NSA Director General Keith Alexander (shown), testifying along with officials from the Justice Department in a rare public oversight hearing by the House Intelligence Committee, claimed that more than 50 terror plots had been discovered and prevented thanks to the highly classified data collections.

But a jury conviction cited in a deputy attorney general's claim about a terrorist plot never occurred, and at least two other cases he cited appear not to support the claim that they were solved through the National Security Agency's massive collection of telephone records and Internet communications

Alexander told the House Intelligence Committee, "In the 12 years since the attacks on Sept. 11, we have lived in relative safety and security as a nation. That security is a direct result of the intelligence community's quiet efforts to better connect the dots and learn from the mistakes that permitted those attacks to occur on 9/11."

Deputy Attorney General Sean Joyce cited four recently declassified cases as examples of how the system worked to apprehend terrorists. In one case, Joyce said, the NSA discovered a terrorist in Yemen was talking to a man in Kansas City, Missouri, named Khalid Ouazzani. With a warrant from the secret Foreign Intelligence Surveillance Court, agents were then able to search Ouazzani's electronic communications. They discovered that he and two other conspirators were "in the very initial stages" of a plot to blow up the New York Stock Exchange, Joyce said.

Rep. Mac Thornberry (R-Texas) wanted to know if it was a serious plot Joyce was describing or merely "something that they kind of dreamed about."

"I think the jury considered it serious, since they were all convicted," Joyce replied. That appeared to settle the matter — but for the fact that there was no trial or jury for Ouazzani, who was neither charged with nor convicted of any domestic terrorist plot. He did admit to sending money to al-Qaeda and pled guilty to bank fraud and "conspiracy to provide material assistance to a terrorist organization." His plea bargain agreement made no mention of the alleged bombing plot, CNN Money reported.

"Khalid Ouazzani had nothing to do with any plot to blow up the New York Stock Exchange," his lawyer, Robin Fowler, said.

Two alleged co-conspirators in the bombing plot, Sabirhan Hasanoff and Wesam El-Hanafi, both from Brooklyn, also pled guilty to providing material support to terrorism. The New York Times cited a sentencing memorandum filed by prosecutors, stating that Hasanoff, "at the direction of a terrorist leader," conducted surveillance on the Stock Exchange and sent the leader a one-page report of it. "The report was rudimentary and of limited use" for any terrorist operation, the memo said, while recommending a 20-year sentence for Hasanoff. Joshua Dratel, the lawyer for Hasanoff, told the Times that Joyce's description of the case to the House committee was "astonishing," since none of the defendants was charged with the stock exchange allegation and there was no jury trial in any of the cases.

Joyce discussed in brief two other cases officials have previously claimed were solved through NSA surveillance. They involved an alleged 2009 conspiracy to detonate a bomb in the New York City subway system and the apprehension of David Headley, a Chicago resident convicted for his role in the 2008 terrorist attack in Mumbai, India. Those cases have also been cited by Sen. Dianne Feinstein (D-Calif.) and Rep.Mike Rogers (R-Mich.), chairmen, respectively, of the Senate and House Intelligence Committees, in defense of the NSA surveillance programs. Rogers was especially emphatic in the case of Najibullah Zazi, the would-be subway bomber.

"I can tell you, in the Zazi case in New York, it's exactly the program that was used," Rogers said in a recent interview on ABC's This Week. "I think the Zazi case is so important, because that's one you can specifically show that this was the key piece that allowed us to stop a bombing in the New York subway system."

But investigators did not discover Zazi and his interest in bomb-making through a daily collection of billions of personal e-mail messages. A 2009 National Public Radio report, citing as sources "law enforcement officials close to the Zazi case," said the Afghanistan native came to the FBI's attention through a tip from officials of the Pakistani government whom Zazi had met with al-Qaeda operatives there. When Zazi, a Denver-area shuttle bus driver, returned to the United States, he was put under surveillance that included court-approved wiretaps and intercepts of his text messages. But it was only after a physical search of the Zazi's car and the laptop computer found in it that investigators concluded they had enough evidence to arrest successfully prosecute him.

Sam Rascoff, who used to work on terrorism cases for the intelligence unit of the New York Police Department, credited the effective use of the "old tools" for Zazi's apprehension. "I think what's striking about the Zazi case is not so much that new tools were being used, but that old tools were being used in a comprehensive fashion," he told NPR. "And that they were being stitched together in a thoughtful, strategic way, so that one tool naturally gave way to another."

Pakistani-American Daood Gilani, aka David Headley, was sentenced to 35 years in prison for his role in scouting out targets for the terrorists who bombed the Taj Mahal Towers Hotel and other sites in the 2008 city-wide attack in Mumbai that killed more than 160 people. Two of Headley's three wives said they had warned U.S. officials of Headley's terrorist ties. One of them said she told federal officials in New York in 2005 that Headley — who authorities later confirmed was an informant for the U.S. Drug Enforcement Administration in Pakistan — had boasted of working for the DEA while training with the militant Pakistani organization Lashkar-e-Taiba, the group behind the Mumbai assault. The other wife, described by the New York Times as a young Moroccan woman, said she told officials at the U.S. Embassy in Islamabad, Pakistan, less than a year before the Mumbai event that her husband was plotting an attack.

"I told them, he's either a terrorist, or he's working for you," she recalled. "Indirectly, they told me to get lost."

The FBI arrested Headley in October of 2009, nearly a year after the Mumbai bombings, following a discovery by British intelligence of his meetings with militants plotting an attack on a Danish newspaper for publishing cartoon depictions of the Prophet Mohammed.

The other case cited by Joyce involved a group of men in San Diego convicted of sending money to an extremist group in Somalia.

General Alexander said the NSA will discuss more cases as the agency determines it is safe to declassify them, though the majority must remain secret for security reasons. Perhaps future disclosures concerning the NSA's success in finding terrorists and preventing terrorist attacks will be more persuasive than what has been revealed thus far.

Photo of NSA Director General Keith Alexander: AP Images