Frankfurt - especially the neighborhood near the Hauptbahnhof - is lousy with Muslims. Germany's open borders will cause its demise.
Yes, for decades and decades, Germany, West Germany has had many Turks... so to begin with, those will be Muslims and then, a lot of Kurds as well. It sounds like the domestic governments of Turkey even pay attention and campaign with these communities in foreign countries to get their vote... let alone, once in awhile, the Kurds are in the news as well. I think the two communities have even had some "skirmishes" with each other.
But I don't want to mislead, these may well have been Syrians, Iraqis and Afghanis. They are not telling us yet at least, in these English-language articles. Apparently, Germany also holds back last names sometimes. Surely, they will tell us.
German police arrest 10 people on suspicion of planning Islamist attack
...
"They had already made contact with different arms dealers, rented a large vehicle and collected financial assets to use for the purchase of guns and the execution of the planned murders," a spokeswoman for the prosecutor's office said.
More: https://www.bbc.com/news/world-europe-47677657
Last October:
Syrian man 'planned bomb attack' in Germany
https://www.bbc.com/news/world-europe-41816348