Gen. Keith Alexander, director of the National Security Agency, disclosed Tuesday, June 18, 2013 that plots to bomb the New York subway system and the New York Stock Exchange were among more than 50 stopped by secret surveillance programs.

Bomb plots targeting the New York Stock Exchange and the city's subway were among more than 50 worldwide thwarted by top-secret surveillance programs since the 2011 al Qaeda attacks on the United States, authorities said on Tuesday.

Gen. Keith Alexander, National Security Agency director, FBI and other officials revealed startling details at a House Intelligence Committee hearing aimed at finding out more about the telephone and e-mail surveillance initiatives that came to light this month through leaks of classified information to newspapers.

It was the most comprehensive and specific defense of those methods that have come under ferocious criticism from civil liberties groups, some members of Congress and others concerned about the reach of government into the private lives of citizens in the interest of national security.

National security and law enforcement officials asserted that the leaks were egregious and carry huge consequences for national security.

"I think it was irreversible and significant damage to this nation," Alexander said when questioned by Rep. Michele Bachmann.

"Has this helped America's enemies?" the Minnesota Republican asked.

"I believe it has and I believe it will hurt us and our allies," Alexander said.

President Barack Obama has defended the programs as necessary in an era of terror, and said they have been vetted by Congress and are subject to strict legal checks.

In an interview with Charlie Rose broadcast on Monday night, Obama said the situation requires a national debate on the balance between security and privacy.

Alexander noted last week in Senate testimony that the surveillance programs helped stop dozens of terror plots.

He briefly mentioned planning to bomb the New York subway system, but fuller details about that and revelations about others emerged on Tuesday in the House.

In all, officials said the controversial surveillance aimed at communications overseas helped to disrupt more than 50 plots globally that were in various stages of planning.

Details of virtually all remain secret, but national security officials said they were working on declassifying more information and could have a report to Congress as early as this week.

"We are revealing in front of you today methods and techniques," said Sean Joyce, deputy FBI director, adding the need to do so reflects the substantial impact the leaks have had on the national security community.

Joyce detailed for committee members e-mail surveillance that helped authorities discover the two New York City plots directed at the stock exchange and the subway.

In the fall of 2009, Joyce said the NSA intercepted an e-mail from a suspected terrorist in Pakistan. That person was talking with someone in the United States "about perfecting a recipe for explosives."

Authorities identified Afghan-born Najibullah Zazi of Denver. The FBI followed him to New York and later broke up planning to attack the subway. Zazi later pleaded guilty and is currently in prison.

In the other New York case, NSA was monitoring a "known extremist" in Yemen who was in contact with a person in the United States. Joyce said the FBI detected "nascent plotting" to bomb the stock exchange, long said by U.S. authorities to be a target of terrorists.

He also said e-mail surveillance also disrupted an effort to attack the office of a Danish newspaper that was threatened for publishing a cartoon of the Prophet Mohammed in 2006.

This one involved David Headley, a U.S citizen living in Chicago. The FBI received intelligence at the time regarding his possible involvement in the 2008 Mumbai terror attack that killed 164 people, Joyce said.

The NSA, through surveillance of an al Qaeda-affiliated terrorist, found that Headley was working on a plot to bomb the newspaper. Headley later confessed to conducting surveillance and was convicted. He also pleaded guilty to conducting surveillance in the Mumbai case.

Lastly, secret surveillance led "tipped us off" to a person who had indirect contacts with a known terrorist group overseas.

"We were able to reopen this investigation, identify additional individuals through the legal process and were able to disrupt this terrorist activity," Joyce said.

Intelligence Committee Chairman Mike Rogers welcomed the testimony.

"I think you have struck the right balance between protecting sources and methods, and maintaining the public's trust, by providing more examples of how these authorities have helped disrupt terrorist plots and connections," Rogers, a Michigan Republican, said.

The hearing came one day after the admitted leaker of documents to Britain's Guardian newspaper and the Washington Post about the classified surveillance programs sought to defend his actions.

In a series of blog posts on the Guardian website, Edward Snowden said he disclosed the information because Obama worsened "abusive" surveillance practices instead of curtailing them as he promised as a presidential candidate.

The former NSA contractor insisted that U.S. authorities have access to phone calls, e-mails and other communications far beyond constitutional bounds.

While he said legal restrictions can be easily skirted by analysts at the NSA, FBI and CIA, Snowden stopped short of accusing authorities of violating specific laws.

Instead, he said toothless regulations and policies were to blame for what he called "suspicionless surveillance," and he warned that policies can be changed to allow further abuses.

Under questioning from Rogers, Alexander said the NSA does not have the authority to listen to phone calls of U.S. citizens or read their e-mails under the two surveillance programs.

He also said there was no technology for a lone analyst to arbitrarily listen to Americans' phone calls or read their e-mails.