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 .
Peter Bergen : Many unresolved questions about how or why brothers carried out bombing . He says experience of other terrorists in the West 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 11 .