The head of European police said Sunday there could be “possibly thousands” who have become radicalized Muslims and could carry out attacks in Europe.

Europol Director Rob Wainwright said European forces were engaged in a “determined police response” following terror attacks in Paris, but cautioned that they were fighting against a foe large in scale and loosely organized.

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“It's spread across so many European countries, perpetrated by community of possibly thousands of people who have been radicalized on the Internet by their conflict experience in Syria and Iraq,” he said on ABC’s “This Week.” “Many of them have returned to European society, perhaps some of them with the intent and capability to carry out the attack.”

Wainwright said he did not believe there to be an organized network of people, but rather loosely affiliated people outside of a traditional command structure, many self-motivated by online resources. That arrangement makes it much more difficult for police to track down and prevent attacks.

“It's something much more difficult now, moving rather insidiously in our communities and across the Internet in particular.  It's a real challenge for police right now,” he said.

To underline that point, Wainwright said the attackers behind the Paris shooting were part of a cell thought to be dormant since 2006. European security forces typically focus investigations on Westerners who have recently returned from areas like Syria and Iraq. But the fact that these attackers, once thought to be out of action, emerged to attack makes their work that much harder, Wainwright said.

“It’s much more complex than we thought,” he said. “It's important that we increase the scale of our international collaboration, also with the U.S. authorities, so that we can get a better picture of the threat that they pose to society at the moment.”