The provision that allowed the NSA to collect phone records has helped thwart “dozens” of terror events, Gen. Keith B. Alexander told a Senate panel. (The Washington Post)

The head of the National Security Agency said Wednesday that his agency’s extensive electronic surveillance programs have played a critical role in thwarting “dozens” of terrorist attacks aimed at U.S. targets and abroad.

Gen. Keith Alexander, in making the assertion, cited the two intelligence collection programs that have been the focus of public scrutiny in recent days, one that allows the government to collect the call records of millions of Americans from U.S. telecommunications companies and another that collects Internet records.

In testimony before the Senate Appropriations Committee, he cited in particular 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, which killed more than 160 people. In both instances, he said, the Internet data-mining program helped unravel the plots.

The Obama administration had defended its extensive collection of telephone and Internet records amid a political furor over the surveillance that has consumed Washington for the past week. It follows disclosures in the The Washington Post and the Guardian newspaper.

President Obama last week said he would welcome a public debate over the surveillance, saying Americans will have to sacrifice some privacy for the sake of security. Alexander vowed to quickly make public records that show the success of the phone program in preventing attacks.

Thomas Drake was indicted after leaking classified information to a journalist. Drake ultimately didn’t serve any jail time but he says the experience changed his life forever. He offers insights on what Edward Snowden now faces. (The Fold/The Washington Post)

“I don’t have those figures today,” he said. “Over the next week it will be our intent to get those figures out...I want the American people to know we’re being transparent here.’’

The order that allowed for the NSA collection of phone records of millions of Americans was based on Section 215 of the Patriot Act, which allows law enforcement to obtain a wide variety of “business records,” including calling records. The program collects customer “metadata,” including the phone numbers dialed and the length of calls, which is used by intelligence analysts to detect patterns and personal connections, on every phone call made or received by U.S. customers of major American phone companies.

The principal source of the recent revelations was Edward Snowden, a 29-year-old NSA contractor who has worked for several government agencies.

A senior intelligence official said Wednesday that the CIA has opened an internal investigation to determine what Snowden was involved in when he worked there. Separately, the Office of the Director of National Intelligence has ordered a damage assessment following Snowden’s disclosures, which he acknowledged on Sunday in interviews with the Post and the Guardian.

“Obviously, Mr. Snowden’s actions over the past week-plus are generating a lot of activity on the part of the intelligence community and the CIA (in terms of taking) a look back and seeing what we can learn about just what he may have been involved in, and also see if there are some lessons we can learn,” said the official, who spoke on condition of anonymity to discuss an ongoing inquiry.

CIA officials did not dispute that Snowden was a CIA employee, though they could not say exactly when he worked there. After leaving the agency, he worked for the technology company Dell and later for Booz Allen Hamilton, the consulting firm. While working for Booz Allen earlier this year, he was assigned to the National Security Agency Threat Center in Honolulu, where he is believed to have downloaded highly classified material, including information about widespread surveillance programs run by the NSA.

Karen DeYoung contributed to this report.