Reverse Warrants: Casting a Data Mining Net into a Sea of Cell Phone Location Data to Catch Criminals Who Commit Pattern Crimes

February 17, 2019

A reverse warrant is a tool used by law enforcement to acquire data about all mobile devices used within a certain geographical area, during a specified time, in order to develop suspects.  They have been used by the Raleigh Police department on several occasions in high profile cases, including a recent fire that was suspected for arson which destroyed a high-rise under construction and shut down several city blocks downtown.  The use of reverse warrants has been questioned on the grounds of whether they contain enough particularity to satisfy the Fourth Amendment: are they a technologically-aided fishing expedition?  No, reverse warrants are instead closer to a modern pen register rather than a wiretap.  Reverse warrants do not acquire identifying information, nor the content of communications, and are therefore constitutional.

So what is a Reverse warrant?  A reverse warrant demands from a technology company, such as Google, a list of mobile devices that were near the scene of a crime when the crime occurred.  The technology that makes a reverse warrant possible (and a powerful tool for law enforcement) is the storage of location data that smartphones collect through normal use.  Every time a smartphone opens up Google Maps, utilizes the browser, updates the weather, or starts using nearly any application on the phone – then the phone’s precise location (longitude and latitude) are logged.  A precise location can be gathered through GPS, a Wi-Fi signal, over cellular data, or even through cell phone tower triangulation – and this location is “accurate to the square foot.”

When getting the list of devices that were in the area during the crime, investigators request that the phone or accounts be identified only by a numerical identifier.  Subsequent warrant requests would be submitted to get identifying information about people who were suspects or possible witnesses to the crime.  This two-step process means that investigators do not obtain identifying information for a person in the initial request sent to Google, but only after eliminating most of the phones and accounts from the returned reverse warrant data and submitting a new warrant.  This also complies with both the Fourth Amendment and Google’s internal way of processing warrants, helpfully described by the company as the “Way of a Warrant.”

Reverse warrants serve the public by giving law enforcement a method for helping to solve crimes, particularly ones that have a discernable pattern, and are a constitutional way of doing so.

Reverse warrants are especially useful when investigating pattern crimes, crimes that are committed by the same suspect over multiple days and times.   This is because the pattern itself narrows down the devices that were located within the search area: out of the possibly thousands of devices identified initially, far fewer would also be in the same area during the second crime, even fewer in the third, and so on.  Because of this narrowing of suspects, in a recent investigation into five robberies of a single store, federal agents bypassed the two-step process and requested the identifying information directly for all devices that were within the targeted area during all the specified times.

The above picture is an example of the geographic search area and time limitation provided to Google in a reverse warrant in a pattern crime submitted by the FBI.  The center of the circle is the location of a Dollar General store which was robbed multiple times, and the yellow circle represents the geographical area covered by the reverse warrant in this case.  The FBI set a radius of 375m, thereby gathering the phone information for every device in an area covering 109 acres.

The most compelling argument that Reverse Warrants are constitutional comes from the Wake County District Attorney, who said “[w]e’re not getting text messages or emails or phone calls without having to go through a different process and having additional information that might lead us to a specific individual.”  This is significant because it is the nature of the information which determines whether a legitimate expectation of privacy has been violated.

When law enforcement uses technological aids to narrow down a list of suspects using information held by Google; not identifying individuals whose devices were in the area of the crime when a crime took place, but instead anonymously identifying devices, it is a constitutional use of government power.  Reverse warrants serve the public by giving law enforcement a method for helping to solve crimes, particularly ones that have a discernable pattern, and are a constitutional way of doing so.

Alex Rutgers, 11 February 2019