Flock’s Optional Gunshot Detection Feature Has Fourth Amendment Implications for Gun Owners

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 [automated license plate recognition] 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.

— Del Schlangen in Flock’s Gunfire Alerts Raise a New Fourth Amendment Question