these plots that were, apparently, broken up? reporter: well, thanks, jenna, good morning. as we just heard from the head of the national security agency, keith alexander, he believes these controversial programs have provided leads that have helped disrupt what he said were 50 potential terrorist events, and more specifically 90% of those cases are linked to the collection of foreign internet communications and not the collection of phone records of american citizens. these programs are limited, focused and subject to rigorous oversight. they have distinct purposes and oversight mechanisms. we have rigorous training programs for our analysts and their supervisors to understand their responsibilities regarding compliance. reporter: witnesses testified this morning that information from the controversial programs helped prevent at least three specific plots. we ve heard before about the new york city subway plot in 2009 with nanny bull zazi, also to