GWEN IFILL: We return now to the Paris attacks, and the story of Abdelhamid Abaaoud, the ringleader who managed to return to Europe undetected after joining Islamic State in Syria.

To help us understand more about him and other radicalized Europeans like him, I'm joined by Wall Street Journal reporter Stacy Meichtry, and Lorenzo Vidino, director of the Program on Extremism at George Washington University.

And welcome to you both, gentlemen.

Stacy Meichtry, how do — tell us everything that we know about Abdelhamid Abaaoud.

STACY MEICHTRY, The Wall Street Journal: Well, he comes from a suburb of Brussels, where his family were — they were business owners. They had a house. And as far as they were concerned, they were a normal family.

As he got older, he started a sort of career of petty crime, getting convicted for assault, breaking and entering, and he ended up serving prison terms in three different prisons. At some point, he decided to travel to Syria. And it was there where he apparently rose to significantly a high rank for a foreign fighter in Islamic State territory.

He was known as an emir of war, which is a military commander.

GWEN IFILL: So, he was part of his parents' retail business in Belgium until 2013, and sometime between now and then, he not only got involved, but he became a leader in this.

How significant is it that he was taken down today?

STACY MEICHTRY: It's very significant, first of all because it deprives ISIS of a significant commander.

Second of all, this is somebody who managed to construct a broad network of associates capable of carrying out attacks. You know, the interior minister of France today said that he was involved in up to five different plots against European soil.

And when they raided the apartment, his hideout, police came up against some significant firepower. And it was a pitched battle that lasted hours. This is someone who was capable of assembling a significant arsenal.

GWEN IFILL: Lorenzo Vidino, how unusual is this, what we're seeing here, what we're having described here?

LORENZO VIDINO, George Washington University: Unfortunately, not that unusual.

We know that there are up to 5,000 Europeans who have gone to Syria and joined ISIS and other jihadist groups. We see quite a few of them that have gone back. Now, of course, not all those that go back are intent on carrying out attacks.

I mean, Abaaoud was somewhat of an exception, in the sense that he was specifically going back and forth between Syria and Europe to develop a network of operatives and mobilize them to carry out attacks.

But, unfortunately, the numbers are very high and there are a lot of problems for European intelligence agencies to monitor such a large number of people.

GWEN IFILL: Well, that's interesting to me, because people knew about him. They knew a little bit about his movements. Why wasn't he tracked, or was he being tracked?

LORENZO VIDINO: Well, we have got to find out the details of exactly what happened.

And, clearly, that was a failure of intelligence on the French part. But, again, I think there are a couple of things to be mentioned here. First of all, as I said, the numbers, it's so difficult to track such a large number of people.

The French alone have up to 1,500 people who have gone to Syria to fight. And they have 11,000 individuals that they consider radicals that they have to monitor. Obviously, the manpower required for that is enormous.

Add to that the fact that people can cross borders in Europe without having to go through any barrier and having to show a passport. So there is freedom of circulation, but intelligence agencies are still divided by countries.

So he could have gone in from a certain European country, and the French would have not been notified by them. So there is this division among different European countries, and the terrorists exploit it.

GWEN IFILL: Stacy Meichtry, what do these attacks and this investigation, what do they tell us about how these kinds of terror networks work?

STACY MEICHTRY: Well, I think what's remarkable about this case is that, previously, we had assumed that Islamic State in particular was almost a local militia. Their focus was to build a caliphate in Syria and Iraq in the Middle East.

What this particular attack demonstrates is that they have really sort of expanded and developed the capability to launch indirect attacks on Western soil. They really have become sort of a global syndicate capable of activating operatives in the field.

And in the case of Abaaoud in particular, this is somebody who was able to slip back and forth across European borders, go to Syria. He was seen evidently as recently as a few months ago in Greece. That was today's revelation. So, you know, this really caught French officials off guard.

GWEN IFILL: And we're discovering now that he had other plots in mind?

STACY MEICHTRY: In fact, we spoke to French officials today, and they told us that they believe that either he or the militants involved with him were planning an additional attack on Montmartre. And that, of course, is the neighborhood that hosts Sacre-Coeur.

It is one of the most cherished neighborhoods in Paris. There were other plots afoot. Apparently, he was also thinking of launching an attack against Paris' business district, La Defense.

GWEN IFILL: Lorenzo Vidino, why Belgium? Why is Belgium such a hotbed for this kind of activity?

LORENZO VIDINO: Well, Belgium, as a tradition, has given asylum some 20 years ago to a lot of very radical people who in a way have radicalized a new generation.

There have been a few groups that operated there. And they were not necessarily violent a few years ago. But when the conflict in Syria broke out, they started recruiting and mobilizing and crossed the threshold from extremism to violence. And the Belgians never really cracked down on them.

It's also a country that has a lot of political problems internally, division between the French-speaking part, the Dutch-speaking part. The divisions, political divisions inside Belgium prevent the formation of a unified law enforcement and intelligence agency.

So, they haven't really cracked down on these networks. There is a lot of links also to organized crime. That is why it's so easy for terrorist groups to get weapons on the black market in Brussels and other places in Belgium. So, it is somewhat, to some degree, the weak link in the chain of counterterrorism in Europe.

GWEN IFILL: We heard that French — I think it was the prime minister, the foreign minister, someone, say today that they were worried about chemical attacks and biological attacks in the works.

Do we know what that's based on? Have we heard, have you heard any reporting, Stacy Meichtry, that suggests that that's the next worry?

GWEN IFILL: That's all right.

Mr. Meichtry first.

STACY MEICHTRY: Yes.

It's a constant concern. We don't know exactly what the prime minister, what his sources were. Obviously, when we're talking about Islamic State, you're talking about a group that now occupies territory where there were the materials to make chemical weapons.

Now, evidently, there was an agreement struck in order to de-weaponize those arms, but you never know. There are concerns always that there are still materials out there.

GWEN IFILL: OK.

Lorenzo Vidino of the George Washington University Program on Extremism, and Stacy Meichtry of The Wall Street Journal, thank you both very much.