terrorists who targeted the american embassy in yemen. >> the strike among the latest in an aggressive anti-terrorism counterterrorism campaign waged by american drones. with the details here is our chief investigative correspondent brian ross. >> reporter: a small but significant sigh of relief from u.s. counterterrorism officials. with a senior u.s. official telling abc news four al qaeda terrorists connected to the latest threat stream against the american embassy in yemen had been killed in a drone strike over the weekend. yemen has become the center of the latest al qaeda plots against the u.s. according to other u.s. officials, about three dozen al qaeda operatives have been killed in drone strikes in yemen over the last two weeks. not among those killed according to u.s. officials is the mastermind bomb maker, al siri. al siri created the underwear bomb meant to bring down a u.s. airliner and last year devised a bomb with ingredients u.s. officials admit would go through screening machines undetected.