washington u.s. force capture militant believe play role 2012 attack u.s. diplomatic compound benghazi libya kill u.s. ambassador christopher stevens three americans u.s. officials say monday officials speak condition anonymity say u.s. special operation force capture militant libya past days two officials identify mustafa al-imam say play role attack ambassador death officials say man custody department justice transport back unite state military add operation authorize president donald trump notify u.n.-backed government national accord statement trump say al-imam face justice unite state allege role september 11 2012 attack nw1n1n000y attorney general jeff sessions say al-imam custody unite state would continue investigate identify involve attack appropriate congressional committees families americans kill 2012 attack also notify officials say attack embassy topic numerous congressional hear republican lawmakers critical way then-secretary state hillary clinton respond attack early month u.s. prosecutors open case suspect ringleader ahmed abu khatallah khatallah await trial since 2014 capture team u.s. military fbi officials libya transport 13-day journey unite state aboard navy vessel militants exploit chaos follow muammar gaddafi 2011 downfall islamic state take sirte early 2015 turn important base outside middle east attract large number foreign fighters city islamic state militants shift desert valleys inland hill southeast tripoli seek exploit libya political divisions defeat sirte unite state carry strike militant group libya strike dozen militants september u.s. capture suspect benghazi attack militant