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Emerging tech and law enforcement: What are geofences and how do they work?

While GPS is not new technology, the use of geofence warrants in law enforcement investigations is relatively recent

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By Prathi Chowdri

Have you ever checked your phone’s Maps app to see a section of the freeway highlighted in red, indicating a traffic slowdown? This is useful information, but how does Google know about the traffic jam in the first place?

Some might suspect Google is monitoring actual traffic conditions, but that assumption is incorrect. Rather, the search giant provides real-time traffic information by using GPS to track the movement of cell phones within their owners’ cars.

Slow phones = slow cars = traffic. By tracking and recording the position of billions of individual devices in real time, Google provides a helpful service to consumers.

Google’s location history

Google uses a feature called Location History to pull this off. According to Google, the feature is turned off by default, requiring a user to affirmatively opt in to make use of location-related information.

Deleting Location History or even pausing being tracked can be confusing for users. If you delete the application that prompted the opt-in, Google may continue to collect GPS data through your account. Even if you turn off Location History in your Google account, the company may sometimes still collect time-stamped location data from your device through other applications or services. For example, if you access Maps — whether it is to get directions or to read reviews for local businesses — your phone may still report its location to Google even if you have opted out. Similarly, searching for just about anything can trigger your phone to report its location to Google.

All told, Google is tracking over a billion devices worldwide, both Android and iOS, and saving that data to help provide services and advertising to its users. And this data can be extremely helpful to law enforcement investigators.

What is a geofence warrant?

A geofence warrant is a formal request from law enforcement to a provider (usually Google) to provide the Location History data of every device within a defined geographic region and within a specific time frame. Those two factors — the location and the time — are the most important elements of a geofence warrant. The purpose of the warrant is to eventually narrow down a single user from the list of devices returned by the provider, with the goal of identifying that device owner as the perpetrator of a crime.

The most obvious way devices are tracked is through the global positioning system, or GPS, which is usually accurate to within a dozen or so feet in optimal conditions. Device locations may also be tracked using cell phone tower triangulation as well as Wi-Fi and Bluetooth connections. Location History information can often reveal a device location much more precisely than cell site location data. Providers like Google collect this data every two minutes or so, so it is not always 100% complete. However, it is generally accurate enough to show that a device (and its owner) was in a specific area at a specific time.

While GPS is not new technology, the use of geofence warrants in law enforcement investigations is relatively recent. Whereas GPS triangulation targets a specific person already known to police, geofence warrants are different in that they cast a fairly wide net. According to a 2022 Slate article, “Google received its first geofence warrant in 2016. Law enforcement’s use of these warrants has grown exponentially since then. Warrants to Google for users’ location information grew 1,500 percent from 2017 to 2018 and 500 percent from 2018 to 2019. In 2019, Google received about 9,000 geofence requests.” That number has continued to grow in the ensuing years.

The geofence warrant process

Picture this: Police receive a report of a shooting at a convenience store. According to eyewitnesses and CCTV, two suspects entered the store, demanded cash from the register, and after an altercation, shot at the store clerk. The incident lasted five minutes. The suspects left the store around 3:15 p.m., ran to a vehicle and drove northbound for two blocks before eyewitnesses lost sight of them.

Based on these facts, police may determine that cell phone location data could be helpful in identifying the suspects. Requesting device information from Google is a three-step process:

Step 1: The investigator sends a warrant to Google requesting information on devices with estimated locations that were within the area (e.g., a two- to three-block perimeter around the convenience store) and time frame (e.g., 3:00 to 3:30 p.m.) defined in the warrant. Google uses Location History data to identify devices matching the warrant criteria. At this point, device owners are still completely anonymous to law enforcement. Instead of user information, the devices are tagged with a code that hides their personal identifying information from investigators.

Step 2: If the investigator can narrow down the results after analyzing the de-identified list of devices and their locations, they may request additional information about specific devices and their locations before and after the time frame in the warrant, as well as outside the geographic “fence” (the original location). For example, if Step 1 provides two devices that were exiting the convenience store at the time and driving in the direction that corresponds with the eyewitnesses, then in Step 2, the investigator may request that Google provide additional information about those devices — specifically, additional Location History data. Now we are outside the original time frame (3:00 to 3:30 p.m.) and the original location (two to three blocks from the convenience store) of the original geofence warrant.

Step 3: Using the information from the first two steps, investigators further narrow down any devices that closely match the pattern of evidence for possible suspects, and request account information (including names and email addresses) from the provider. Slate points out that “Google ‘prefers’ that this third set of users be narrowed from the second set, but it’s possible, at least, that sometimes that doesn’t happen.”

Note that a judge-approved warrant is currently only required before Step 1 in the process. The rest of the steps happen without any additional oversight from the courts. The process appears, rather, to be managed by a Google employee and the law enforcement investigator. Google appears to be using this process to respect users’ privacy and limit the data released to law enforcement. Nonetheless, some people see this as problematic.

Privacy and accuracy concerns

Geofence warrants can be useful in investigations, but they are not without their controversy. One relates to privacy. By their very nature, geofence warrants return data on devices owned by innocent people who may not have even been witnesses to the crime.

In a much-criticized warrant relating to the Capitol riots on January 6, 2021, the FBI requested and received from Google a list of 5,723 devices that was eventually pared down to 1,535 names and used to identify and prosecute participants. A Reason article about the case notes that a warrant of this kind “arguably allows law enforcement to work backward, to say, We think a crime was committed around this place and this time. Let’s sweep up location data for everyone who was there and investigate them all.” This process, author Bonnie Kristian notes, is uncomfortably similar to a “general warrant,” which is exactly what the Fourth Amendment was written to prevent.

Another issue is accuracy. While GPS data appears to be accurate within 10 or 11 feet, that is not always the case. Trees, buildings, bridges and other structures can interfere with the connection to the satellites that allow the system to work. This can cause “dirty data” that may result in innocent people being swept up in the investigation who were not even in the location of the crime when it occurred. Conversely, this potential for inaccuracy can exclude the actual perpetrators. In one case, Jorge Molina was arrested in Arizona and held in jail for six days after location data falsely tied him to an Avondale murder — in spite of Molina having a solid alibi. He subsequently filed a $1.5-million lawsuit against the city of Avondale, its chief of police, and several officers.

Important cases

Two cases illustrate both the concerns and successes regarding geofence warrants.

United States v. Chatrie

On May 20, 2019, a gunman robbed the Call Federal Credit Union in Midlothian, Virginia. The suspect got away with $195,000, escaping on foot. As part of its investigation, law enforcement filed a geofence warrant with Google, which produced responsive Location History information. An analysis of the data led police to arrest Okello Chatrie and charge him with the robbery. Chatrie’s defense team filed a motion to suppress the location data on Fourth Amendment and other grounds.

In her ruling on the motion to suppress, U.S. District Court Judge Hannah Lauck called into question the validity and legality of the sweeping warrant. In part, she said, “Astoundingly, the Government claims that law enforcement established probable cause to obtain all information (Steps 1, 2, and 3) from all users within the geofence without any narrowing measures. Yet the warrant simply did not include any facts to establish probable cause to collect such broad and intrusive data from each one of these individuals.” Since investigators gathered location information on 19 cell phones, only one of which may have been the suspect’s, the judge indicated that the warrant violated the constitutional rights of at least 18 innocent people.

Another concern was the size and scope of the geofence: The perimeter was roughly the length of three football fields, and the time frame was an hour. The judge thought this constituted an overly large dragnet from which to collect device location data.

In spite of her reservations, though, the judge denied the motion and permitted the data gleaned from the geofence warrant on the basis that the detective who had been working with Google in Steps 1, 2, and 3 had acted in good faith. In the conclusion of her decision, she stated: “If the Government is to continue to employ these warrants, it must take care to establish particularized probable cause. As the legal landscape confronts newly developed technology and further illuminates Fourth Amendment rights in the face of geofence practices, future geofence warrants may require additional efforts to seek court approval in between steps, or to limit the geographic and temporal information sought. But in light of the complex legal issues that lead to this Court’s conclusion, the Court cannot say that Det. Hylton’s reliance on the Geofence Warrant was objectively unreasonable.”

In the end, Okello Chatrie was convicted of the bank robbery charges and sentenced to nearly 12 years in federal detention.

Price v. Superior Court of Riverside County

On Oct. 29, 2019, a man referred to in court records as “Jovany R.” was shot and killed on the front porch of his home in Jurupa Valley near Riverside, California. According to investigators, Jovany had been growing marijuana in the home, and they believed the crime was related to his grow house operation. A man named Ahmad Raheem Price was arrested and charged with first-degree murder and unlawful possession of the firearm used to commit the murder.

During their investigation, law enforcement submitted a geofence warrant to Google. In contrast to the Chatrie warrant, the area in this case was limited to “the front porch area where the shooting occurred, and the street in front of the house … for the lengths of two houses in each direction (north and south), between 10:00 p.m. and 10:22 p.m. … a 22-minute period.” This time and location was based on eyewitness testimony as well as 911 calls that provided specific information regarding where and when the crime occurred.

In advance of the trial, Price’s legal team filed a motion to suppress (among other things) the information gained from the geofence warrant on the grounds that the request “(1) failed to satisfy the Fourth Amendment’s probable cause and particularity requirements; (2) had to be traversed based on material factual omissions in their affidavits; and (3) violated the particularity and notice requirements of the California Electronic Communications Privacy Act.”

Unlike the federal court in the Chatrie case, the Price court held that the warrant was “narrowly tailored” and did not question whether the warrant left too much discretion in the hands of law enforcement. This was because the court found that this geofence warrant “was a model of particularity in geographic scope and time period.” In denying the petition, the judge praised the investigators for how the warrant was drafted to “minimize the potential for capturing location data for uninvolved individuals.”

The motion to suppress was denied and Price was tried and convicted in early 2023.

Key takeaways

If you are a law enforcement investigator and want to use geofence warrants in your investigations, here are some tips for helping to ensure your geofence warrants stand up to scrutiny in a future prosecution:

  • Narrow geography: Draw your geofence as tightly as you can, based on the information you have about the crime and who might have committed it. The smaller the geographic area, the more likely the warrant will be considered particularized, and, hopefully, sustain Fourth Amendment challenges. For example, the Chatrie court suggested that instead of simply circling a radius around the crime scene, which can sweep up innocent devices, investigators should have used what they knew about the crime to draw a specific polygon around the area instead. In other words, try to eliminate extraneous areas if you reasonably can.
  • Limited time period: Similarly, be sure to use the narrowest possible time frame, within reason, to limit the number of irrelevant devices (and their owners) returned in the results of Step 1.
  • Justify each step: When you request additional information in Steps 2 and 3, document the logic you used to narrow down the initial list. This will help ensure that you are not “fishing” and help you justify your requests for additional information from the provider.
  • Involve the prosecutor: Do not attempt to do all this by yourself. Work with the prosecutor or your agency counsel as you draft your geofence warrant and work your way through the steps. You may get some good advice during the process, leading to a properly completed and successful investigation.

Geofence warrants are just one of several fascinating new tools being used to investigate crimes. The more you know about them, and the better you adhere to the best practices, the more successful you will be as you put them to work in your investigations.

About the author

Prathi Chowdri joined the Lexipol legal team in September 2015. Prior to Lexipol, Prathi worked as a federal trial attorney defending the NYPD against claims of false arrest, excessive force, wrongful conviction and malicious prosecution in the Southern and Eastern Districts of New York. Many of her cases as Senior Counsel involved complex civil litigation with extensive e-discovery and high-profile claims against the city, its police officers, prosecutors, and corrections officers. During her career, she also worked as an associate at a private law firm in New York City where she continued federal trial litigation in both 42 USC § 1983 and medical malpractice. Prathi has also served as an Adjunct Professor in Constitutional Law at Florida Atlantic University.