All together, there are 14 million Muslims in Europe.
However, in France, Germany, and Belgium, three countries in close proximity to one another, there are 10.2 million or 73% of all the Muslims in Europe. There have been 2 significant terror attacks, one in France and one in Belgium, in just a span of months. But, be aware that it is Germany that has the highest Muslim population of all three, at 4.8 million or 38% of all Muslims in Europe. That is significantly higher than the U.S. whose Muslim population of 3.3 million is in a country with a population 4 times greater than Germany's.
What we know is that radicals within Islam are somewhat proportional to the size of the Muslim population and not necessarily size dependent on the social environment they live in. One study believes, that modestly, 1% of Muslims in Europe are radicalized. Therefore, in Germany, there could be as many as 53,000 radicals who would be either willing to die for their cause or carry out some type of terrorist attack.
So, this begs the question. Is Germany the next to be significantly hit in Europe?
5 facts about the Muslim population in Europe: http://www.pewresearch.org/fact-tank/2015/11/17/5-facts-about-the-muslim-population-in-europe/
How many Muslim extremists are there? Just the facts, please: http://www.csmonitor.com/World/Security-Watch/terrorism-security/2015/0113/How-many-Muslim-extremists-are-there-Just-the-facts-please
German airports on high alert after Brussels explosions: http://www.thelocal.de/20160322/germany-tightens-airport-controls-after-brussels-explosions
Terrorism in Germany: https://en.wikipedia.org/wiki/Terrorism_in_Germany