Officials in Brussels on Wednesday named a pair of brothers as two of the suicide bombers in Tuesday’s bloody attacks in the Belgian capital.

It was the third terrorist attack in Europe in a little over a year involving two male siblings, adding to questions about how individuals become radicalized and what authorities can do to stop it.

In January 2015, in an assault linked to al-Qaeda, brothers Said and Chérif Kouachi stormed a satirical publication in Paris, killing a dozen people. In November, attacks plotted by a cell of militants aligned with the Islamic State, including brothers Brahim and Salah Abdeslam, rocked Paris. The former blew himself up at a Paris cafe; the latter was arrested last week in Brussels after months on the run.

The sibling phenomenon is not limited to Europe. In 2013, brothers Tamerlan and Dzhokhar Tsarnaev carried out the deadly bombings at the Boston Marathon.

“Jihad has become a family business,” said Christina Schori Liang, a senior fellow at the Geneva Center for Security Policy. “If one family member becomes radicalized, it is predictable that others will become militant as well.”

According to Liang, more than a quarter of Westerners who have gone to fight in Syria have had a family connection to others involved in the armed Islamist cause, often a relative who also had gone to fight there.

It’s not known whether any of the recent attackers were radicalized by their siblings or whether they happened to find similar appeal in the same ideology.

Like some of the others involved in recent attacks in Europe, the Abdeslam and the Bakraoui brothers (identified in the Brussels attacks) shared criminal records. All four had been convicted of robbery or other financially motivated crimes before they turned to militancy.

Faysal Itani, a scholar at the Atlantic Council in Washington, said siblings may be radicalized by proximity or may choose to pursue militant action with siblings because of the high level of trust required for such activity.

Abdelhamid Abaaoud, the mastermind of the Paris attacks, is thought to have taken his teenage brother to Syria to fight alongside the Islamic State. Abaaoud was killed during a standoff with police a few days after the Nov. 13 string of shootings and explosions in Paris.

According to recent media reports, Abaaoud’s brother Younes, now about 15, may be plotting revenge for his death from Syria.
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