National Security Agency Director Gen. Keith Alexander, second from right, testifies on Capitol Hill Tuesday. (J. Scott Applewhite/AP)

Intelligence officials said Tuesday that the government's sweeping surveillance efforts have helped thwart "potential terrorist events" more than 50 times since the Sept. 11, 2001 attacks, and the officials detailed two new examples to illustrate the utility of the programs.

In testimony before the House Intelligence Committee on Tuesday, officials cited a nascent plot to blow up the New York Stock Exchange and a case involving an individual providing financial support to an overseas terrorist group.

"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 the terrorist -- the potential terrorist events over 50 times since 9/11," National Security Agency Director Gen. Keith Alexander told the committee.

He said at least 10 of the plots targeted the United States.

FBI Deputy Director Sean Joyce said Tuesday that a provision in the Foreign Intelligence Surveillance Act helped officials monitor a "known extremist in Yemen" who was in contact with an individual in the United States. The information led to disruption of the New York Stock Exchange plot, Joyce said.

Joyce also said that the use of a FISA business record provision helped officials with an investigation involving an individual who was communicating with an overseas terrorist.

"The NSA, using the business record FISA, tipped us off that this individual had indirect contacts with a known terrorist overseas," said Joyce. "We were able to reopen this investigation, identify additional individuals through a legal process and were able to disrupt this terrorist activity."

"So that's four cases total that we have put out publicly," Alexander said Tuesday.

The Washington Post and Britain's Guardian newspaper recently revealed the sweeping Internet and telephone surveillance techniques the NSA has utilized in recent years.

Several of the witnesses testifying Tuesday said the disclosure of the surveillance programs by admitted leaker Edward Snowden had made the world a more dangerous place.

“We are now faced with a situation that because this information has been made public, we run the risk of losing these collection capabilities,” said Robert S. Litt, general counsel of the Office of the Director of National Intelligence. “We’re not going to know for many months whether these leaks in fact have caused us to lose these capabilities, but if they do have that effect, there is no doubt that they will cause our national security to be affected.”

Alexander had previously said the intelligence gathering helped in the cases of Najibullah Zazi, an Afghan American who pleaded guilty to planning suicide attacks in New York, and Pakistani American David Headley, who conducted surveillance in support of the 2008 attacks in Mumbai, India. Joyce elaborated on the two previously revealed cases on Tuesday.

Alexander said he would provide details of the 50 examples he cited Tuesday to lawmakers in a classified setting on Wednesday.

"Those 50 cases right now have been looked at by the FBI, CIA and other partners within the community, and the National Counterterrorism Center is validating all the points so that you know that what we've put in there is exactly right," said Alexander.

Alexander also said that if the surveillance programs had been in place before the Sept. 11 attacks, the United States would have known that hijacker Khalid Muhammad Abdallah al-Mihdhar was in San Diego and communicating with a known al Qaeda safehouse in Yemen.

Alexander's testimony came a day after President Obama defended his administration’s right to engage in such surveillance in an interview with PBS host Charlie Rose, saying the programs had adequate checks and balances.

The Foreign Intelligence Surveillance Court, Obama argued, provided sufficient oversight of the National Security Agency’s activities and said the government was “making the right trade-offs” in balancing privacy rights with national security prerogatives.

“What I can say unequivocally is that if you are a U.S. person, the NSA cannot listen to your telephone calls, and the NSA cannot target your e-mails,” he added, before Rose interjected, “And have not.”

“And have not,” Obama reiterated. “They cannot and have not, by law and by rule, and unless they — and usually it wouldn’t be ‘they,’ it’d be the FBI — go to a court, and obtain a warrant, and seek probable cause, the same way it’s always been, the same way when we were growing up and we were watching movies, you want to go set up a wiretap, you got to go to a judge, show probable cause.”

During the interview — which aired Monday night  — the president took pains to distinguish his national security approach from those of former president George W. Bush and former vice president Richard B. Cheney.

“The whole point of my concern, before I was president — because some people say, ‘Well, you know, Obama was this raving liberal before. Now he’s, you know, Dick Cheney.’ Dick Cheney sometimes says, ‘Yeah, you know? He took it all lock, stock and barrel,’ ” Obama said, according to a transcript provided by PBS. “My concern has always been not that we shouldn’t do intelligence gathering to prevent terrorism, but rather are we setting up a system of checks and balances?”

Juliet Eilperin contributed to this report.

Updated at 2:21 p.m.