Flock's Gunfire Alerts Raise a New Fourth Amendment Question
Gun owners should be paying attention to how police use gunfire microphones in conjunction with license plate cameras.
Flock Safety, the main supplier of the recently controversial roadside cameras that scan passing cars, offers more than automated license plate readers (ALPRs). The company also sells a surveillance bundle of sorts called the "Safe City" subscription. Billed as Flock's "Most Complete Solution," the "Safe City" subscription includes the much-discussed ALPRs, along with a live video camera—and a gunfire microphone. Bundling these together risks turning lawful gun owners into criminal suspects based only on proximity.
It is hard to find reliable information about the number of active gunfire microphones in the United States. The Electronic Frontier Foundation's Atlas of Surveillance documents nearly 250 gunshot-detection deployments and purchase plans nationwide, although that includes programs that have since ended. Cities also have deployed firearm detection systems that rely on AI to detect visible firearms on video or combine acoustic and dual-sensor infrared cameras to detect gunshots.
Flock is just one of many suppliers in the global gunshot detection market, which has an estimated total value of $1.5 billion. But the company's marketing material raises concerns about the technology's capabilities, which go beyond simply detecting gunfire. In Flock's own words, a confirmed gunshot "can surface the vehicles seen entering or leaving the area around the time of the sound, using nearby Flock license plate reader cameras." That statement deserves scrutiny, since it describes a mechanism for turning suspected gunfire into a list of nearby vehicles.
Innocent gun owners could end up on such lists. Suppose a bang recorded by one of these microphones gets classified as gunfire (which it may not actually be, given the possibility of a false positive from a similar noise). That "confirmed event" can trigger an ALPR inquiry that sweeps in drivers who had nothing to do with the noise. In a country where more than 20 million people have carry permits and 29 states do not require them, some of those drivers will be lawfully armed. The proximity of those innocent drivers could be mistaken for evidence that they fired the shot.
This possibility raises two legal concerns for lawful gun owners. First, what justifies stopping a person on one of these lists? Second, is querying the database a search under the Fourth Amendment? Recent cases provide some answers.
The Supreme Court's 2022 decision in New York State Rifle & Pistol Association v. Bruen protects lawful public carry. In the 2024 case United States v. Daniels, which involved the detention of an unarmed man who did not match a 911 caller's description of armed men, the U.S. Court of Appeals for the 10th Circuit noted that no crime was reported. Citing Bruen, the court said it "cannot look with suspicion on citizens presumably exercising their Second Amendment rights in a lawful way."
Last May in Commonwealth v. Foster, by contrast, the Pennsylvania Supreme Court upheld a stop where police responded within seconds to late-night alerts from a gunfire microphone, found only two people, and observed conduct the court considered evasive. An investigative stop requires reasonable suspicion based on specific facts connecting someone to suspected criminal activity. The risk here is that an alert combined with a plate match could be treated as sufficient for reasonable suspicion even though it only establishes proximity.
A recent case from New Jersey illustrates that risk. Last March, in State v. Carrillo, a state appeals court ordered suppression of evidence (an unlawfully possessed handgun) discovered in a minivan after a traffic stop. The police searched the minivan after obtaining a warrant. But the court held that a gunshot alert and an ALPR hit did not adequately connect the minivan to a shooting the day before. Prosecutors conceded that video supposedly showing muzzle flashes from the minivan at the time of the gunfire actually showed none.
When police look for evidence inside a car, that obviously counts as a search. But is that also true when they query an ALPR database?
Last March in United States v. Porter, the U.S. Court of Appeals for the 5th Circuit held that using a 10-camera ALPR network to locate a known vehicle was not a search. Three months later in Chatrie v. United States, by contrast, the Supreme Court held that obtaining Google's phone location records to identify people near a crime scene was a search. Roadside camera observations differ from detailed phone location data. But an alert-triggered ALPR query seems closer to the situation in Chatrie.
Defenders of gunshot microphones will say that gunfire on city streets is usually illegal and that responding quickly saves lives and reduces violence. But a federally funded evaluation of the gunfire detector ShotSpotter in Chicago and Kansas City, Missouri found that the microphones, while associated with faster responses and more recovered firearms, did not result in fewer shootings or better clearance rates.
With state legislatures slow to act, city governments have to make their own decisions about how to balance safety with privacy. Short of enacting a total ban on every part of the Flock bundle, city councils should adopt four rules to preserve their constituents' rights.
First, except in exigent circumstances, police should be required to obtain a warrant, including time and distance limits, before running an alert-triggered ALPR query. Second, no city should link gun detection alerts to ALPR data without first holding a public hearing and a vote. Third, every query should be logged for auditing, and information about uninvolved vehicles should be deleted when it is no longer needed as evidence. Fourth, a stop or search based solely on "alert plus proximity" should not be allowed.
Every city using gunshot detection has a contract renewal or replacement decision coming. Those decisions should include publicly disclosed rules about how the system can be used with ALPRs. Because these technologies can be bundled, these questions matter regardless of which vendor a city chooses. Whether a city is expanding an existing network or considering a replacement, the limits on surveillance should be settled before the next contract is signed.