fort meade md u.s. government publicize existence program vacuum bulk americans telephone call data existence leak former contractor edward snowden national security agency deputy director say tuesday richard ledgett retire next month say interview reuters disclose secret program would difficult say might mitigate damage do snowden one might say yes ledgett say office nsa headquarter fort meade maryland one maybe would less shock snowden do ledgett comment echo sentiments former top u.s. officials come u.s. intelligence community step efforts convince congress reauthorize controversial surveillance program program allow intelligence agencies collect vast amount digital communications foreigners live overseas incidentally scoop communications unknown number americans program authorize section 702 foreign intelligence surveillance act expire dec. 31 unless congress take action ledgett say nsa whose mission intercept adversaries communications protect sensitive u.s. government network learn snowden experience open section 702 program say privacy advocate repeatedly demand government share estimate many americans ensnare program authorize section 702. intelligence officials decline ledgett remark early tuesday forum sponsor aspen institute say yes ask estimate would provide year end snowden revelation bulk telephone data program capture information call number time stamp actual content prompt congressional hear court battle 2015 congress pass law replace limit system instal new transparency measure u.s. surveillance activity u.s. officials argue program lawful aim find violent militants adversaries communication ally inside unite state 50-minute interview nsa large campus outside washington ledgett express concern vulnerability u.s. critical infrastructure cyber attack another nation major cyber threat actors worry efforts place capability say take traffic system absolutely take comm unication systems absolutely take gas oil distribution energy distribution systems absolutely ... vulnerable most also give small insight secret cyber battle play globally around-the-clock late 2015 say unclassified u.s. government computer network penetrate another nation nsa call help past adversary realize discover would withdraw like turtle touch pull shell ledgett say happen fight back kind become hand-to-hand combat thing say remove malware would deploy new malware even though know system try get around us decline name attacker computer system involve top nsa official say telephone surveillance disclose