BOB GARFIELD:  When the Edward Snowden leaks broke last year, the most discussed and lamented disclosure was the National Security Agency's gathering of metadata from domestic phone calls. Here's what Snowden told the Guardian website last June.

 [CLIP]:

EDWARD SNOWDEN:  Now, increasingly, we see that it's happening domestically. And to do that, they, the NSA specifically, targets the communications of everyone. It ingests them by default. It collects them in its system and it filters them and it analyzes them and it measures them and it stores them for periods of time, simply because that's the easiest, most efficient and most valuable way to achieve these ends.

  [END CLIP]

BOB GARFIELD:  To listen to the media on the subject, including this program, the public might have assumed that virtually no phone call metadata went unmonitored by the NSA. But, maybe not. Last week, the Washington Post reported that just 30 percent of the call metadata is gathered because, the Post tells us, the government selects the technological and legal wherewithal to capture the cell phone traffic that now dominates telecommunication. This has wide implications for how effective the program might be and changes the narrative of an all-knowing NSA.

Ellen Nakashima wrote that story for the Washington Post. She says there are a couple of possible reasons why the NSA never denied the charges that they were collecting every phone call.

ELLEN NAKASHIMA:  They consistently described how they needed to have the entire haystack in order to find the needle, that the program was premised on breadth and depth, so they needed to have as broad and comprehensive a set of data as possible, in order to make sure they didn't miss that one number that could be the clue to the terrorist plot in the making.

At the same time, I don't think they wanted to give any adversaries or bad guys a roadmap to what companies or what types of communications they should use or avoid, and so weren’t about to say, well, this is exactly the number of, of records we do collect and these are the companies we do collect from.

BOB GARFIELD:  You used the “needle in the haystack” metaphor. If you only have a third of a haystack to go through, does that not diminish the chances of locating [LAUGHS] that needle?

ELLEN NAKASHIMA:  To hear officials say it, they feel that even at 20 to 30 percent they are still getting some value. For instance, as long as the collection is spread across different vendors and as long as people are making calls between phone companies their feeling is you’re bound to pick some of that up.

From their point of view, the program is also useful in helping us learn whether or not there is a network that is poised to strike. For instance, with the Boston Marathon bombings, they used the collection to determine whether or not the Tsarnaev brothers were part of a larger network that was about to carry out another attack. They determined, they say, quickly that there wasn't, thanks to this, this program. So that’s sort of what some have called the peace of mind metric.

BOB GARFIELD:  But you’ve raised a sort of “scare the living daylights out of you” metric by reminding us of an - another time when the, the haystack was too small.

ELLEN NAKASHIMA:  The government has said that the reason they created this program, in the first place, was to prevent a repeat of what happened in 2001, when there was, in fact, a 9/11 hijacker in San Diego. So he was calling to an Al Qaeda safe house in Yemen. They had the content of the call. They just didn't know that the other end of the call was of someone sitting in San Diego. So they like to say that had they had the database of phone records, they might have discovered that there was the person in San Diego and gotten the FBI to move on him.

BOB GARFIELD:  So, we have both privacy breaches and an ineffectual security apparatus, the worst of both worlds.

ELLEN NAKASHIMA:  The administration, the government has said they are aiming to get closer to 100 percent. Now, they were, at one point in time, closer to 100 percent. It's just that over the years, as more and more people turned to cell phones and fewer people used land lines, and because most of their collection had been initially focused on land lines, the percentages flipped. That’s not to say they’re not collecting any cell phone calls. They are. It’s just that they’re not collecting as many as, as they would like.

BOB GARFIELD:  What is it about cell phones and voiceover Internet protocol communications that makes it harder for the NSA to, to get the metadata on?

ELLEN NAKASHIMA:  In some cases, phone companies have had difficulty separating out cell phone tower or geo-location data from the so-called metadata.

BOB GARFIELD:  And the government is not allowed to collect the cell tower data.

ELLEN NAKASHIMA:  That’s correct, as of this point, no. And then beyond that, there have also been issues with storage. Believe it or not, the NSA is struggling [LAUGHS] to keep up with all of the amounts of data that they might be able to collect, and storage costs money. And in order to get that money in your budget, you have to plan ahead for it in the government and do so in a way that would meet all of the, the rules for compliance set up by this Surveillance Court.

BOB GARFIELD:  You know, Ellen, there's a couple of ways this could shake out, it seems to me, from a PR perspective. One is that the NSA could be seen as kind of horrible, subject to the ebb and flow of technology and budgets and other bureaucratic difficulties, just like any other government agency. Ah, so maybe it isn’t so bad, after all. And the other is, oh my God, they’re not that good, after all.

ELLEN NAKASHIMA:  I’m sure they’d want to be seen by the American people as an agency that can be trusted. And they'd probably rather not be seen by the terrorists, at all. But the bottom line here is, if they're going to restore the people's trust, then they really can't continue to foster a perception that is so out of sync with reality.

BOB GARFIELD:  All right. Ellen, thank you.

ELLEN NAKASHIMA:  Thank you.

BOB GARFIELD:  Ellen Nakashima covers national security for the Washington Post.