Peter Bergen : Many unresolved questions about how or why brothers carry out attacks . He says experience of other brothers in the past could shed some light . What exactly prompted the FBI to interview Tamerlan Tsarnaev in 2011 , following a tip from Russia that he was `` a follower of radical Islam '' and was contemplating leaving the United States to join a clandestine organization ? Some who spent formative years in U.S. were radicalized over the Internet , he says . Bergen : Three pairs of brothers were among 19 hijackers on September 11th .
Peter Bergen : Many unresolved questions about how or why brothers carry out attacks . He says experience of other brothers in the past could shed some light . Some who spent formative years in U.S. were radicalized over the Internet , he says . Bergen : Three pairs of brothers were among 19 hijackers on September 11th .