A crime occurs outside a business at 9:17 p.m.
There are no clear fingerprints.
No witness gets a good look at the person responsible.
A security camera captures movement, but not enough detail to identify anyone.
Investigators once might have started by asking who was seen in the area.
Modern technology creates another possibility.
Instead of beginning with a known suspect and asking where that person's phone was, investigators may seek information about which devices were near the crime scene during a particular period of time.
That reverses the traditional direction of the investigation.
The search does not necessarily begin with a person.
It begins with a place.
The results can potentially include the person investigators are looking for.
They can also include everyone else who happened to be nearby.
The Investigation Can Begin With a Digital Boundary
This investigative technique has commonly been associated with what are called geofence warrants.
A geofence is essentially a geographic boundary.
Investigators identify an area surrounding a crime scene and a period of time relevant to the event. They then seek location information associated with devices that were detected within that boundary.
Imagine a convenience store is robbed between 10:02 and 10:06 p.m.
Investigators might be interested in devices appearing near the store around that time.
The resulting information could potentially include the robber's device.
But it could also include the phones of the cashier, customers, people living nearby, drivers stopped at an intersection and someone walking a dog across the street.
The technology does not initially know which person matters.
It identifies devices based on location.
That Is Very Different From Tracking a Known Suspect
Traditional location investigations often begin with an identified person.
Police suspect John Doe committed a crime.
Investigators seek information associated with John Doe's phone.
The target is known before the location information is obtained.
A geofence investigation can operate in the opposite direction.
Investigators know where the crime happened but do not necessarily know who committed it.
They use location information in an effort to identify possible people who were there.
That difference has generated major Fourth Amendment questions.
The Fourth Amendment generally requires warrants to be supported by probable cause and to particularly describe the place to be searched and the persons or things to be seized.
Geofence warrants challenge courts to apply those requirements when investigators are seeking information associated with a group of initially unidentified devices.
Your Phone Can Appear in the Results Even If You Did Nothing Wrong
Suppose you live in an apartment above a store.
The store is robbed.
Your phone was at home during the robbery.
That location could make your device relevant to a geographic search even though you never entered the store and had nothing to do with the crime.
Or imagine you were sitting in a restaurant next door.
You might appear for the same reason.
This is an important characteristic of location-based investigative searches:
Presence in the dataset does not establish involvement in the crime.
It may establish only that a device generated location information placing it within or near an area of interest.
Investigators still need to determine what that information means.
A Phone's Location Is Not Necessarily Its Owner's Location
Even accurate device-location information has an important limitation.
It describes the device.
People sometimes leave phones behind.
A family member may borrow a phone.
Someone can carry multiple devices.
A tablet or other connected device may be associated with the same account.
Location estimates themselves can also vary in precision.
So even if investigators establish that a particular device was near a crime scene, another question remains:
Who actually possessed it?
That distinction is easy to overlook because phones are so personal that people often treat the device and its owner as interchangeable.
Legally and factually, they are not always the same thing.
Google Became Central to the Geofence Debate
Google historically became particularly important to geofence investigations because location information associated with its services could be stored in a system that investigators sought through legal process.
The company's handling of that information changed substantially over time.
Google announced changes that moved Maps Timeline data to users' devices and reduced the default period before certain Timeline information was automatically deleted. The company also said the shift meant it would no longer have the same ability to respond to broad geofence warrants using centrally stored Timeline information.
That technological change illustrates something important about digital evidence.
An investigative technique can depend not only on constitutional law but also on how a private company designs its systems.
If the architecture changes, the evidence available to investigators can change with it.
Courts Have Struggled With Whether These Warrants Are Constitutional
Geofence warrants have produced substantial disagreement in federal courts.
In 2024, the Fifth Circuit considered the issue in United States v. Smith and concluded that the geofence warrant involved in that case was unconstitutional under the Fourth Amendment. The court described geofence warrants as searches that can identify people based on their presence within a geographic area rather than beginning with an identified suspect.
The court nevertheless did not suppress the evidence because it concluded that the officers had acted in good-faith reliance on the warrant under the circumstances of the case.
Other courts examining geofence practices have taken different approaches, and the constitutional analysis can depend on the warrant's scope and the investigative process involved.
That disagreement is one reason broad statements such as “geofence warrants are legal” or “geofence warrants are illegal” can be misleading.
The issue has been developing through litigation rather than governed by one simple nationwide rule covering every possible form of location-based warrant.
The Fifth Circuit Case Began With a Postal Robbery
The facts of United States v. Smith show why investigators were attracted to the technique.
The case involved the armed robbery of a postal worker in Mississippi.
Investigators did not initially know who committed the robbery.
They obtained a geofence warrant seeking information associated with devices present within a geographic area around the crime during a specified period.
The investigation eventually contributed to identifying suspects.
But the Fifth Circuit focused on the broader constitutional implications of obtaining location information associated with people who were not identified before the search began.
The case put two important interests into direct tension.
Law enforcement had a serious crime to solve.
At the same time, the investigative method could expose location information belonging to people merely because they happened to be within the geographic boundary.
One Location Point Can Be Less Revealing Than a Pattern
A single observation may not say much.
Your phone appeared near a grocery store at 4:15 p.m.
That is a small piece of information.
But location information becomes more revealing as observations accumulate.
Suppose records show a device near a residence every night.
Near a particular workplace every weekday.
At a medical clinic on Tuesday.
At a religious service on Saturday.
At a political meeting the following week.
At another person's home repeatedly.
The individual data points begin to reveal a pattern of life.
That concern was central to another major location-privacy case, Carpenter v. United States.
Carpenter Changed the Legal Treatment of Historical Phone Location Records
In Carpenter v. United States, investigators obtained historical cell-site location information associated with Timothy Carpenter's phone.
The government argued in part that the records were held by a third-party cellular provider.
Traditionally, information voluntarily conveyed to a third party has often received less Fourth Amendment protection.
But the Supreme Court declined to apply that principle mechanically to the extensive historical cell-site records at issue.
In 2018, the Court held that the government's acquisition of Carpenter's historical cell-site location information constituted a Fourth Amendment search and generally required a warrant supported by probable cause.
The Court emphasized the deeply revealing nature of long-term location information.
But it also described its decision as narrow.
It did not resolve every future question involving digital location data.
Geofence warrants became one of those future questions.
Carpenter Started With a Person. A Geofence Can Start With Everyone Nearby.
The difference is significant.
In Carpenter, investigators knew whose records they wanted.
They sought historical location information associated with a particular phone.
A geofence search can begin without knowing whose phone will appear.
Investigators define the location and time.
The search identifies devices meeting those conditions.
That means innocent people can be included at the beginning of the process.
Investigators may later narrow the results, but the initial collection is what creates much of the constitutional controversy.
Critics of broad geofence warrants argue that this resembles searching a group of people to discover which one might be suspicious.
Supporters of the investigative technique have emphasized its usefulness in serious cases where the perpetrator is unknown and location evidence may provide an otherwise unavailable lead.
Courts have had to determine how those competing considerations fit within the Fourth Amendment's requirements.
The Size of the Boundary Matters
Not all geographic searches are equally broad.
Imagine two warrants.
One covers a small section of an isolated parking lot for five minutes.
Another covers several city blocks for three hours.
The second could capture information associated with far more people.
The duration matters.
The geographic radius matters.
Population density matters.
Nearby homes and businesses matter.
The type of location matters.
A geographic area surrounding an isolated warehouse may produce a very different privacy problem from an area containing apartments, restaurants, medical offices and hundreds of pedestrians.
This is why the details of the warrant can be more important than the label attached to it.
An Innocent Person Can Still Require Investigation
Suppose a geographic search returns fifteen devices.
One belongs to an employee working inside the business.
One belongs to a resident upstairs.
Three belong to customers.
Another belongs to a delivery driver.
One device remains unexplained.
Investigators may begin trying to determine which devices have innocent explanations and which deserve further investigation.
That process can require additional information.
A digital identifier can become associated with an account.
An account can become associated with a person.
That person's movements may be compared with other evidence.
The original location record may therefore be only the first step in identifying an individual.
For innocent people, that means simply being in the wrong place at the wrong time can potentially place their digital information inside an investigation.
Location Data Can Also Clear Someone
The same evidence can work in the opposite direction.
Suppose investigators suspect someone committed a robbery in Los Angeles at 8:00 p.m.
Reliable location evidence shows that person's phone was active in San Diego around the same time.
That does not automatically establish an alibi—the phone and person are not necessarily inseparable—but it can become important evidence.
Location information can contradict a witness.
It can challenge a timeline.
It can point investigators toward another suspect.
Digital location evidence is not inherently prosecution evidence.
It is information about where a device appears to have been.
Its significance depends on the facts surrounding it.
A Geographic Search Can Produce False Confidence
Maps look authoritative.
A dot appears on a screen.
The temptation is to treat that dot as an exact statement of where a person stood.
Real location systems can be more complicated.
Different technologies estimate location in different ways.
Accuracy can vary.
Buildings can affect signals.
A reported point can represent an estimate rather than a perfectly measured position.
The time associated with a location observation also matters.
Investigators and courts therefore need to understand what the underlying technology actually establishes.
A location record placing a device “near” a building is not necessarily the same as evidence establishing that its owner entered the building.
The Technique Extends Beyond One Company's Database
Google's changes reduced the practical availability of the particular centrally stored location history that had fueled many geofence investigations.
But the underlying concept is larger than one company.
Phones interact with many systems.
Applications can collect location information.
Vehicles generate location-related data.
License-plate readers record cars at particular places.
Wireless networks can create connection records.
Advertising technology can involve location-related information.
The legal rules governing those sources are not necessarily identical.
Neither are their technical capabilities.
The disappearance or restriction of one source of location information does not mean location-based investigation disappears.
It means investigators may look elsewhere.
The Reverse Search Idea Can Apply to More Than Location
Geofence warrants are part of a broader category of investigative techniques sometimes described as reverse searches.
Traditional investigation begins with a suspect and searches for evidence connected to that person.
A reverse search begins with evidence and asks which people match it.
Location is one example.
Another controversial form has involved requests seeking information about users who searched for particular words or addresses online during a relevant period—sometimes called keyword warrants.
The common issue is the reversal.
Instead of saying:
“We have probable cause to investigate this person.”
The process can resemble:
“We have evidence associated with this activity; tell us which people match it.”
That reversal creates constitutional questions that courts will continue confronting as digital databases become more powerful.
The Database Can Know Who Was Nearby Even When No Witness Does
Imagine a crime committed on an empty street at 3:00 a.m.
There are no witnesses.
Nobody remembers a vehicle.
No usable camera captured the perpetrator.
In the physical world, investigators may have almost nowhere to begin.
Digital records can change that.
A database might contain information associated with devices in the area.
A toll system might have recorded vehicles approaching the neighborhood.
License-plate cameras may have captured traffic several blocks away.
A cellular provider may possess other records obtainable through appropriate legal process.
Technology can therefore create investigative leads where human observation created none.
That is precisely why these techniques are attractive to investigators.
It is also why their scope matters so much.
The Fourth Amendment Was Written Before Anyone Could Search a Crowd This Way
The Constitution's authors lived in a world of physical searches.
Searching many people required physically confronting many people.
Following someone required officers to actually follow.
Learning who visited a location required witnesses or surveillance.
Digital databases changed the economics of investigation.
A computer can search enormous collections of information in seconds.
That capability forces courts to confront an enduring constitutional question in a completely new setting:
When does a powerful investigative tool become too broad a search?
The answer cannot come from technology alone.
It comes from applying constitutional principles to what the technology actually does.
Being Near a Crime Is Not the Same as Being Connected to It
This is ultimately the most important distinction.
A phone can be near a crime because its owner committed the crime.
It can also be there because its owner lives nearby.
Works nearby.
Was making a delivery.
Was sitting in traffic.
Was eating dinner.
Was walking home.
Or simply passed through at exactly the wrong moment.
Location information can tell investigators where to look.
It does not necessarily tell them what happened.
That requires additional evidence.
And as location databases become capable of identifying people who were never suspects when an investigation began, courts face a question that would have been technologically impossible for most of American history:
How broadly may the government search through the digital traces of innocent people in order to find the one person it is looking for?