What can explain the chilling ongoing pattern of brother terrorist teams? Genetics, upbringing and the peculiar chemistry of sibling relationships, say experts.
Brother Khalid and Brahim el-Bakraoui have been identified as two of the three men responsible for the Brussels airport and metro attacks that killed 34 people. Investigators believe they blew themselves up. Security forces are hunting for the third man.
The el-Bakraoui brothers, both Belgian nationals, are only the most recent set of siblings involved in terrorism. Dzhokhar Tsarnaev and his big brother, Tamerlan, carried out the Boston Marathon bombings, and brothers Said and Cherif Kouachi perpetrated the deadly Charlie Hebdo attack in Paris.
Six of the 19 terrorists in the 9/11 attacks were brothers, and they worked together in tandem, according to the 9/11 Commission report. Three of the perpetrators responsible for the 2002 bombings in Bali that killed 202 people were brothers. The list goes on and it's a long one.
US military intelligence concluded in a report 10 years ago that having a close family member engaged in violent militancy was the biggest predictor of whether a person would also get involved, the Guardian has reported. The reasons are likely linked to both genetics and environment.
"There are numerous factors" that contribute to terror-related murders, "including neurobiological, personality, social and cultural," neuropsychologist Robert Hanlon of the Northwestern University Feinberg School of Medicine told Discovery.
Particular genes have been linked to an increased risk for violent behavior, and may be passed on to children from parents. In addition, brothers are also raised in a similar environment where they may both repeatedly witness violent crime on the street or domestic abuse within their homes and become desensitised to it.
The terror pairs are almost inevitably male, Hanlon noted, adding that men commit 90% of murders annually in the US alone. "Due to biological, psychological and socio-developmental differences between the genders, men are far more violent than women," he said.
Terrorism can be nurtured in the peculiar chemistry between brothers. "One brother is usually a stronger character and the leader of the two," Hanlon explained. "The other brother is a follower and is influenced by the leader-brother. The follower wants to please the leader and obtain the respect of the leader."
Such sibling pairs can fortify a terror cell. But the leader "big brother" can also serve as a powerful "older brother figure" stand in to others in the group who aren't biologically related to him, according to counterterrorism experts. Members of the group then serve to further radicalise one another.