The terrorist attacks in Brussels mark the third major assault in the heart of Europe in just over a year and raise a troubling question: Are European states prepared to deal with a sustained onslaught?

The satirical magazine Charlie Hebdo was hit in January 2015. Terrorists rampaged through Paris again in November. And now, Brussels has suffered bomb attacks at the airport and the subway, claiming more than 30 lives.

"This is not over," French President François Hollande said just last Friday. He spoke in Brussels shortly after the arrest of Salah Abdeslam, believed to be the lone survivor of the Paris attack last fall. Hollande added that French and Belgian authorities had uncovered a much wider network of jihadists during their recent investigations.

Yet this heightened state of alert did not prevent Tuesday's bombings in Brussels.

Analysts point to several factors behind the current attacks and the difficulty in stopping them.

Paris and Brussels are major crossroads, with huge numbers of travelers crossing national boundaries, making it difficult to track extremists. Prior to last year, Western Europe had not been hit hard by terrorists for a decade and therefore may have been underprepared. And the extremism so prevalent in the Middle East has now taken root in Europe.

"It's really only with the rise of ISIS in Syria and the attacks in Paris, and now this attack, that we've seen Western Europe facing such a concentrated, deadly and really sophisticated threat," Michael Leiter, the former director of the U.S. National Counterterrorism Center, told NPR's Morning Edition.

"We're seeing the challenges of relatively open borders and a fractured intelligence system that makes it hard to detect and stop these attacks," he added.

What's striking is that all three attacks were carried out in places that security experts considered potential targets.

The Charlie Hebdo office was destroyed by a firebomb back in 2011, and after that, the magazine increased security as it carried on with its provocative cartoons that often lampooned Islam and Muslim figures.

In the wake of the deadly 2015 attack on Charlie Hebdo, security was ramped up dramatically throughout Paris, including troops in the streets. Yet that didn't prevent the November attack at multiple sites that left 130 dead.

And as French authorities investigated the Paris bloodbath, attention shifted to Brussels, where several attackers had been living and Abdeslam was captured last Friday.

Analysts had warned that Western Europe was not immune. France and other European countries have large and often restive Muslim populations. Thousands of young Muslim men in Europe have traveled to and from Europe to join the Islamic State, according to various estimates.

An estimated 200 have gone from Belgium, which gives it one of the highest per capita rates in Europe.

"When I heard about the number of foreign fighters going in and out of Syria in 2013, I felt this wave would be coming to Europe and likely the United States," said Barry Pavel, director of the Brent Scowcroft Center at the Atlantic Council and a former member of the National Security Council under both President Obama and President Bush.

"I'm not saying these attackers today came out of Syria. I don't know that. But as long as the Syria war continues, it's going to be an incubator for extremists," he added.

Belgian officials hailed last Friday's arrest of Abdeslam as an important breakthrough. By taking him alive, they said, he could provide valuable information about the attack he's been linked to as well as other potential attacks.

Yet it took authorities almost four months to find him in the neighborhood where he grew up, suggesting a strong network of sympathizers who protected him.

Belgium's Foreign Minister Didier Reynders said Sunday that police had found heavy weapons last week in raids that led to Abdeslam's arrest. The suspect has been cooperating and told authorities that at least 30 jihadists remained at large in the city.

"He was ready to restart something from Brussels," Reynders said. "That is maybe the reality, because we have found a lot of heavy weapons and we have seen a new network around him."

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