syria and Iraq have changed that a bit recently but those new migrants (war refugees) are not guaranteed citizenship and probably won't receive it outside of those educated ones that the societies can use to their benefit economically.
