The White House also confirmed assertions by US officials and members of Congress that electronic eavesdropping by the National Security Agency had helped foil a plot by Islamist militants to bomb the New York subway system in 2009.
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The revelations came as government officials continued to defend the broad, secret surveillance programs that were revealed last week by former NSA contractor Edward Snowden. Snowden's leak revealing the NSA's collection of data from billions of communications each day ignited a national debate over whether the government is intruding too far into Americans' lives in the name of national security.
Some members of Congress - including US Senator Rand Paul, who on Thursday sought to promote a class-action lawsuit against the NSA - have accused President Barack Obama's administration of not telling Congress enough about such surveillance.
Obama and several congressional leaders have argued that lawmakers were kept informed, and that such surveillance was authorized by Congress as part of dramatic security changes that followed the hijacked airline attacks of Sept. 11, 2001.
US authorities have said in recent days that the data collected from telephone and Internet companies has help to thwart "dozens" of attacks.
Pressed on Thursday to defend the data collection programs, White House spokesman Jay Carney pointed to the arrest of David Headley at Chicago's O'Hare International Airport in 2009.
Headley pleaded guilty to terrorism charges in a plot to bomb the Danish newspaper Jyllands-Posten, which published cartoons of the Prophet Mohammad that offended many Muslims.
Headley, a US citizen of Pakistani descent, also admitted to scouting targets for the 2008 Islamic militant raid in Mumbai, India, that killed more than 160 people. He was sentenced in a US court in January to 35 years in prison, escaping the death penalty because of his cooperation with authorities.
"People often ask - because these are by necessity classified programs - you know, can we demonstrate that they're effective. Are they really in our national security interest?" Carney said during his daily briefing for White House reporters.
"There was an effort undertaken to declassify these instances to demonstrate to you and the American people that there are concrete results from these programs," Carney said.
Carney also affirmed claims by legislators and officials that NSA surveillance had helped foil a plot by Afghan-born militant Najibullah Zazi to bomb New York City's subway system in 2009.
A government source familiar with that investigation said that US intelligence had picked up an email from the United States to a known militant suspect in Pakistan in which the US sender talked about how a "wedding was ready."
American investigators traced the email to Zazi, found him in Denver, then followed him across the country to New York, the source said. Zazi subsequently pleaded guilty to terrorism-related charges.
On Wednesday, General Keith Alexander, director of the NSA, told a congressional committee that US electronic eavesdropping had helped to foil "dozens" of possible attacks.
Senator Dianne Feinstein, head of the Senate Intelligence Committee, said Alexander had pledged to make public on Monday a list of attack plots that had been thwarted by the programs.
A US official said the Obama administration had given Congress a detailed, classified catalog with examples of thwarted plots. The administration was working on declassifying some of the material for the public, said the official, who was not authorized to comment publicly.
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