The National Security Agency is gathering nearly 5 billion records a day on the location of cellphones around the world. Ashkan Soltani, a Washington Post contributor and an independent privacy and security researcher, sat down with The Post's Alice Rhee to explain. (Thomas LeGro/The Washington Post)

The National Security Agency is monitoring the locations of most of the world’s cellphones, examining billions of records daily in an effort to identify associates of surveillance targets, Barton Gellman and Ashkan Soltani report. Documents describing the bulk collection were given to The Washington Post by former NSA contractor Edward Snowden.

Senior intelligence officials said that the program, known as CO-TRAVELER, does not operate inside the United States, but U.S. cellphones used abroad are visible to the system.

The NSA has little interest in most of the world’s population, but it collects information about where they are anyway to identify people who may be associated with those who the agency believes are dangerous:

The NSA has no reason to suspect that the movements of the overwhelming majority of cellphone users would be relevant to national security. Rather, it collects locations in bulk because its most powerful analytic tools — known collectively as CO-TRAVELER — allow it to look for unknown associates of known intelligence targets by tracking people whose movements intersect.

Still, location data, especially when aggregated over time, are widely regarded among privacy advocates as uniquely sensitive. Sophisticated mathematical tech­niques enable NSA analysts to map cellphone owners’ relationships by correlating their patterns of movement over time with thousands or millions of other phone users who cross their paths. Cellphones broadcast their locations even when they are not being used to place a call or send a text message.

CO-TRAVELER and related tools require the methodical collection and storage of location data on what amounts to a planetary scale. The government is tracking people from afar into confidential business meetings or personal visits to medical facilities, hotel rooms, private homes and other traditionally protected spaces.

“One of the key components of location data, and why it’s so sensitive, is that the laws of physics don’t let you keep it private,” said Chris Soghoian, principal technologist at the American Civil Liberties Union. People who value their privacy can encrypt their e-mails and disguise their online identities, but “the only way to hide your location is to disconnect from our modern communication system and live in a cave.”

The NSA cannot know in advance which tiny fraction of 1 percent of the records it may need, so it collects and keeps as many as it can — 27 terabytes, by one account, or more than double the text content of the Library of Congress’s print collection. . . .

The NSA’s capabilities to track location are staggering, based on the Snowden documents, and indicate that the agency is able to render most efforts at communications security effectively futile.

Barton Gellman and Ashkan Soltani

Recently, the government has argued for its power to collect “metadata” on U.S. cellphone communications, such as the numbers dialed and the locations of phones, by citing a 1979 Supreme Court case. In Smith v. Maryland , authorities used evidence that a robber had called a victim, Patricia McDonough, at home to arrest and convict him:

Without getting a warrant, the police requested the telephone company install a pen register device to record the numbers dialed from Smith’s home. The pen register revealed a call to McDonough, and Smith was arrested.

Smith argued that police violated his Fourth Amendment right to privacy by failing to get a warrant for the pen register. But the Supreme Court disagreed with him. The high court ruled that the audio of the phone call is protected by the Fourth Amendment, but the numbers he dialed is not. Ever since then, law enforcement agencies have invoked Smith v. Maryland to argue that while the contents of communications enjoy Constitutional protection, “metadata” like phone numbers dialed does not. The NSA argues that the same ruling applies to location metadata.

But Smith v. Maryland was a very different case in a very different time than the intelligence activities laid bare by documents from former NSA contractor Edward Snowden. For one thing, Smith v. Maryland involved the very narrow targeting of data collection about a specific person the police already suspected of committing a crime, bulk collection and long-term storage of data about huge numbers of innocent people. But more importantly, the surveillance capabilities of current technology were almost unthinkable in 1979. . . .

Because everyone was using landlines when Smith v. Maryland was decided, getting metadata didn’t mean getting information about whenever a cellphone connected to which tower or transmitted GPS coordinates to a provider. So back then, location tracking was a much more onerous affair, requiring so many resources it was only used for the most serious investigations.

Andrea Peterson

For complete coverage of the documents provided to the media by Snowden and what they reveal about the National Security Agency, visit this page.