In Katz v. United States, 389 U.S. 347 (1967), the Court held that “the Fourth Amendment protects people, not places,” stating that there is protection in situations where a person has a reasonable expectation of privacy and, in fact, intends to keep something private, even in spaces accessible to the public. In Kyllo v. United States, 533 U.S. 27 (2001), the Court ruled that using technology not in general public use (in this case, a thermal imager) was a search requiring a warrant. In United States v. Knotts, 460 U.S. 276 (1983), the Court sanctioned warrantless tracking of a vehicle’s movements on public roads and visual observations of a home’s exterior, reasoning that such movements could have been observed by any member of the public. In Smith v. Maryland, 442 U.S. 735 (1979), the Court held that law enforcement’s collection of phone numbers dialed from a particular phone did not constitute a search, because individuals lack a reasonable expectation of privacy in information voluntarily conveyed to third parties (i.e., the phone company).

In United States v. Jones, 565 U.S. 400 (2012), the Supreme Court ruled that attaching a GPS device to a vehicle (private property under the Fourth Amendment) and monitoring it constituted a “search,” requiring a valid warrant. Although the facts in Jones are similar to Knotts, the Court points to key differences, resulting in a different outcome in its Fourth Amendment test. Where the Court found that Knotts did not have a reasonable expectation of privacy on a public road, it found that the government’s surveillance of Jones using a device attached to private property violated the Fourth Amendment. The holding in Knotts only addressed the “reasonable expectation of privacy test” and not the “common-law trespassory test:” This doctrine is the idea that the Fourth Amendment protects against the physical intrusion of private spaces where there is a reasonable expectation of privacy. The physical installation of a GPS onto a car (an effect under the Fourth Amendment) to obtain information about someone’s movements constituted a search.  In Riley v. California, 573 U.S. 373 (2014), the Court held that the police generally may not, without a warrant, search digital information on a cell phone seized from an individual who has been arrested, since digital data stored on a cell phone does not present any risks to the officer’s safety or to the preservation of evidence. And, in Carpenter v. United States, 585 U.S. 296 (2018), the Court held that police need a warrant to access historical cell phone location data, highlighting that a cell phone is “almost a feature of human anatomy” and presents greater privacy concerns than GPS monitoring of a vehicle in Jones or a bugged container in Knotts. 

So far, there is no comprehensive legal framework or Supreme Court precedent governing the constitutionality of tools such as real-time social media monitoring, predictive policing software, or gang databases, and existing court decisions have only touched on these technologies in limited or indirect ways. While these cases above reflect an emerging recognition of privacy rights in the digital age, they focus on physical tracking and private digital data, not public online activity. Posting on social media, appearing in public housing, or associating with certain people can all become grounds for suspicion, even without evidence of a crime. These tools often rely on open-source or third-party data, the kind of information tracked by platforms like Facebook and Instagram. Under the third-party doctrine, such information generally falls outside Fourth Amendment protection. Because this information is considered voluntarily shared, law enforcement can often access and analyze it without a warrant or other judicial authorization, sidestepping traditional legal frameworks. While some lower courts have begun to question the unchecked use of surveillance in public spaces, particularly when it chills speech or disproportionately affects specific communities, no comprehensive federal protections exist.