The rapid progress of technology challenges traditional application of the Fourth Amendment. As law enforcement’s technological capabilities advance, judges have had to wrangle with what constitutes a search and seizure. Past precedent generally remains unhelpful unless new technology can be analogized to past technology. To combat this, the Supreme Court recognized the need to introduce an administrable test that could balance citizens’ inherent right to privacy and law enforcement’s need for power in combating crime. In Katz v. United States, the Supreme Court introduced the reasonable expectation of privacy (REP) test.1 The two-pronged REP test, emerging from Justice Harlan’s concurrence in Katz, evaluated the defendant’s exhibition of a subjective expectation of privacy and whether society would objectively find that expectation to be reasonable.2 After nearly half a century of REP analysis, the Court reintroduced “trespass to chattels” into Fourth Amendment jurisprudence in 2012 in United States v. Jones.3 Trespass theory finds that the government’s physical intrusion upon a constitutionally protected area to obtain information constitutes a Fourth Amendment search.4

This created two tests for analyzing violations of Fourth Amendment claims. In United States v. Ackerman, the Tenth Circuit applied both tests and held that the government conducts a warrantless search that implicates the Fourth Amendment when it opens unopened emails provided by Internet Service Providers (ISPs).5

This comment will discuss several important considerations from the Tenth Circuit’s decision in Ackerman. First, it will discuss the Katz REP test and Jones trespass test. Next, it will analyze the Tenth Circuit’s rationale for both applying the respective tests and the use of analogy in comparing emails to traditional postal mail. Finally, this paper will discuss the possible impact of the Ackerman analysis on future Supreme Court interpretations of Fourth Amendment cases related to the government’s use of technology.


The reasonable expectation of privacy test emerged in 1967 from the seminal case Katz v. United States.6 In Katz, Federal Bureau of Investigation (FBI) agents placed an electronic listening device on a telephone booth to gather information about the defendant’s bookmaking scheme.7 The Court held that the government listening to and recording the defendant in a public payphone violated the defendant’s justifiable right to privacy and thus the government conducted an unreasonable “search and seizure” under the Fourth Amendment.8 The Court famously stated that the Fourth Amendment “protects people, not places,” thus distancing the Fourth Amendment from physical trespass of property.9 However, it was Justice Harlan’s concurrence that brought the REP test to the forefront of Fourth Amendment analysis. The two-pronged test requires that a person exhibit an actual—or subjective— expectation of privacy.10 The second prong evaluates whether society is prepared to recognize that expectation as “reasonable.”11

After nearly half a century, in United States v. Jones, the Court reintroduced the trespass theory, holding that the installation of a GPS tracking device on the undercarriage of a motor vehicle to monitor its movement on public streets was a search under the Fourth Amendment.12 Under this doctrine, when the government willfully gathers information by physically encroaching on a constitutionally protected area, a Fourth Amendment search has occurred.13 Thus, attempting to find information by trespassing on a possessory interest is the central issue rather than the trespass itself. The physical trespass theory officially originated from Olmstead v. United States, which held the wiretapping of a suspected alcohol runner during the Prohibition era did not amount to a search and seizure under the Fourth Amendment because there was not a search and seizure of defendant’s papers or effects or “an actual physical invasion of his house ‘or curtilage’ for the purpose of making a seizure.”14 Olmstead applied an originalist approach to the Fourth Amendment jurisprudence, which may partially explain why the Court brought the doctrine back to life.

This resulted in a bifurcated approach to Fourth Amendment search analysis: Judges could apply the Katz REP test or the trespass test from Jones. The issue now for judicial decision-makers is which test they should apply, particularly when technology is involved. Interestingly, in Ackerman, the Tenth Circuit applied both tests and reached the same conclusion: that a government entity conducted a search and seizure that both trespassed on the defendant’s property interest in his information and that he had a reasonable expectation of privacy in said information.15


In Ackerman, AOL scanned the outgoing emails of its users and screened for the transmission of child pornography.16 The pornographic images in this case would have a string of data associated with them. A hash value generates a unique shorter string of characters from these images, and acts much like a digital fingerprint.17 As the Tenth Circuit noted, the hash values are “calculated in a way that makes it highly unlikely another set of data will produce the same value.”18 Here, AOL ran an algorithm that matched and filtered the hash value of images deemed to be child pornography by AOL employees.19 Employees compared the hash values of the outgoing email to the hash values of known child pornography. 20

AOL forwarded Walter Ackerman’s email—along with four attachments—after identifying a hash for child pornography to the National Center for Missing and Exploited Children (NCMEC). 21 The NCMEC is a “national clearinghouse for information regarding missing or exploited children.”22 In this case, the organization opened the email sent by AOL and all four attachments, three of which had not been opened by AOL.23 Afterwards, the NCMEC sent the information to law enforcement authorities.24 The district court held that the NCMEC was a “private actor,” not accountable as a state actor under the Fourth Amendment.25 In the alternative, it found if the NCMEC was a state actor, the NCMEC did not go beyond the scope of the private-search reconstruction doctrine through AOL’s private search.26

Tenth Circuit’s Application of REP Doctrine

As a threshold issue, the Tenth Circuit found that the NCMEC constituted a government entity.27 However, this case comment will focus on the importance of the reasoning for applying both Fourth Amendment tests. After determining that NCMEC was a government entity, the court turned to whether the NCMEC violated Walter Ackerman’s Fourth Amendment rights by opening the emails without a warrant.28 If opening the unopened attachments in the email constituted a search, then it required a warrant. Here, the Tenth Circuit overruled the district court and found that opening the attachments was a warrantless search.29

Before delving into Katz analysis, it is important to discuss an important exception to REP—the third-party doctrine. The third-party doctrine finds that an individual does not display a subjective intent to privacy when she discloses information to a third party. For example, individuals do not have a reasonable expectation of privacy in the telephone numbers they dial because they generally know that third-party telephone companies have records of such numbers.30 The lower court did not apply third-party doctrinal analysis, which seems peculiar in this case given that the general similarity between sending an email through an ISP and dialing a telephone number through a phone company.31 Nevertheless, because the lower court did not reason through third-party analysis, the Tenth Circuit also did not apply it in Ackerman.32

The government argued it could search Ackerman’s emails under the private search doctrine because AOL—a private enterprise—had conducted a prior search.33 The government found support for this in a similar case, United States v. Jacobsen, where FedEx employees passed a package containing plastic bags of white powder to law enforcement.34 The DEA agents reopened the package to test it for cocaine.35 The Supreme Court found that no search took place because the government would with “virtual certainty” not find anything else of significance in the package and that there is no legitimate privacy interest in contraband under Katz.36 Here, unlike Jacobsen, the government opened the email but surpassed the private search AOL had conducted.37 The Court in this case argued that the unopened email attachments could have disclosed information “previously unknown to the government” besides information about contraband.38 Whereas in Jacobsen the FedEx employee opened a package that only contained and could only contain white powdery substance similar to cocaine, here the email attachments could have revealed non-contraband material that would be private to the defendant. Because the government search in Ackerman exceeded the scope of the private search, the Court held that the Ackerman had an expectation of privacy in the content of the information.39 The fact that opening all the attachments risked the government exposing private non-contraband information not previously examined was enough to decide the case.

Correct Application under Jones Trespass

The Tenth Circuit continued analyzing the issue at hand by applying the trespass theory under Jones, analogizing unopened email attachments to sealed letters sent through postal services.40 The Court invoked a familiar analogy between physical mail and electronic mail, thereby easing the issue into a “papers and effects” argument from the Fourth Amendment.41 The Court found that the warrantless opening and searching of private correspondence in the email was “exactly the type of trespass to chattels that the Framers sought to prevent when they adopted the Fourth Amendment.”42

Under the Fourth Amendment, unreasonable searches of “papers[] and effects” require the government to use a warrant.43 NCMEC conducted a search of “papers[] and effects” when it opened the attachments in the email and viewed each of them.44 The Court, by drawing an analogy to traditional papers, found that the government conducted a warrantless search when it opened and reviewed electronic paper.45 Therefore, the government also violated the defendant’s Fourth Amendment right from unreasonable searches through the trespass theory.46

The Tenth Circuit’s application of the Jones test surprised court observers because the court had already found a Fourth Amendment violation through the more established REP test. Additionally, the Court’s “trespass to chattels” theory was applied to an electronic communication—not to a tangible device as it was in Jones. There are three strong possible reasons for the Tenth Circuit applying both tests. First, it may be that the court wanted to support its holding, that even if its REP analysis does not find support, the trespass theory aids their holding. Second, the court may have wanted to provide a foundation for the trespass theory to be applied as a coequal to the REP test.47 The trespass theory’s reintroduction into Fourth Amendment jurisprudence faced critique from the Justice Alito’s concurrence in Jones, which outlined the repeated criticism of trespass theory in the Court’s history.48 The Tenth Circuit may therefore have been attempting to give the trespass theory more weight by treating it as an additional necessary test to determine whether a search occurred.

Impact of Ackerman in Future Supreme Court Decisions

Justice Gorsuch, who wrote the opinion in Ackerman, may well push the Supreme Court towards favoring a pro-search warrant approach in technology related searches. In United States v. Denson, Justice Gorsuch scrutinized the government’s warrantless use of a radar device—capable of detecting human movement and even breathing within a home—as “pos[ing] grave Fourth Amendment questions.”49

The most critical indicator of how Justice Gorsuch may approach Fourth Amendment issues will appear in the upcoming Carpenter v. United States, where the defendant appeals his conviction of armed robberies after law enforcement used cell-site locational data to place him near the site of the robberies.50 The question before the Court is whether the warrantless search and seizure of historical cell phone records revealing the location and movements of a cell phone user over the course of 127 days is permitted by the Fourth Amendment. If Justice Gorsuch remains consistent with his approaches to Ackerman and Denson, then we should expect him to (a) analyze both “trespass to chattels” and reasonable expectation of privacy tests under the Fourth Amendment, (b) find a search occurs when the government uses technology to track, monitor and discover electronic communications, data and movements of defendants and (c) analogize to past technology in Carpenter and future Supreme Court cases.


As explained, the Tenth Circuit reversed the holdings of the lower court and held that the NCMEC was a governmental agency that had conducted a warrantless search. The case’s importance in providing insight into Justice Gorsuch’s judicial philosophy cannot be underestimated. When future Supreme Court Fourth Amendment cases are analyzed, scholars and observers will return to United States v. Ackerman to find the Justices’ beginning as an “analogizer” of technology and an applier of the two fundamental tests of Forth Amendment analysis.