Alexander said at least ten of the attacks were set to take place in the United States. NSA: PRISM stopped NYSE attack

Recently leaked communication surveillance programs have helped thwart more than 50 “potential terrorist events” around the world since the Sept. 11 attacks, National Security Agency Director Keith Alexander said Tuesday.

Alexander said at least 10 of the attacks were set to take place in the United States, suggesting that most of the terrorism disrupted by the program had been set to occur abroad.

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The NSA also disclosed that counterterrorism officials targeted fewer than 300 phone numbers or other “identifiers” last year in the massive call-tracking database secretly assembled by the U.S. government.

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Alexander said the programs were subject to “extraordinary oversight.”

”This isn’t some rogue operation that a group of guys up at NSA are running,” the spy agency’s chief added.

The data on use of the call-tracking data came in a fact sheet released to reporters in connection with a public House Intelligence Committee hearing exploring the recently leaked telephone data mining program and another surveillance effort focused on Web traffic generated by foreigners.

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Alexander said 90 percent of the potential terrorist incidents were disrupted by the Web traffic program known as PRISM. He was less clear about how many incidents the call-tracking effort had helped to avert.

Deputy FBI Director Sean Joyce said the Web traffic program had contributed to arrests averting a plot to bomb the New York Stock Exchange that resulted in criminal charges in 2008.

Joyce also indicated that the PRISM program was essential to disrupting a plot to bomb the New York City subways in 2009. “Without the [Section] 702 tool, we would not have identified Najibullah Zazi,” Joyce said.

However, President Barack Obama acknowledged in an interview aired Monday that it is impossible to know whether the subway plot might have been foiled by other methods.

”We might have caught him some other way. We might have disrupted it because a New York cop saw he was suspicious. Maybe he turned out to be incompetent and the bomb didn’t go off. But at the margins we are increasing our chances of preventing a catastrophe like that through these programs,” Obama told Charlie Rose on PBS.

At the hearing, Alexander detailed the scope and safeguards of the programs, while Deputy Attorney General James Cole laid out the legal basis for the surveillance.

“This is not a program that’s off the books, that’s been hidden away,” Cole said of the call-tracking program, which was classified “top secret” prior to recent leaks. He noted that the Patriot Act provision found to authorize it has been twice reauthorized by Congress.

“All of us in the national security [community] are constantly trying to balance protecting public safety with protecting people’s civil liberties,” Cole said.

NSA Deputy Director Chris Inglis said a very limited number of individuals are authorized to access the call-tracking database.