GWEN IFILL: We return now to the hunt for terrorists in Europe.

Today's raids against armed militants in Belgium highlighted the trend of European Muslims traveling to Syria and Iraq to fight. Europol says that as many as 5,000 Europeans have joined the conflict there. Hundreds have returned to Europe, where authorities fear they may use their military training to launch attacks.

For more on this, I'm joined by Lorenzo Vidino of the European Foundation for Democracy. He studies Islamism and political violence in Europe and North America.

So, officials say there is no direct connection to the Paris attacks, Mr. Vidino, but they seem awfully similar.

LORENZO VIDINO, European Foundation for Democracy: Yes, absolutely.

What we see right now in Belgium is two different situations. It was the — allegedly the arrest of the individual who provided the weapons to Coulibaly, the man who attacked the supermarket in Paris, but they also allegedly thwarted a plot which traces its routes to Syria to carry out attacks in Belgium, so a very tense situation many Belgium with two ongoing situations.

GWEN IFILL: Is the Syria connection particularly problematic in Europe, where the borders are so porous?

LORENZO VIDINO: Oh, absolutely.

First of all, the borders are porous when it comes to Turkey. So, it's very easy for Europeans to go to Turkey. In many cases, you don't even need a passport. Just with an I.D., you can travel to Turkey. And then it's easy from the Turkish border the make your way in into Syria.

Equally easy is to come back from Syria to Turkey and then to European countries. And, finally, it's very easy to go from one country and another. There is no border anymore between European countries. One travels like you travel from state to state in the U.S. The problem is that the police does not have the same power, so there are borders for authorities.

So French police cannot investigate and operate in let's say Belgium or Spain, but the terrorists move freely. That's a big problem for Europe.

GWEN IFILL: You're in Milan tonight in Italy and you work in Belgium. Is there — from the people you talk to on the continent, is there a particular nervousness, tension about the potential for these kinds of attacks or plots to spread?

LORENZO VIDINO: Yes, obviously, you do see a larger presence of police in many potential targets.

I don't think there's panic in terms of population, but obviously that's what people talk about. It's in the media. And you do see some tensions in society. So, obviously, these are tense times. It's nothing new in European history. We have dealt with Islamist terrorism for the last 15 years. We have had other forms of terrorism, right-wing, left-wing, nationalist, in the past.

So it's really nothing new. At the same time, obviously, there are some tensions. And it's a new form of terrorism which is in some way particularly insidious. So it's problematic, indeed.

GWEN IFILL: In Brussels lately and in Belgium in general, there have been — there's a history of some attacks recently.

LORENZO VIDINO: There was indeed another attack back in May, where another returnee from Syria, French individual who was radicalized in prison, traveled to Syria, fought for one year in Syria, then came back to Europe, managed to get an AK-47 and went inside a Jewish museum in Brussels and killed four people. He was then subsequently arrested.

A lot of the dynamics that we have seen now with France and Belgium were already visible back then. We do see these sort of dynamics throughout Europe, where there have been several plots thwarted in many countries, from the U.K. to even peaceful Switzerland and Sweden. And so it is a problem.

The numbers you mentioned are particularly problematic, up to 5,000 individuals. That's a huge number that creates a lot of problems for authorities to monitor them. There's a lack of manpower and there is obviously a legal problem in monitoring all these people.

GWEN IFILL: Is there a distinction or a distinction without a difference between Islamic State militants and al-Qaida militants in terms of trying to trace the source of all of this activism and terrorism?

LORENZO VIDINO: No, I think it's — there's obviously an importance.

We know the two groups are at odds. They are actually in competition on some level, and at the leadership level, there is a bit of a rivalry between the two. And in a way, the argument is that one is trying to outdo the other by carrying out spectacular and symbolic attacks.

I think, at the grassroots level, when it comes to aspiring terrorists, aspiring jihadists, for them, those distinctions don't really make much sense. People who want to sort of get involved a bit, embrace this ideology and want to mobilize, to them, al-Qaida, the Islamic State make little sense. It's a distinction, but it doesn't really matter.

What they want to do is do something, whether it's in Syria or here in New York. Whether they do it with al-Qaida and the Islamic State, it matters very little to them.

GWEN IFILL: Lorenzo Vidino of the European Foundation for Democracy in Brussels, thank you very much.

LORENZO VIDINO: Thank you.