NSA chief says surveillance has blocked dozens of terror attacks Keith Alexander said the U.S. is less safe now than it was before Edward Snowden leaked information about surveillance systems 'If we tell terrorists what we're going to do, Americans will die' He said the systems helped thwart attack on the New York subway He added he had 'great concerns' about Snowden's qualifications The director of the National Security Agency has defended government programs that monitor phone calls and emails, claiming they have disrupted 'dozens' of terrorist attacks. NSA director Keith Alexander defended the programs as 'critical' at a Senate hearing on Wednesday - a week after a former contractor, Edward Snowden, leaked information about the snooping. While he refused to discuss specifics, he did say federal data mining appeared to play a role in helping disrupt a recent plot to attack the New York subway system. He said information developed overseas was passed along to the FBI and used to identify suspect Najibullah Zazi in Colorado and uncover the plot. He pleaded guilty to terror-related charges in 2010. At the hearing on Capitol Hill, Alexander also expressed his concerns about Snowden's qualifications. He was entrusted with the government's top secrets despite dropping out of high school. I do have concerns about that. I have great concerns about that,' he said. Defense: Gen. Keith B. Alexander, National Security Agency Chief, defends surveillance programs on Capitol Hill on Wednesday, saying they have disrupted dozens of terrorist attacks Alexander insisted that the programs were key for the nation's security and following Snowden's leaks, the country is much less safe than it was two weeks ago. 'We're trying to be transparent here, protect civil liberties and privacy, but also the security of the U.S.,' he said. 'If we tell the terrorists what we're going to do, they will get through and Americans will die.' Sen. Patrick Leahy, who led the hearing on Capitol Hill, pressed Alexander for further details on the activities that had been disrupted, but Alexander said much of the information was classified. Alexander said he hopes to reveal details of the cases in a week and pledged that the agency would be more transparent with the phone records program in the future. He said the NSA is amassing data and 'only in specific times' with the okay of justice and a court, do they actually access it. Insistence: Alexander said that the country is much less safe now than it was before the leaks occurred In 2009, Najibullah Zazi plotted with two friends to detonate bombs in New York City during rush hour in the middle of trains. Last week, sources told Reuters that thanks to the surveillance systems, authorities were monitoring an e-mail address known to belong to a member of Al Qaeda. Zazi e-mailed that address and revealed himself and his plans. He was arrested and now faces life in prison. When news of the phone-records program broke, officials quickly credited it with thwarting the attack. But the use of PRISM to catch Zazi does little to resolve one of the key questions in the debate: whether the government needs to take such vast amounts of data to keep the country safe. That's because, even before the surveillance laws of 2007 and 2008, the FBI had the authority to monitor email accounts linked to terrorists. The only difference was, before the laws changed, the government needed a warrant. To get a warrant, the law requires that the government show that the target is a suspected member of a terrorist group or foreign government, something that had been well established at that point in the Zazi case. In response, Leahy said the intelligence community believes that these surveillance programs are critical, but he added that he thinks they should not be made permanent and that Congress should be able to review and debate them periodically. Alexander spoke out after Snowden, a former CIA analyst, leaked information last week about programs that track phone calls and information from large internet companies. At the hearing, Sen. Collins asked Alexander when Snowden would have been able to tap into virtually any call or email as claimed in an interview. 'False,' Alexander responded. Ahead of the questioning, he said U.S. critical infrastructure - which ranges from telecommunications to water to energy supplies - is not well prepared to handle a destructive cyber attack. 'On a scale of one to 10, with 10 being strongly defended, our critical infrastructure's preparedness to withstand a destructive cyber attack is about a three based on my experience,' Alexander wrote in the statement prepared for the Senate Appropriations Committee for a hearing on cybersecurity. Alexander said the U.S. has been and continues to be a target of cyberattacks by foreign nations. About 90 per cent of the nation's critical infrastructure is owned by the private sector, and therefore is not under the control of the U.S. government or military. 'Traitor': Edward Snowden, a former CIA analyst, leaked information about the surveillance systems last week While he made no mention of the leaks about NSA surveillance programs in this opening statement, Alexander said it was vital to have a strong Defense Department role in cyberspace in light of what he called real and growing threats. 'While we feel confident that most foreign leaders believe that a devastating attack on the critical infrastructure and population of the United States by cyber means would elicit a prompt and proportionate response, it is possible, however, that some regime or cyber actor could misjudge the impact and the certainty of our resolve,' he said. Alexander told the Reuters Cybersecurity Summit last month that the United States was increasingly vulnerable to attacks like those that destroyed data on tens of thousands of computers in Saudi Arabia and South Korea in the past year.