During an exclusive interview with ABC News' George Stephanopoulos, National Security Agency Director Gen. Keith Alexander said this morning on "This Week" that NSA leaker Edward Snowden has caused "irreversible and significant damage" to the U.S. with his actions. But Alexander could not say why the NSA's systems were not able to prevent Snowden from stealing and leaking highly classified documents, saying "the system did not work as it should have."

When asked by Stephanopoulos, "Do you understand why the system did not blink red in a way that could prevent Snowden from leaving Hawaii in the first place with those secrets?" Alexander responded, "No, I don't."

"It's clearly an individual who has betrayed the trust and confidence we had in him," Alexander said of Snowden, who has fled Hong Kong in route to Russia today and faces espionage and theft charges for leaking classified U.S. documents on the NSA's secret surveillance programs. "This is an individual who is not acting, in my opinion, with noble intent."

READ THE FULL INTERVIEW TRANSCRIPT HERE

"What Snowden has revealed has caused irreversible and significant damage to our country and to our allies," Alexander added in his first Sunday morning interview as NSA director.

When asked if there is anything that could prevent another private contractor from accessing and leaking classified information from the NSA's systems, Alexander said, "This is a key issue we've got to work our way through. Clearly, the system did not work as it should have."

"[Snowden] betrayed the trust and confidence we had in him. This was an individual with top secret clearance whose duty it was to administer these networks. He betrayed that confidence and stole some of our secrets," Alexander added. "We are now putting in place actions that would give us the ability to track our system administrators, what they are doing, what they're taking, a two-man rule. We've changed the passwords. But at the end of the day, we have to trust that our people are gonna do the right thing."

But Alexander defended the NSA's surveillance programs revealed by Snowden, saying they have been critical in disrupting terrorist plots since Sept. 11.

"My first responsibility to the American people is to defend this nation… The intel community failed to connect the dots in 9-11. And much of what we've done since then were to give us the capabilities… that help us connect the dots," Alexander said of the NSA's domestic phone and overseas internet surveillance programs. "These are two of the most important things, from my perspective, that helps us understand what the terrorists are trying to do," Alexander said.

Alexander testified before the House Intelligence Committee last week that the surveillance programs helped prevent 50 terrorist attacks worldwide. But while critics in Congress agree the PRISM overseas computer surveillance program has been effective, they question whether the domestic phone data collection program has produced tangible results. Alexander says the phone program has been key in disrupting 10 plots.

"If you look at it just across the board, across these 50, the business record FISA [phone collection] is only going to focus on those that had a nexus in the United States, and that's a little over 10," Alexander told Stephanopoulos. "And in those, I think it contributed in the majority of cases. It allowed us to form some of the dots. And I think that's key in what we're trying to do."

And Alexander said his agency follows the law in how it monitors phone conversations involving overseas contacts with individuals in the U.S., requiring a court order before monitoring.

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"In order for the NSA to target the content of a U.S. person's communications anywhere in the world, anywhere, NSA requires probable cause and a court order, a specific court order," Alexander said.

"We train our people how to do this right. We get oversight by Justice, we get oversight by the courts, we get oversight by the administration and by Congress, all three parts of government," he added.

The Cyber Threat

Alexander, a four-star general who has headed the National Security Agency since 2005, also took charge of U.S. Cyber Command in 2010, presiding over military efforts on cyber warfare operations.

Alexander explained the extent of his authority on cyber warfare, after documents leaked by Snowden outlined Alexander's emergency authority to act on imminent cyber threats.

"To be clear, what I can do on my own right now is, within our networks, to launch offensive measures to stop somebody from getting into the networks," Alexander said. "Anything that I want to do outside the networks that is offensive in nature we would have to call the Secretary [of Defense] and the President to get their approval. So there are things that we can do to stop packets in flight, but from our perspective, any actions that's offensive in nature would require the policymakers."

Asked to respond to House Intelligence Committee chair Mike Rogers' comments that the U.S. is losing the cyber war to China, Alexander said, "I think our nation has been significantly impacted with intellectual property, the theft of intellectual property by China and others. That is the most significant transfer of wealth in history."

"Who's taking our information is one of the things I believe the American people would expect me to know," Alexander added. "That's one of my missions. Who's doing this to us and why? … Who's coming after us? We need to know that so we can defend this nation."

In a statement this morning on Edward Snowden's departure from China, the government of Hong Kong said it had "no legal basis to restrict Mr. Snowden from leaving Hong Kong," while also seeking clarification on reports based on Snowden's leaks "about the hacking of computer systems in Hong Kong by U.S. government agencies."

Alexander responded that "we have an interest in those who collect on us, as an intelligence agency," but said that the NSA follows U.S. law in its overseas activities.

"The fact is, what we're trying to do is get the information our nation needs, the foreign intelligence," Alexander said. "I'm confident that we're following the laws that our country has in doing what we do. We have a set of laws that guide how NSA acts. We follow those laws."

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