It’s no surprise that a pair of brothers was among the suspects believed to have carried out the deadly attacks in Brussels Tuesday, terror experts say.
This is hardly the first time that brothers have been part of such a plot.
In addition to the El-Bakraoui brothers -- Khalid and Ibrahim -- who died in Tuesday's suicide bombings, other sets of brothers were involved in the November terror attacks in Paris, the Charlie Hebdo shooting in Paris in January 2015, the Boston Marathon bombing in 2013 and even in the Sept. 11 attacks, where three sets of brothers were among the group of hijackers.
"Typically, what we're seeing is someone in a family will be drawn to a cause or drawn to a terrorist organization and they will then, ultimately -- as they are becoming more and more focused in doing something in furtherance of that cause -- they'll bring in someone who they have a close connection to," said John Cohen, an ABC News consultant and former acting Homeland Security undersecretary.
Why Sibling Terrorists Work
The trust that exists between siblings is ideal for terror groups who are focused on ensuring that their plots are not leaked, said Nick Casale, the former director of counter terrorism for the New York State Metropolitan Transit Authority.
Casale noted that there is "absolute secrecy" that comes with including a sibling in the plot, but also a ready target for radicalization.
"When one person gets influenced, there is no better way to influence somebody then by recommendation," said Casale, who now works as an ABC News consultant. "Do you look at the Yellow Pages for a doctor or do you go by what a friend says?"
Though there is an inherent familial bond between siblings, Cohen said that a similar relationship could be applied to spouses -- like in the San Bernardino shootings in November -- or close friends -- like in the Columbine school shooting.
"This is not an uncommon sort of dynamic. We've seen it with gangs and we've seen it with other mass casualty incidents," Cohen said.
"Since they were doing [the prep work and radicalization] in their own home, who was observing their behaviors and making a report?" he said, specifically referencing to Syed Rizwan Farook and Tashfeen Malik, the husband-wife killers from San Bernardino.
Breaking Familial Bonds Proves Problematic
Another reason sibling pairs -- which have tended to be brothers rather than sisters in these violence cases – have been effective is that their closeness does not spark any red flags for law enforcement.
"One of the challenges this poses for law enforcement, in particular, is communication between two brothers, or a husband and wife that are also co-conspirators, is difficult to pick up," Cohen said.
"That communication in itself is not suspicious activity.”
A Long History of Familial Fighters
Even though only one set of brothers -- Ibrahim and Khalid El-Bakraoui -- were directly involved in Tuesday's attacks in Belgium, authorities have also tied Khalid to Salah Abdeslam, the Paris attack suspect who was arrested this month in Brussels.
Abdeslam and brother Ibrahim both participated in November's coordinated attacks in Paris. Ibrahim was a suicide bomber who detonated his device on Boulevard Voltaire.
Earlier that year in Paris, Said and Cherif Kouachi both fled the Charlie Hebdo offices in Paris before leading French law enforcement on a manhunt that led to their deaths in a shootout after a hostage standoff in Dammartin-en Goele, France.
Tamerlan and Dzhokhar Tsarnaev are the clearest examples of brothers taking up a deadly cause, as they were found responsible for the Boston Marathon bombing in 2013. Dzhokhar Tsarnaev ended up running over older brother Tamerlan during a police shootout while they were on the run. Tamerlan died and Dzhokhar was eventually apprehended.
He was sentenced to death in May 2015.
And on the FBI's official list of the 19 hijackers aboard the four planes on Sept. 11, 2001, there were three sets of brothers included. One set of siblings each were on board the planes that hit both the World Trade Center towers and the Pentagon.