bob? >> reporter: maurice, that threat surfaced in the past week when intelligence analysts intercepted electronic communications between top al qaeda leaders discussing a plot against an unspecified target. sources say the head of al qaeda ayman al-zawahiri, was pressing the leader of al qaeda in the arabian peninsula to step up american targets. they referenced something big, noting an attack could happen as early as yesterday. nasir al-wuhayshi, once the personal secretary to osama bin laden, runs the yemen based branch of al qaeda that has proven to be the most aggressive in plotting against the u.s. bound airplanes with bombs hidden inside underwear and computer printers.