From genetics to life at home, there appear to be a number of factors that predispose some brothers to violent behavior.

So it may not be a coincidence that two brothers are suspected of participating in yesterday’s terrorist attacks in Belgium. Federal prosecutor Frederic Van Leeuw announced that Ibrahim El Bakraoui is suspected as being one of the Brussels Airport suicide bombers. His brother Khalid El Bakraoui is suspected of inflicting the deadly suicide bombing an hour later at the Maelbeek metro station.

Other sets of brothers have previously been identified as terrorists. Just a few examples include Dzhokhar Tsarnaev and his slain brother, Tamerlan Tsarnaev, for the Boston Marathon bombing, and brothers Saïd Kouachi and Cherif Kouachi for the killing of 12 people at the offices of the Charlie Hebdo newspaper in Paris.