Editor's note: This story ran originally on Dec. 2, 2015. It has been updated to reflect this week's bombings in Brussels.
When police announced that Khalid el-Bakraoui and his brother, Ibrahim, had blown themselves up in two separate attacks in Brussels on Tuesday, the pair became the latest example of terrorists who share fraternal bonds.
Salah Abdeslam, the only surviving participant of the Nov. 13 Paris attacks, was also working with his brother, Brahim. In that case, Brahim went through with a suicide bombing at a cafe. Salah, for reasons still unexplained, didn't detonate his vest. He was arrested in Brussels last week.
The January 2015 murders at the Charlie Hebdo magazine offices in Paris were also carried out by brothers, Cherif and Said Kouachi.
Closer to home, the Boston Marathon bombings in 2013 were planned and executed by Tamerlan Tsarnaev and his younger brother, Dzhokar. And more than a decade earlier, three pairs of brothers from Saudi Arabia were among the 19 hijackers in the attacks of Sept. 11, 2001.
It's no accident that high-profile terrorist attacks involve brothers. Intelligence officials say that small terrorist cells made up of relatives or close friends are the hardest to spot. They are not only difficult to infiltrate, but because the terrorists are often living in close quarters, perhaps even under the same roof, it is also difficult to pick up their communications or signs of their plan.
That's what happened in the case of the Boston Marathon bombings, when the Tsarnaev brothers built pressure-cooker bombs and ignited them near the race's finish line.
The eldest of the Abdeslam brothers, Mohamed, went on Belgian television in November and said that Salah and Brahim had started to change about six months before the Paris attacks. Mohamed had no role in the plot and called on Salah to turn himself in. Police do not believe he knew where Salah was hiding before his arrest last week.
Mohamed said he noticed that his brothers started praying more, and they announced to the family that they had quit drinking. But that, said Mohamed, seemed like something to celebrate. He thought his brothers were just maturing, that they had started to take their religion, and their lives, more seriously.
"When your brother tells you that he's stopped drinking," he told Belgian television last year, "it's not a change that says something is wrong."
But in hindsight, the change appears to have had at least some connection to the Nov. 13 Paris attacks, in which terrorists opened fire on cafes and a concert hall and detonated explosive vests, killing 130 and wounded hundreds more.
Abdeslam has also been linked to the Brussels plot and his fingerprints were found in an apartment that had been rented by the Bakraoui brothers.
An Intense Feedback Loop
Brothers and terrorism have gone hand-in-hand for years. Reid Sawyer, who used to run the Combating Terrorism Center at West Point, said foreign fighters and jihadists often had close ties to other family members who had taken the same path.
"When you find one person that radicalizes in a family, it is a great predictor that somebody else in that family — either immediately a brother or a cousin — is likely to participate," he says. "And that's exactly what we've seen in this [Nov. 13] Paris attack."
The process of radicalization is a fairly intense one; when family dynamics are added into the mix, it is even more so.
Family members not only feel compelled to show they are committed to a particular ideology, but there is also an extra pressure to prove oneself — to show a brother or a cousin or other relatives that they can count on you. The feedback loop makes for an intense commitment.
Rik Coolsaet, a professor of international relations at Ghent University in Belgium, who has been studying radicalization there — and in Europe more generally — sees much of this radicalization through the lens of sociology.
"It is not new," he says. "In the study of terrorism, it is very often the case that you are not, as an individual, radicalizing all by yourself. Often it is a case that you radicalize due to small-group dynamics — kinship and friendship bonds."
His research suggests that people don't initially embrace extremist groups because of their ideology. Instead, he says, what's more important is whether their friends or family members already have joined.
"This might be a bunch of guys in my neighborhood, this might be family members," he says. "And so it is this dynamic that pushes you forward on this road, on this journey to violent terrorism."
Neighborhood Bonds
The Abdeslam brothers lived in a Brussels neighborhood called Molenbeek. Since November, the neighborhood had become ground zero for Belgian counterterrorism efforts. Nearly all the Nov. 13 Paris attackers had a connection to the neighborhood or some of the men who lived there. The Bakraoui brothers lived in Schaerbeek, just a short drive away.
Molenbeek doesn't look particularly sinister, with its cluster of low-slung apartment buildings and its small shops and awnings decorated with Arabic script. It is a high-crime and high-unemployment area, just a short two-stop subway ride to the central square in Brussels. But Coolsaet says the neighborhood might as well be a world away.
"These are neighborhoods where it is difficult to live," he says, "where you feel an atmosphere among youngsters of no future. They think they will never be accepted into Belgian society."
That makes these young men particularly vulnerable, he says, to messages like those ISIS sends that suggest they can belong to the worldwide ummah, orcommunity of Muslims.
In fact, the man thought to have been a key organizer of the Paris attacks was from Molenbeek as well.
Abdelhamid Abaaoud is thought to have helped plot terrorist attacks in Europe for over a year. He was sent by ISIS leaders in Raqqa, Syria, U.S. and European counterterrorism officials say, and his early attempts to launch attacks fizzled. Early last year, police raided a safe house in the Belgian city of Verviers, where he had been stockpiling weapons and explosives.
With the investigation continuing and raids underway throughout Belgium, it is too early to tell who is involved this time around, and whether they will have the same Molenbeek connections. It is also unclear if this team of terrorists forged their bonds in Syria and then returned home to attack.
Dina Temple-Raston is NPR's Counterterrorism correspondent. Follow her @NPRDina.
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