THe Supreme Court ruled Friday that law enforcement cannot track people's movements for periods of weeks or months without a warrant.
In a 5-4 ruling, the court held that the acquisition of cell-site records by government officials is covered under the Fourth Amendment.
Chief Justice John Roberts who wrote the opinion sided with the court's four liberal judges; Sonia Sotomayor, Elena Kagan, Ruth Bader Ginsburg and Stephen Breyer - while Justice Anthony Kennedy, Clarence Thomas, Samuel Alito and Neil Gorsuch dissented.
While stressing that their decision doesn't question longstanding surveillance techniques and tools such as security cameras, Roberts said that historical cell-site records present even greater privacy concerns than monitoring via GPS.
"Here the progress of science has afforded law enforcement a powerful new tool to carry out its important responsibilities," Roberts said, adding "While individuals regularly leave their vehicles, they compulsively carry cell phones with them all the time."
"A cell phone faithfully follows its owner beyond public thoroughfares and into private residences, doctor’s offices, political headquarters, and other potentially revealing locales."
The conservative judges strongly objected - writing four times as much in their dissents than Roberts did for the court's majority.
Justice Anthony Kennedy said the government's search of cellphone location records was permissible because they were held by the service provider, not the individual. "The court's new and uncharted course will inhibit law enforcement."
Justice Samuel Alito called it a "revolutionary" ruling that "guarantees a blizzard of litigation while threatening many legitimate and valuable investigative practices upon which law enforcement has rightfully come to rely." -USA Today
The foundation of the ruling stems from a series of armed robberies which took place in Michigan and Ohio between 2010 and 2011. To prosecute the suspect, Timothy Carpenter, the government used the Stored Communications Act of 1986 to obtain his cell phone location information - revealing his movement for more than 127 days and placing him in proximity to the crimes.
According to the Stored Communications Act, cell phone companies can turn over records to the government if there are "reasonable grounds" to believe they will help in a criminal investigation. This is a much lower standard than a warrant.
Prior to Friday's ruling, lower courts upheld the search of Carpenter's records under the "third-party doctrine," which has been used in earlier Supreme Court cases to justify government access to suspects' bank records and landline phone numbers.
In 2016, police made some 125,000 requests for cellphone location data from Verizon and AT&T alone, often involving several suspects over periods of months. Courts routinely grant those requests under the 1986 law. -USA Today
Privacy groups say that cellphone location data is different, however, and could lead to a slippery slope of privacy violations involving email, text messages, social media posts and internet browsing histories of suspects.
In his opinion, Roberts noted that the intersection of privacy and technology will be "the real challenge for the next 50 years."
"What does the Fourth Amendment mean when you can, through technology, literally see through walls with heat imaging?"