British authorities still have many questions about the Monday night concert bombing in Manchester. They don't yet know if the suicide bomber had any helpers or how he obtained his explosives.
But this much is clear: Western European cities have become regular targets over the past two years, a period coinciding with the rise of the Islamic State and its calls for supporters to strike anywhere they can and with whatever weapons are at hand.
The Manchester bombing is the 13th terrorist attack in Western Europe since the beginning of 2015. Collectively, they have claimed more than 300 lives. ISIS has been linked to most of the attacks, and on Tuesday, claimed responsibility for the concert bombing.
By comparison, the United States has suffered fewer than 10 deadly terrorist attacks, with fewer than 100 deaths, since the Sept. 11, 2001, attacks.
Calls to strike Europe
ISIS makes no secret of its desire to hit Europe. Its online magazine, previously named Dabiq after a town in Syria, is now called Rumiyah, or Rome. That's because the group wants to eventually seize Rome, as the symbolic home of the pope and Christianity.
"We will not rest from our jihad except beneath the olive trees of Rome," says the quotation under the magazine's title. It's attributed to Abu Hamzah al-Muhajir, one of the group's leaders, who was killed in Iraq in 2010.
After surging across large swaths of Syria and Iraq in 2013 and 2014, ISIS declared a caliphate, an Islamic nation, in the territory it controlled.
Since then, it has been pushed back in its core territory by the U.S. air campaign and by allied forces on the ground. But it has managed to claim terror attacks far from its base.
"The global reach of ISIS right now remains is largely intact, despite the extremely effective work that has been done to degrade ISIS in its caliphate," Nicholas Rasmussen, head of the U.S. National Counterterrorism Center, said earlier this month in Washington.
ISIS claimed responsibility for the Manchester bombing via the group's Amaq media channel, the same way it has announced attacks in the past.
British authorities say a suicide bomber set off his explosive in the entrance hall of the arena as the crowd was leaving the concert by American pop star Ariana Grande. The ISIS statement does not mention a suicide attacker, but says one of its "soldiers" placed a bomb at the arena.
Attacks began in Paris
The European spree began in France, when a gunman went on a shooting rampage on Jan. 8, 2015, at the Paris offices of the satirical magazine Charlie Hebdo. The attack killed 17 people.
The steady stream of assaults has included large-scale attacks orchestrated in great detail by ISIS, like coordinated shootings and bombings in Paris in November 2015 that left 130 dead.
There have also been individual attacks, like one in March by a driver who plowed into pedestrians on London's Westminster Bridge and then began stabbing people as he tried to enter the British Parliament grounds. ISIS claimed responsibility for that attack, though British authorities have said they believe the man was acting alone and it's not clear whether he had any links to the group.
Five of the dozen European attacks since 2105 have been carried out by gunmen, vehicles have been used four times and bombs were detonated in three instances.
With the smaller scale attacks carried out by individuals, government officials and counterterrorism analysts say it can be difficult to determine the exact links, if any, between an attacker and ISIS.
An attacker may simply be a long-distance admirer inspired by the ISIS call to strike in its name. Analysts say it's also possible that ISIS may claim some attacks — even if had no knowledge of them — to suggest an ability to hit far and wide.
Also, government officials and analysts say that as ISIS continues to lose ground in its core areas of Iraq and Syria, the group is likely to place even greater emphasis on attacks further afield in an attempt to show its relevance.
Fighters returning home
Thousands of European Muslims went to fight for ISIS and security officials have been concerned that they will return home and try to carry out attacks. Rasmussen, the U.S. official, says this remains a big worry, though the thinking is evolving.
"Intelligence agencies were initially concerned about the sheer numbers of ISIS returning home," he said. "But now the focus is on what's expected to be a small number who have skills to recruit or carry out sophisticated attacks."
ISIS is struggling to hold on to parts of western Mosul in northern Iraq, the last urban area where the group has a foothold in that country. In Syria, the U.S. and its partners have been advancing toward Raqqa, the de facto ISIS capital in the north-central part of the country.
Even if ISIS is driven from the territory it controls, the group is expected to carry out guerrilla-style attacks in both Syria and Iraq, as well as terrorist strikes elsewhere.
Greg Myre is a national security correspondent. Follow him @gregmyre1.
STEVE INSKEEP, HOST:
So what can we learn from the larger pattern of ISIS-claimed attacks in recent months and years? We're going to put that question to NPR's national security correspondent Greg Myre, who's in our studios. Hi, Greg.
GREG MYRE, BYLINE: Good morning, Steve.
INSKEEP: What do you see when you study multiple attacks at once?
MYRE: Well, just the way that this is baked into the ISIS ideology - they really came to prominence in 2014. The U.S. then started its air campaign, and then the key moment was in January, 2015, the attack on Charlie Hebdo, the French satirical magazine. Since that point, until this - 'til Monday, 13 separate deadly terrorist attacks in Western European cities...
MYRE: ...London, Paris, Berlin, Nice, Stockholm, Copenhagen - it's an absolute fundamental part of the ISIS ideology.
INSKEEP: But help me out here, Greg, because you note that number, which is startling - 13 different attacks. People are bombed. People are killed, city after city. But the survivors go on. Life goes on. It seems to me the same as it was when European cities were being bombed far more severely in World War II. People still went on. What does ISIS get out of those 13 attacks?
MYRE: Well, it shows that it's relevant, that it's powerful. It's not just a group fighting for this core territory and wanting to create its own state in the Middle East, that it has a presence in Europe. It can hit in Europe. It can recruit in Europe. It can inspire people to act on their own in Europe. So this may be self-defeating in the sense that it is getting more powerful enemies for ISIS. More Western countries are willing to participate in different ways in the battle. But it is - as I said, it's very, very central to their ideology. And I spoke with Rita Katz. She's the head of the SITE Intelligence Group, which constantly monitors ISIS on social media, and here's how she described it.
RITA KATZ: ISIS is making sure to import the war that they're facing in Iraq and Syria and Afghanistan to the West.
INSKEEP: They want to be on the offensive I guess is the way that they would phrase that. They're trying to do that through their followers, who are in Europe. A lot these people are citizens of the countries involved. What is the message they're sending to their followers?
MYRE: Well, their message is we can fight back in a lot of ways, and we want to keep growing and reaching out to more people. One of their great strengths is social media, which allows them to - has allowed them to grow. But it also allows you to follow them quite closely. And Katz noted something very interesting. When ISIS carries out an attack, it releases a statement directly from the group. When there's a lone wolf attack, it won't comment usually, and it allows its 'Amaq News Agency to do it. And it's - 'Amaq is a propaganda arm of ISIS, but they make this distinction. And for her, that allows - that's this clear pattern that she's seen. And so she thinks it's very consistent in the way it releases information. That's why she thinks this attack on Monday was directly by ISIS. And here's how she describes the way that the group deals with information.
KATZ: We have not been able to find, like, a real lie from ISIS. Despite the fact that they are a terrorist organization, they want to provide their followers and supporters authentic information.
INSKEEP: Greg Myre, let me ask one other thing because the United States is trying harder to get troops into Raqqa - allied troops into Raqqa, which is the ISIS capital in northern Syria. If Raqqa's taken, if ISIS territory is taken, does it still have the ability to lash out in other countries?
MYRE: The assumption is yeah. So we're seeing these two conflicting patterns. On the battlefields of the Middle East, ISIS territory is shrinking. It has less power. It's being squeezed in many ways, but it still shows an ability to carry out attacks in Europe. And all the indications are it's going to press ahead with that attempt.
INSKEEP: OK, Greg, thanks very much as always.
MYRE: Thanks, Steve.
INSKEEP: That's NPR national security correspondent Greg Myre this morning. Transcript provided by NPR, Copyright NPR.