He also asked government and industry executives, gathered at a cybersecurity summit in Washington, for their support in maintaining the NSA's data-collection and surveillance efforts.
"In the last week, over 950 people were killed in Kenya, Iraq, Yemen" and elsewhere in the world as a result of terrorist attacks, he said. "We've been fortunate to have avoided that in the U.S., but it's not just because of luck," he added, referring to the work of analysts and agents at the NSA, the FBI, the Department of Homeland Security and other agencies.
Alexander said the data gathering and analytic tools the U.S. intelligence community has assembled since the Sept. 11, 2001 attacks have been instrumental in averting at least 54 terrorist attacks in the U.S. and overseas. But in light of growing demands by legislators and privacy advocates to end the NSA's data collection practices, he acknowledged, "We're going to have a debate in this country on do we give up those tools. I'm concerned we're going to make the wrong choice."
[ Is the NSA tapping your smartphone? Read NSA Vs. Your Smartphone: 5 Facts. ]
The NSA director tried to dispel what he called sensationalized media reports about the NSA's activities, explaining that when the NSA collects phone records, it only sees the phone numbers, time of day and duration of each call. "There is no content and no names," he said, insisting NSA analysts are not collecting the content of America's communications.
"We'd need a warrant to do that," said Alexander, pointing to provisions in the Foreign Intelligence Surveillance Act (FISA), authorized in 2008. Warrants are issued when Americans are shown to be in contact with foreign targets overseas, and that occurred fewer than 300 times in 2012, he said. Alexander acknowledged that NSA analysts had made technical and operational errors that counted as conduct violations, but insisted that over the past decade, "we've had only 12 willful violations" where individuals used NSA systems wrongfully, mainly in pursuit of foreign nationals, and "we held them accountable."
Information released by former NSA contractor Edward Snowden has exposed the NSA to criticism that NSA analysts have been able to skirt FISA rules. Lawmakers, including Sens. Patrick Leahy (D-Vt.) and Ron Wyden (D-Ore.), have introduced legislation that would end the program that allows the NSA to collect domestic phone records.