In conversations with The Washington Post over Barton Gellman and Ashkan Soltani's recent story on cellphone location tracking, an intelligence agency lawyer told Gellman, "obviously there is no Fourth Amendment expectation in communications metadata.” But some experts say it's far from obvious that the 1979 Supreme Court case on which the administration bases this view gives the government unfettered power to scoop up Americans' cellphone location data.

That Supreme Court case, called Smith v. Maryland, started with a 1976 robbery of a woman named Patricia McDonough in Baltimore. Soon afterward, she began receiving threatening calls from a man who identified himself as the robber. In one of those calls, the man on the line asked her to come outside, where she saw a 1975 Monte Carlo she had earlier described to the police drive by slowly.

The police spotted a man matching McDonough's description of the robber driving a 1975 Monte Carlo in her neighborhood later that month and traced the license plate to a Michael Lee Smith. 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.

"The big technological change is that computers have allowed law enforcement to monitor on a much broader scale," says Orin Kerr, a law professor at George Washington University. Despite those changes, Kerr agrees with the government that Smith v. Maryland allows warrantless collection of location information. But others disagree.

Michelle Richardson, an attorney at the American Civil Liberties Union, points out that the type of metadata collected in 1979 was much more limited in scope than what telecom companies have access to now. This is particularly true on the issue of location information, she argues. "It reflects who you know, what you do -- it can reflect if you attend a certain church, or a political rally, or AA meetings."

"It's incredibly rich data, which is why they want it so bad," says Richardson. 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.  Given these new capabilities, Richardson calls Smith v. Maryland "way out of date," adding "it was before cellphones, before the Internet, before services that collect intensely personal information."

And there's some reason to believe that a majority of the current Supreme Court justices might agree with her on the location data aspect of metadata. The most recent Supreme Court case involving location tracking, United States v. Jones was settled on narrow trespassing grounds in 2012. But five Supreme Court justices signed on to concurring opinions that questioned whether Smith v. Maryland holds up in the face of modern technology.  An opinion concurring in judgment with the Jones decision written by Justice Samuel Alito, and joined by Justices Ruth Bader Ginsburg, Stephen Breyer and Elena Kagan specifically noted the prevalence of smartphones and argued that "the use of longer term GPS monitoring in investigations of most offenses impinges on expectations of privacy."

A separate concurring opinion from a fifth justice, Sonia Sotomayor made many of the same arguments, saying "fundamentally, it may be necessary to reconsider the premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties" -- and even went further by arguing that "awareness that the Government may be watching chills associational and expressive freedoms."

Of course, as Kerr notes, those comments are "not a holding of the Court" and until there is a ruling it's impossible to divine how they might navigate the multitude of technical aspects involved in such a case. So until the Supreme Court hears a case applying Smith v. Maryland to location data, we won't know if the Constitution allows the government to obtain this kind of information without a warrant.

And in the case of the massive cellphone location tracking program revealed by Gellman and Soltani, Richardson believes the administration would push to stay the course, regardless of if courts specifically ruled in favor of location data having Fourth Amendment protections. Since the Protect America and FISA Amendment Acts of 2008, intelligence agencies have adopted a legal argument that as long as the targets of a program are foreign nationals the Fourth Amendment doesn't apply, says Richardson, even if those programs "incidentally" collect information about American citizens.

"I think that they're minimizing the impact by calling it incidental," says Richardson, adding that that theory has not been tested in the courts as of yet. "When you're taking about billions of billions even a tiny fraction means a huge number of Americans are in there."