Nightly News   |  June 18, 2013

Surveillance prevented at least 10 U.S. terror attacks, NSA says

The director of the National Security Agency, Gen. Keith Alexander, defended the controversial surveillance programs exposed by Edward Snowden and revealed details of two previously unreported cases he said were cracked with the help of the programs. NBC’s Andrea Mitchell reports.

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>>> feeling at your worst, allowing a close friend to stay with you at the hospital. "nightly news" begins now.

>>> good evening, after damaging leaks from a young american who has since fled overseas, today it was the government's turn. the head of the nsa told congress and tried to reassure the american people they're not only not listening in on the conversations of average americans, but the communications they have monitor monitored through data mining operations have in fact stopped, they say, dozens of preplanned terrorist attacks in the post 9/11 era. all of this as individual americans are still sorting out how they feel about how much information is out there and how many people are monitoring all of it for suspicious activity. it's where we begin again tonight with our chief foreign affairs correspondent andrea mitchell .

>> unclassified information.

>> reporter: the head of the nsa defended the controversial surveillance program and announced eric snowden for releasing them.

>> it will have a long and irreversible impact on our nation's security and on that of our allies.

>> reporter: he revealed details of only two previously unreported cases the fbi says were cracked with the help of these programs. a plot to blow up the new york stock exchange between 2007 and 2010 . a kansas city man, a naturalized citizen was channeling money to al kads de operatives in yemen, who were planning to blow up the exchange. he pled guilty to bank fraud and money laundering .

>> would you say their intention to blow up the new york stock exchange was a serious plot? or is this something that they kind of dreamed about, you know, talking among their buddies?

>> i think the jury considered it serious since they were all convicted.

>> reporter: a second case of a san diego man allegedly funding a terrorist group in somalia. enter edward snowden . a foot soldier with top secret clearances. his leaks jeopardized these intelligence tools.

>> we have too many people who have access to top secret clearances. as a result of that you have people who are not fully vetted and this may be the case with mr. snow den.

>> a former cia official says you can't stop leakers.

>> it suggests can you stop someone from thinking they should do something like edward snowden has done. and stop them at the border before they do it. i don't understand how you can stop this. you can't police someone's thoughts.

>> reporter: they're rethinking the system.

>> we have to look and say, how did he do that, what did he do, where was that.

>> reporter: even the president on pbs defended them.

>> my concern has always been not that we shouldn't do intelligence gathering to prevent terrorism, but rather, are we setting up a systems of checks and balances.

>> tonight there are reports still that snow den is still believed to be in hong kong has asked an intermediary to seek asylum in iceland. even though officials believed he made off with 1 million separate documents.

>> andrea mitchell , thanks.