Snowden claims US government is 'using citizenship as a weapon'Reuters

The chief of United States National Security Agency (NSA) has justified the activities of its PRISM surveillance program saying it has helped fend off many terrorist attacks within the country and abroad since the September 9/11 attack. 

At a rare public hearing in Washington, NSA Director Keith Alexander said  that more than 50  terrorist attacks in over 20 countries were prevented by data collected through phone records (section 215 of the FISA Amendments Act) or 'business records' including emails (section 702).

The disrupted attacks include 'little over 10' plots with a 'domestic nexus'. FBI Deputy Director Sean Joyce described four declassified instances where NSA's surveillance prevented terror plots by arresting the accused including the capture of David Headley, the accused in the 26/11 Mumbai attack.

The first one, a plan to bomb New York's subway system, led to the arrests of Afghan-American Najibullah Zazi in Colorado and his two accomplices in the Big Apple. The PRISM program identified an email sent by Zazi to an address in Pakistan known to be associated with al-Qaeda. Alexander said the email surveillance carried out was "not just critical, it was the one that developed the lead on," reported The Guardian.

"This was the first core Al Qaeda plot since 9/11 directed from Pakistan," said Joyce.

Another example was of Khalid Ouazzani of Moroccan origin who had a 'nascent' plan to bomb the New York Stock Exchange. 

In the third example, FBI received intelligence of Headley's involvement in the 26/11 Mumbai bombings. Using 702 authority NSA found that "Headley was plotting to bomb a Danish newspaper that published cartoons depicting the Prophet Mohammed," said Joyce. On his capture, Headley confessed scouting the newspaper site. In the end, Joyce said a terrorist activity was disrupted through an NSA business record FISA search where an individual in the US had a "known contact to terrorists overseas." 

Alexander, the NSA director, was supported by some of the most senior intelligence officials in the House of Intelligence Committee in defending the broad surveillance programs as a vital security tool.

He denied the claim by Edward Snowden, the whistleblower who leaked the NSA's surveillance program to The Guardian and Washington Post, that the NSA had the ability to 'tap into virtually any American's phone or emails'.

"I know of no way to do that," said Alexander.

Moreover, there were repeated assurances that none of the law enforcement agencies involved can access personal records of a US citizen without (FISA) court approval.

Surrounded by senior FBI officials from the FBI, Justice Department and Office of the Director of National Intelligence, Alexander warned that the disclosure of the surveillance programs by the two international dailies could cause "a long and irreversible impact on our nation's security and on that of our allies."