Has the NSA's massive collection of metadata thwarted any terrorist attacks?
It depends which senator you ask. And evidence that would help settle the matter is, yes, classified.
Sen. Mark Udall, D-Colo., told CNN on Sunday, "It's unclear to me that we've developed any intelligence through the metadata program that's led to the disruption of plots that we could [not] have developed through other data and other intelligence."
He said he could not elaborate on his case "without further declassification."
Sen. Feinstein told ABC that the collection of phone records described in the Verizon order had been "used" in the case of would-be New York subway bomber Najibullah Zazi. Later in the interview, Feinstein said she couldn't disclose more because the information is classified. (It's worth noting that there's also evidence that old-fashioned police work helped solve the Zazi case 2014 and that other reports suggest the Prism program, not the phone records, helped solve the case.)
How much information, and from whom, is the government sweeping up through Prism?
It's not clear.
Intelligence director Clapper said in his declassified description that the government can't get information using Prism unless there is an "appropriate, and documented, foreign intelligence purpose for the acquisition (such as for the prevention of terrorism, hostile cyber activities, or nuclear proliferation) and the foreign target is reasonably believed to be outside the United States."
One thing we don't know is how the government determines who is a "foreign target." The Washington Post reported that NSA analysts use "search terms" to try to achieve "51 percent confidence" in a target's "foreignness." How do they do that? Unclear.
We've also never seen a court order related to Prism -- they are secret -- so we don't know how broad they are. The Post reported that the court orders can be sweeping, and apply for up to a year. Though Google has maintained it has not "received blanket orders of the kind being discussed in the media."
So, how does Prism work?
In his statement Saturday, Clapper described Prism as a computer system that allows the government to collect "foreign intelligence information from electronic communication service providers under court supervision."
That much seems clear. But the exact role of the tech companies is still murky.
Relying on a leaked PowerPoint presentation, the Washington Post originally described Prism as an FBI and NSA program to tap "directly into the central servers" of nine tech companies including Google and Facebook. Some of the companies denied giving the government "direct access" to their servers. In a later story, published Saturday, the newspaper cited unnamed intelligence sources saying that the description from the PowerPoint was technically inaccurate.
The Post quotes a classified NSA report saying that Prism allows "collection managers [to send] content tasking instructions directly to equipment installed at company-controlled locations," not the company servers themselves. So what does any of that mean? We don't know.