GWEN IFILL: We return to the attacks in Brussels, and what they said about the growing Islamic State threat in Europe and elsewhere.

Daniel Benjamin was coordinator for counterterrorism at the State Department during the first term of the Obama administration. He's now a professor at Dartmouth College. And Joby Warrick is a national security correspondent at The Washington Post. He's also the author of the book "Black Flags: The Rise of ISIS."

Daniel Benjamin, was this a nightmare scenario that could have been foreseen? Yesterday, we heard people saying, this is what we feared.

DANIEL BENJAMIN, Former State Department Official: I think that people who have been watching terrorism have been fearing this for many years, actually.

The recognition that Europe had a problem with extremism in its midst and the recognition that Europe hadn't taken security arrangements as seriously as it should have, I think, has been common. That observation has been common in the security community for many years now.

GWEN IFILL: Joby Warrick, the Associated Press, among others, have been reporting today that there were as many as 400 people being trained by ISIS to carry out these attacks in Europe. So did they not leave any footprints or any signs?

JOBY WARRICK, The Washington Post: Well, the number's a little bit soft, I think, but it's not unrealistic to think that if you have 5,000 Europeans that went to Iraq and Syria to fight, if only 10 percent of them came back home, you know, that's a pretty good base for future attacks.

You know, Abaaoud, the guy who carried out the November attacks in Paris, claimed that there were 90 people that he knew of involved in cells getting ready to carry out attacks. So it's not something that can be easily dismissed.

GWEN IFILL: From what you know about from writing your book on ISIS, is this the sort of thing that the United States should be wary of? Are there footprints as well leading here?

JOBY WARRICK: Well, it's a different situation for us. Fortunately, we don't have the kind of radical communities that you see in Europe. We have radicalized individuals, but not so much a community problem.

And we also have, fortunately, oceans separating us from some of these jihadists. I think Europeans have a much more acute problem. They're really waking up to it now. It's almost too late for some of the action they're taking, because the threat is really at hand.

GWEN IFILL: So, Daniel Benjamin, you agree that this idea of alienation makes it different with Muslim communities or the radical Muslim communities in Europe and here?

DANIEL BENJAMIN: There's no question that there's a higher level of alienation and of radicalization.

European Muslims are several times more likely to go to fight in Iraq and Syria, perhaps as much as 10 times as much. And if you look at the — just the casualties that we have seen since 9/11, you know, there's been vastly more violence in Europe, 10 times as much at least, compared to the United States, where, despite all the concern, we have only had about 45 casualties, 45 deaths, due to jihadist violence.

GWEN IFILL: So, is it law enforcement that is dropping the ball in Europe?

DANIEL BENJAMIN: It's a much larger problem than law enforcement.

You have a very different kind of set of Muslim communities. They tend to be much poorer, much less educated. They have much less social mobility, much less access to education and even less political representation. So, it's really a societal problem.

It's important to note that the large majority of European Muslims are very, very peaceful and love their countries. In fact, when polls are taken, they tend to be more patriotic than the non-Muslim populations. But, because of these conditions, it is easier for a smaller number of extremists, still much larger than we have in the U.S., to swim in the same seas, if you will.

GWEN IFILL: Well, Joby Warrick, Daniel Benjamin mentions that there are only 45 casualties since 2004, including Madrid and Paris and…

DANIEL BENJAMIN: No, no, I'm sorry. In Europe, you have had closer to 500.

GWEN IFILL: In Europe, oh, 500.

DANIEL BENJAMIN: In the United States, you have had about 45.

GWEN IFILL: The other way around, that's right.

So, making the point — thank you — that there are that many people and that many signs, is this something that is operating so far under the radar that Belgian authorities or Europol or French authorities couldn't have had a heads-up about it, especially — I just want to add, especially since President Erdogan of Turkey today is saying that he made a warning?

JOBY WARRICK: Well, yes, I think the Belgians, in particular, have a problem. And they're very aware of it. And we're often saying these days that you're only as strong as your weakest link.

In the case of Belgium, you have got a country that — it's got a fairly small police force. If they have 100 — by conservative estimates, about 100 ISIS veterans have returned to that country alone, just so the manpower involved in conducting surveillance and watching all these people, watching their phone calls, conversations, it sort of taxes their police forces beyond their capability, not — let alone the problem of dealing with counterradicalization, and just all the tracking and surveillance they need to be doing.

So, yes, they have a problem there. They have a problem with coordinating with other agencies and other governments. And so, yes, that sets a huge problem for them. And I think they really are aware of this now, but it's kind of late to the party.

GWEN IFILL: Has the migrant crisis that we have seen, we have spent a lot of time talking about on this program, has that exacerbated the problem?

DANIEL BENJAMIN: It absolutely has.

We know, for example, in the case of the Paris attacks, that at least one of the perpetrators came in with the flow of migrants. And, you know, it's had a pretty destabilizing effect. And it's quite possible — you spoke earlier of a report of 400, which I find to be really high number of potential infiltrators.

But, certainly, Europe has had very weak external borders. Its internal borders in the Schengen area are nonexistent, so it's very easy for bad actors to move around, and the migrant crisis has really exacerbated things.

GWEN IFILL: Joby Warrick, what do you think about that?

JOBY WARRICK: Yes.

And it's obviously something that people talk about politically in this country, but it's a very different situation for us here. There's really much more vetting of refugees, potential refugees that come into this country. It hasn't really existed in Europe. And now we have got a huge problem that already exists, and it's in they're having to retroactively go back and try to figure out who some of these people are, and it is clear that some bad people came in with the rest.

GWEN IFILL: So, would you both say that maybe the root of this is more likely to be the civil war in Syria than because of this, or the civil war in Syria, which is exacerbating, which is causing the migrant crisis and also causing this kind of radicalization?

DANIEL BENJAMIN: So, there is multiple causality.

Certainly, the Syrian civil war has made a huge difference and had a profound effect on radicalizing people who have wanted to go to Syria and fight to defend Sunni Muslims who they found were being treated appallingly, as we have seen.

At the same time, you know, security services in Europe for the most part — and their bureaucracies in general — never had that catalytic moment that we had after 9/11, and they have never removed their stovepipes. They have never really forced themselves to cooperate as well.

There are times in the U.S. government when you would find that part of a particular foreign European government had one set of information, but didn't want another part to know, but we knew. And, you know, it's a remarkable situation, but, really, until you have one of these horrific attacks, there isn't a sufficient prod to reorganize.

GWEN IFILL: Daniel Benjamin of Dartmouth College, and Joby Warrick, author of the book "Black Flags: The Rise of ISIS," thank you both very much.