Brussels attacks: Coincidence or planned?
Dozens of people were killed in three explosions at a subway station and airport.
Here, CNN's International Diplomatic Editor Nic Robertson gives his analysis of the attack.
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Does this attack show the hallmarks of a particular terror group?
Nic Robertson: The first assessment of counter-terrorist experts is that this has to be ISIS -- that's the logical explanation.
When Salah Abdeslam -- a man believed to be involved in last year's Paris attacks -- was arrested in Brussels last week, detonating equipment was also discovered in the house he had been hiding in.
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Is this retaliation for Abdeslam's arrest in Brussels on Friday?
NR: It's potentially a revenge attack, because ISIS would not want to be seen as losing.
But Abdeslam has also been on the run for four months, so we also have to assume that he and ISIS would already have had plans for what they would do after he was arrested.
The explosives that they're using would be very difficult to make over the space of a weekend. So potentially the equipment of this attack was available before his arrest.
Are Brussels airport departure halls secure?
There has been additional security -- although that's been more in the number of police present.
But you can check in your bags for a flight without there being any security at all. So therefore the suicide bomber could walk into that area, and mix with people, without any risk of security checks.
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What does this attack tell us about the way these types of groups operate?
NR: It's got resilient networks in Belgium, and indeed Europe, and wants to escalate its campaign here.
The numbers of young fighters from Europe that joined Isis in Iraq and Syria is estimated at more than 1,000.
And each of those fighters that comes back to Europe is potentially a hero in their own community -- as well as a recruiter.
These fighters get to know each other while training, and when they return to Europe, those large networks of trust are intact.
And the depth of their capabilities and resources?
NR: They've proven that they're quite capable of making explosives -- as seen in the Paris and Brussels attacks.
We've seen ISIS' growing ability to export strategies for complex terror attacks, techniques for making explosives, and the use of military-style weapons.
They also obviously want to plan complex attacks -- meaning, attacks in multiple locations.
What's the background to terror attacks in Belgium?
NR: There have been recruitment cells for veteran Islamist terror group al Qaeda in the past. Malika El-Aroud, often seen as a jihadi hero, was recruiting Belgian men to go to Afghanistan and Pakistan in the mid-2000s.
El-Aroud became a high-profile recruiter after her late husband helped to kill Ahmed Shah Massoud, a leader of the Northern Alliance, in a suicide mission ordered by Osama Bin Laden.
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So there's a history of al Qaeda in Belgium talking about targeting sporting events and the metro network.
And they've really tapped into the alienation of second- and third-generation Muslim immigrants.
What are the consequences of this attack?
NR: Increased security at airports everywhere in Europe. Along with tougher security at airports, I think we'll also see increased police on the subways.
We can be pretty sure there'll be special forces units deployed to the outskirts of capitals, if not in the capitals already. This is what happened after the Paris attacks.
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NR: My gut tells me this attack in Brussels is over. But I think it could potentially trigger others in the next few days in Europe.