AMNA NAWAZ: Taylor Swift's Eras Tour has been called off in Austria after officials arrested two teenagers accused of plotting a terrorist attack.

The concert promoter, Barracuda Music, said that -- quote -- "We have no choice but to cancel the three scheduled shows for everyone's safety."

Roughly 200,000 people were scheduled to attend the sold-out Vienna concerts.

For a closer look at this, I'm joined by Juliette Kayyem, professor at the Harvard Kennedy School of Government and former assistant secretary at the Department of Homeland Security.

Juliette, thanks for joining us.

The fact that these organizers had to cancel the shows, what does that say to you about how serious, how imminent these threats were?

JULIETTE KAYYEM, Former U.S. Department of Homeland Security Assistant Secretary: It says two things, first, that not only was it serious, but that this was disrupted likely hours before there could have been extreme violence.

And I think they canceled for two reasons.

One is, there was an insider threat.

One of the two teenagers had some affiliation with probably some -- one of the sub-vendors, may have had credentials that would have allowed him to go in and out.

Once you have an insider threat, you close the thing down, because you have no idea who else could be in the room.

The second is that the threat appears to have been in the soft outside perimeter, rather than inside the concert hall.

Anyone who's seen these pictures of the Eras Tour knows that there's lots of people with tickets.

There's lots of people without tickets who want the experience, want to hear her, want to be part of the Taylor Swift movement.

If -- and that -- those parts are not protected.

And so I think it was right and correct, given the threat that we know right now of both insider threat capacity and targeting the soft areas of the concert security, that you would want to shut it down until you understood what the threat was.

AMNA NAWAZ: So, as we mentioned, we know two people have been arrested.

Bomb-making materials were reportedly found in their homes.

One suspect, as you mentioned, even had that job inside concert operations.

What do we know from what authorities have shared so far and what's been reported about what they were trying to do exactly?

What was the threat?

JULIETTE KAYYEM: So I'm going to put this in terms of the global threat right now.

So ever since the Israeli-Hamas war started, everyone has known that the global terror threat has increased.

Part of that is just in response to the war itself.

Part of it is just sort of the radicalization movement gets inspired by what is going on in Gaza and Israel.

So we knew that already existed.

So almost every area is in elevated threat environment.

If you move over to France, the Paris Olympics understood this.

They are at a higher threat level.

Then you have the recruitment efforts by an organization like ISIS trying to prove its relevancy, trying to get a high-profile event, but very difficult to get people to move.

Say, like 15 or 20 years ago or 15 years ago, ISIS' tactic was they wanted people to come fight in a war.

There's no more war like that anymore.

So what they're trying to do is to recruit young men, isolated, maybe affiliated, getting radicalized online, and they radicalize in a very short period.

That's the amazing thing.

And tell them, do something catastrophic in your own country.

You don't need to move across borders and risk getting caught.

So that's what we know right now.

And we will continue for the foreseeable future targeting these high-profile events to make a statement and to show ISIS' relevance in an environment that has changed over the last couple of years for them.

AMNA NAWAZ: And so, Juliette, when you say they were inspired by or they were radicalized, what do we know about links between a group like ISIS or a group like al-Qaida, I know, was also mentioned as an inspiration for them and these two teenagers?

Were they directly actually directed to do something?

Or was it more vague than that?

JULIETTE KAYYEM: Now, we -- we -- what we understand now, and this could change, is one of the teenagers was known to law enforcement at the time.

So something had gone on in which he is at least known to intelligence agents at the time.

There is also some reporting regarding the United States coming to understand that there might have been a threat against the concert specifically.

So, they had -- there was something public or at least part of the surveillance network that was captured.

The specific individual who was arrested with the explosives was unknown, relatively -- or unknown to law enforcement there, is radicalized, from what we understand from family members, in a relatively short period of time, and is probably just going down the rabbit hole of radicalization.

They find you, they bring you into back channels, into Web sites or the Dark Web that we don't -- that you and I don't even follow and basically launch what could have been a terrible incident, not just -- I mean, not just for Europeans or others, and we know how international concerts are, but Taylor Swift is an American icon.

This was a focus on an image of and a reflection of America's openness, America's talent, youth, gender, everything that she represents.

And we're pretty lucky that it was stopped.

AMNA NAWAZ: Juliette Kayyem, national security analyst, thank you so much for joining us tonight.

Really appreciate your insights and time.

JULIETTE KAYYEM: Thank you.