The head of the U.S. National Security Agency on Tuesday defended the broad surveillance of Americans' phone and Internet data as a vital security tool and said it had helped disrupt possible attacks more than 50 times since Sept. 11, 2001.
Justifying the surveillance programs that were disclosed by an NSA leaker earlier this month, General Keith Alexander said he would give lawmakers classified details of all of the thwarted incidents within 24 hours.
Alexander, speaking at a hearing before sympathetic members of the House of Representatives Intelligence Committee, had last week given an estimate of "dozens" of foiled attacks.
Sean Joyce, deputy FBI director, offered information on two of the cases - a plot to bomb the New York Stock Exchange and the provision of financial assistance to a terrorist group in Somalia that conducted suicide bombings.
The disclosure of the programs by former NSA contractor Edward Snowden ignited a political furor, with civil liberties groups and privacy advocates blasting the surveillance as government overreach that lacked proper independent oversight.
But Alexander said Snowden had endangered national security by revealing the U.S. phone and Internet data tracking programs and said they were legal, closely supervised and crucial to defending America.
"I would much rather be here today debating this point than trying to explain how we failed to prevent another 9/11," Alexander told the committee in his second public appearance before Congress since the programs were exposed.
"In recent years these programs, together with other intelligence, have protected the U.S. and our allies from terrorist threats across the globe to include helping prevent ... potential terrorist events over 50 times since 9/11," he said.
Officials had revealed last week two such potential attacks: a 2009 plan to bomb a Danish newspaper that had published cartoons of the Prophet Mohammad and a plot by Islamist militants to bomb the New York subway the same year.
Members of the committee, which has oversight over the surveillance programs, said they were holding the hearing to set the record straight about how the programs operated and their importance for national security.
Representative Dutch Ruppersberger, the top Democrat on the committee, said the leaks "put our country and our allies in danger by giving the terrorists a really good look at the playbook that we use to protect our country. The terrorists now know many of our sources and methods."
Committee Chairman Mike Rogers, a Republican, defended the NSA. "People at the NSA in particular have heard a constant public drumbeat about a laundry list of nefarious things they are alleged to be doing to spy on Americans -- all of them wrong," he said.
Snowden, a former employee of government contractor Booz Allen Hamilton who worked in an NSA facility in Hawaii, defended his actions in an Internet chat on Monday and vowed to release more details on the extent of the agency's access.
Snowden is believed to be in hiding in Hong Kong as the U.S. Justice Department proceeds on a criminal investigation into the leaks.
Rogers said Snowden's continued efforts to speak out and release intelligence information posed more risk for the United States. "Anything that he talks about is dangerous," Rogers said on NBC's "Today" program before the hearing.
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