On Wednesday and Thursday, police in a Paris suburb arrested three men who were in possession of explosives. A construction worker raised the alarm after seeing suspicious materials through the window of an apartment belonging to one of the suspects. French authorities say the men were planning attacks on banks and had been talking with someone in Syria.

Coming just two weeks after the terror attacks in Barcelona, these arrests suggest there might be an undetected Islamic State cell operating on European soil. The possibility has to be considered for a number of reasons.