Police in France, Belgium and Germany have swept up more than two dozen suspected terrorists in the last couple of days as investigators search Western Europe for possible militants. The threat of hundreds of radicalized European citizens returning from Syria, Iraq and Yemen seems urgent.

Earlier this week, the head of Europol said that as many as 5,000 Europeans have traveled to Syria to join the fight there. And the challenge now is how to track these individuals, as well as what are called lone actors who might be able to evade scrutiny.

Raffaello Pantucci is the director of international security studies at the Royal United Services Institute. He joins us from London. Thanks so much for being with us.

RAFFAELLO PANTUCCI: Thank you.

SIMON: How effective are these raids as a generalization?

PANTUCCI: Well, I mean, I think, certainly, the raids that we've seen in the past week in France, Belgium and Germany in particular seem to have been targeted on very specific networks. And at least when we're looking at the Belgian case, they seem to disrupt what looked like a very active group of individuals who were plotting an atrocity.

SIMON: At the same time, do some of these raids just tip off other members of the network and they go to ground?

PANTUCCI: Well, I think the risk always is when a terrorist operation like what we saw happen in Paris happens, immediately there's a sort of pulse of concern we'll go through; both the groups of individuals are concerned, but also security services. For security services, the question of looking again at the books and reassessing maybe people you were looking at before and just making sure that you have sight of all of your networks.

On the other side of the equation, for individuals who might have been thinking of doing a terrorist act, this sort of immediate wake of a large and successful incident like what we saw in Paris is something that they will try to capitalize on.

SIMON: And Mr. Pantucci, what are security services in Europe able to arrest them for if all they're doing is planning?

PANTUCCI: I think it depends what they're planning to do. I think in those other countries now you're increasingly seeing legislation that means you can be convicted for planning to go join an organization or conduct a terrorist act abroad. And I think some of the arrests we saw take place this week were linked to that.

On the other side, if you've got people who are, you know, planning a direct terrorist act, the more complicated people are the individuals who've maybe returned from a battlefield like Syria and Iraq. And if the intelligence services know that they've been out there, but at the same time this individual hasn't got any, you know, photographic evidence or other evidence on their person that shows that they were there, then, you know, it becomes a much, much harder picture to actually know what you can exactly arrest them for.

SIMON: How do security agencies try and track someone who comes back from Syria, say?

PANTUCCI: So in a lot of cases, you know, authorities have some awareness of them because a family member might have mentioned it because the individual may have been on a sort of broader security radar, and then suddenly their telephone goes off in Turkey.

You know, so there are some indicators of people they should be watching out for. But the reality is that when you're looking at the numbers that we're seeing going back and forth and the relative ease of mobility from a lot of continental European countries to Turkey and ultimately to Syria and Iraq, it does become very difficult to know that you've caught everyone or that you're watching everyone.

SIMON: What are some European countries doing or perhaps contemplating that they figure they ought to do to disrupt terrorist networks?

PANTUCCI: I mean, a lot of what you're seeing done is, frankly, traditional counter-terrorism work. I think what's noble about what we're seeing in Syria and Iraq is the sheer volume of it and the sheer volume of potential people of concern. And that in itself is something that has really magnified an existing problem because the problem from a European security force perspective is that, you know, you not only have to deal with this huge issue of Syria and Iraq, but you've also got enduring problems from before.

As I think we're potentially seeing in Paris, where there was some sort of connection to al-Qaida in Yemen in the group. You know, there are other terrorist groups out there that are keen to try to launch attacks. And those have continued while sort of the Syria and Iraq questions has sort of grown the problem even more. And so in some ways, it's a capacity question.

SIMON: Mr. Pantucci, what keeps you up at night?

PANTUCCI: I think the biggest concern that people have increasingly started to circle around is what's described as the lone actor terrorist threats. These are the sort of terrorist threats that don't appear on existing radars. But in some ways, I think what's of greater concern is the fact that if you look at some of the recent incidents of terrorist plots we've seen that have been connected to things that happened before Syria and Iraq, you can see that you've got a community of people who are very radicalized and have maintained these sorts of violent, radical ideas in their heads for very long periods of time. And the point at which they sort of become interested in these ideas to the point at which they take action is a process that can stretch over almost a decade.

SIMON: Raffaello Pantucci is director of the international security studies at the Royal United Services Institute in London. Thanks so much for being with us.

PANTUCCI: Thank you.