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 . Comment: Could he have received some kind of bomb-training from Chechen militants who are experienced in making explosives ? Three pairs of brothers were among 19 hijackers on September 11 .
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 . Comment: Three pairs of brothers were among 19 hijackers on September 11 .