NSA headquarters at Fort Meade. (NSA)

My colleagues Bart Gellman and Ashkan Soltani have a new story reporting that the National Security Agency collects 5 billion cellphone location records every day. Documents released by whistleblower Edward Snowden reveal that the information includes some data about the locations of Americans, but didn't provide specific estimates about how many Americans were affected. When asked about it, an NSA spokeswoman claimed it would be impossible to come up with an estimate:

“It’s awkward for us to try to provide any specific numbers,” one intelligence official said in a telephone interview. An NSA spokeswoman who took part in the call cut in to say the agency has no way to calculate such a figure.

This seems difficult to believe. The metadata the NSA collects about mobile devices should contain more than enough information to make an educated guess about whether they are associated with an American. By examining a random sample of devices it is tracking, the agency should be able to come up with a reasonable estimate of how many of those devices are associated with a U.S. person.

That doesn't translate to a precise count of the number of Americans subject to surveillance. Some Americans own multiple cellphones, and some foreigners might sign up for U.S. cellphone plans. But it should give at least a rough estimate of how many Americans are having their location data scooped up by the NSA.

This isn't the first time the NSA has claimed it wasn't possible to estimate how many Americans were the targets of its surveillance programs. Last year, two U.S. senators pressed the NSA to provide a "ballpark estimate" of the number of Americans who had been monitored under the 2008 FISA Amendments Act. In response, the NSA said that "obtaining such an estimate was beyond the capacity of this office," and that attempting to estimate the number would violate the privacy of Americans.

This reflects a problem with the way that the U.S. intelligence system is currently structured. Publishing estimates about the number of U.S.-based devices that inadvertently wind up in NSA databases would improve oversight of the spy agency without tipping U.S. adversaries off to the NSA's "sources and methods" of information collection. But the secretive way that NSA programs have been designed and implemented have allowed the agency to avoid this kind of basic accountability.