THE HAGUE, Netherlands -- 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, the head of European Union police agency Europol said Friday.

Europe's security and law enforcement agencies need to co-operate more closely to prevent attacks like last week's deadly rampage in Paris, Europol chief Rob Wainwright told The Associated Press in an interview.

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

At least 2,500 and possibly up to 5,000 suspects have travelled from Europe to conflicts in Syria and Iraq, he said.

"The sheer numbers of people involved, the way in which they've been radicalized on the Internet, radicalized by their engagement in the conflict in Syria and Iraq, makes this extremely difficult for the police to contain it in a complete way," he said.

In a grim assessment by the man who leads efforts to co-ordinate crime- fighting across the European Union, Wainwright said the threat is evolving as cells controlled by a clear commander have been replaced by "thousands of independent actors or semi-independent terrorist suspects" who are difficult to track.

"That's the real problem I think that the intelligence community faces right now," he said.

Wainwright praised Belgian authorities for the counterterror operation Thursday night that they said foiled an imminent attack and left two suspects dead in the eastern town of Verviers.

Wainwright said Europol was in close contact with Belgian authorities in the days before Thursday's operation in Verviers.

"I can say that a number of very effective police and security service operations have been carried out in recent years," he said, but wouldn't identify any. "The number of those has increased in the last year especially."

In the face of the threat, "we have to make sure therefore that we can work together in a better way across Europe," he said. That could include legislation under consideration in the European Union to share airline passenger data.

"I think it would add value to the counterterrorist program," Wainwright said.