'Groundbreaking Victory' for Privacy as Supreme Court Rules Against Warrantless Cellphone Tracking

In a decision that digital rights advocates called "a groundbreaking victory for Americans' privacy rights," the U.S. Supreme Court ruled on Friday that police generally must obtain a warrant before collecting cellphone records that can be used to track a person's movements.

The American Civil Liberties Union (ACLU), which represented the petitioner, predicted the "historic" win "will have a ripple effect for privacy," particularly as it applies to data held by third parties. As the group explained in a series of tweets, "It will help protect all sorts of digital information stored online, from emails to data from smart home appliances."

Nate Freed Wessler, the ACLU staff attorney who argued the case, discussed the ruling and what it could mean for future cases in a short video posted on Twitter:

Famed government whistleblower Edward Snowden tweeted the decision was a "victory for ACLU--and America."

Chief Justice John Roberts--joined by Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor, and Elena Kagan--wrote the majority opinion (pdf), which explained: "Cellphones perform their wide and growing variety of functions by continuously connecting to a set of radio antennas called 'cell sites.' Each time a phone connects to a cell site, it generates a time-stamped record known as cell-site location information (CSLI). Wireless carriers collect and store this information."

The case in question involved Timothy Carpenter, who was sentenced to serve more than 100 years in prison for multiple robberies in Michigan and Ohio. With a court order that has a lower standard than a warrant, investigators obtained more than four months of location records from Carpenter's cellphone company, and used that information to build a case against him.

Considering that CSLI data "is detailed, encyclopedic, and effortlessly compiled," and that "individuals have a reasonable expectation of privacy in the whole of their physical movements," the court determined that "the government will generally need a warrant to access CSLI" to comply with the Fourth Amendment.

The narrow ruling, Roberts noted, does not apply to "other business records that might incidentally reveal location information" or "other collection techniques involving foreign affairs or national security." He added that warrantless searches still may be conducted under "exigent circumstances."

The Constitution Project called the ruling a "long-overdue advance for privacy rights" but also accused the court of "taking baby steps as tech sprints ahead."

The high court's other four conservative justices--Samuel Alito, Neil Gorsuch, Anthony Kennedy, and Clarence Thomas--each wrote dissenting opinions. The ruling on Friday follows a 2014 decision in which the justices unanimously determined that police must obtain a warrant before they can search the cellphone of an arrested person.