The director of the National Security Agency, Gen. Keith Alexander, defended the controversial surveillance programs exposed by Edward Snowden and revealed details of two previously unreported cases he said were cracked with the help of the programs. NBC's Andrea Mitchell reports.

The National Security Agency is reviewing whether to stop collecting a vast stockpile of records of Americans’ telephone calls — the most controversial component of its surveillance programs — by allowing telecommunications companies to retain the data until U.S. intelligence officials have a specific reason to review it for possible connections to terror plots, U.S. officials said Tuesday

The NSA’s director, Gen. Keith Alexander, disclosed the review during a hearing before the House Intelligence Committee, saying the agency and the FBI are jointly re-examining “how we actually do this program.”

Asked by Rep. Adam Schiff, D-Calif., if the records of phone calls – known as metadata -- could be left in the hands of telecommunications firms and then reviewed only when there is a suspicion “of a foreign terrorist connection,” Alexander replied: “I do think that that’s something that we’ve agreed to look at and that we’ll do. It’s just going to take some time. We want to do it right.”

The NSA’s sweeping collection tens of millions of phone records was disclosed on June 5 by the Guardian newspaper after ex-NSA contractor Edward Snowden leaked to the paper a top secret order from the Foreign Intelligence Surveillance Court requiring Verizon to turn over information on all calls in its system to the NSA.

Under the program, NSA does not eavesdrop on actual phone calls. Instead, it collects the metadata —phone numbers, the time and length of each call – from telecommunications companies. The firms have been secretly turning over the data to the NSA under FISC court orders for years based on a provision of the Patriot Act that forbids the companies from disclosing the NSA’s collection to their customers, officials say.

Schiff, the congressman who questioned Alexander at the hearing, said the agency’s review of the program was prompted by the public backlash over disclosure of the scope of the agency’s sweeping collections of Americans’ data.

“I think it’s coming from the highest level — the public pressure,” he said after the hearing. “I think they realize there’s a need to maintain public support for what they’re doing.”

Director of National Intelligence James Clapper first hinted at the review in a recent interview with NBC News correspondent Andrea Mitchell, saying that Senate Intelligence Committee chair Sen. Dianne Feinstein had asked the intelligence community to examine if it can “refine” its collection procedures. “We owe her an answer in about a month,” he said.

Separately, a U.S. government official told NBC News on Tuesday that Alexander and FBI Director Robert Mueller are now jointly conducting the review.

The collection of the metadata is separate from another program revealed by Snowden. Under that program, known as PRISM, the NSA intercepts the phone calls and read the emails of individuals in the United States only after obtaining FISC warrants showing they have connections to suspected terrorists overseas.

Together, Alexander testified Tuesday, the programs are “immensely valuable” and provided “critical leads to help prevent over 50 potential terrorist events in more than 20 countries.” Over 90 percent of the cases involved information generated by the program targeting foreign terrorists—not the domestic collection of telephone metadata.

As outlined by Alexander and other officials, the domestic telephone metadata is “queried” (to see if those numbers have called or received calls from others suspected of a terrorist connection) by NSA analysts under tight controls. Fewer than 300 such queries were made last year, he and other officials said.

While acknowledging the program is under review, Alexander also said there may be operational problems in allowing the telecommunications companies to retain the data, rather than the NSA.

“The concern is speed in crisis,” said Alexander, suggesting that it might take longer to retrieve the data from the telecommunications firms than simply checking in the agency’s own database.

In some cases, the data may not be available at all. Officials also testified Tuesday that, under the current NSA program, the telephone metadata is retained for five years. But Chris Inglis, deputy director of NSA, said that by contrast the phone companies only retain the data “from six to 18 months.”

The U.S. government official who discussed the review with NBC News, speaking on condition of anonymity, noted that there is currently no legal requirement for the phone companies to retain the data at all. So, as a first step, Congress might need to pass legislation mandating the phone companies to keep its records of phone calls for a longer period than they current do, the official said.

One other potential operational hurdle noted by the official: When NSA has evidence of possible terrorist connections to a phone number, it might have to query multiple phone companies to get the information it can now easily access through one database.

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