The head of European Union police agency Europol says the large number of radicalized Muslim extremists across Europe, their lack of command structure and growing sophistication make it "extremely difficult" for law enforcement agencies to foil every terror attack.

Europol chief Rob Wainwright told The Associated Press in an interview Friday that Europe needs to cooperate more closely to prevent attacks like last week's deadly rampage in Paris.

Wainwright says that "the scale of the problem, the diffuse nature of the network, the scale of the people involved makes this extremely difficult for even very well-functioning counterterrorist agencies such as we have in France to stop every attack."

Wainwright says at least 2,500 and possibly up to 5,000 suspects have traveled from Europe to conflicts in Syria and Iraq.