Black Lives Matter protesters sit in front of a federal courthouse in St. Louis on August 10.Jeff Roberson / AP

On Monday, 57 protesters from the Black Lives Matter movement were arrested during a demonstration at the Thomas F. Eagleton Courthouse in St. Louis. Among them were Johnetta Elzie and DeRay Mckesson, two prominent activists who have campaigned against police abuses. Elzie and Mckesson, both prodigious users and consumers of social media, pointed out a peculiar aspect of their arrests after their release.

Since when can law-enforcement officials take DNA samples from citizens they arrest? The answer lies in Maryland v. King, a 2012 U.S. Supreme Court case that received little notice at the time, but carried significant Fourth Amendment implications.

First, some background. The Fourth Amendment prohibits unreasonable searches and seizures without a warrant, although the Supreme Court has ruled from time to time that there are exceptions in certain circumstances. The search-incident-to-arrest exception is one of the most significant: It allows law-enforcement personnel to search people they arrest without a warrant, and without the need to demonstrate probable cause. The Court outlined its rationale in the 1969 case Chimel v. California:

When an arrest is made, it is reasonable for the arresting officer to search the person arrested in order to remove any weapons that the latter might seek to use in order to resist arrest or effect his escape. Otherwise, the officer’s safety might well be endangered, and the arrest itself frustrated. In addition, it is entirely reasonable for the arresting officer to search for and seize any evidence on the arrestee’s person in order to prevent its concealment or destruction.

In 2009, Maryland expanded its DNA Collection Act to include individuals under arrest for certain serious crimes. DNA-collection efforts previously covered only those convicted and incarcerated, and the change raised new constitutional questions.

In King, Maryland police took a DNA swab of Alonzo King during his arrest. The police then ran his DNA against a database, matched it to evidence from an unsolved rape, and used that match to charge and convict him. King appealed his conviction, arguing that the warrantless DNA swab violated his Fourth Amendment rights.

The Supreme Court disagreed by a 5-4 vote. Collecting the DNA of defendants upon arrest, argued Justice Anthony Kennedy in his majority opinion, is no different than photographing and fingerprinting them. “The legitimate government interest served by the Maryland DNA Collection Act is one that is well established: the need for law enforcement officers in a safe and accurate way to process and identify the persons and possessions they must take into custody,” he explained. Chief Justice John Roberts, and Justices Clarence Thomas, Stephen Breyer, and Samuel Alito, agreed.

Kennedy’s majority opinion prompted a thundering dissent from Justice Antonin Scalia, who joined with Justices Ruth Bader Ginsburg, Sonia Sotomayor, and Elena Kagan. “It is obvious that no such noninvestigative motive exists in this case,” he shot back. “The Court’s assertion that DNA is being taken, not to solve crimes, but to identify those in the State’s custody, taxes the credulity of the credulous.”

Scalia’s dissent thoroughly gutted the majority’s logic. Central to Kennedy’s argument was the assertion that collecting King’s DNA helped identify him, thereby avoiding “inordinate risks for facility staff” or “existing detainee populations.” Scalia countered by quoting the statute itself, which prohibited testing arrestee DNA before the first arraignment date. Moreover, he asked, “Does the Court really believe that Maryland did not know whom it was arraigning?”

Yes, Scalia noted, Maryland’s DNA-collection statute could help solve crimes—but that doesn’t justify violating an arrestee’s Fourth Amendment rights. “No matter the degree of invasiveness, suspicionless searches are never allowed if their principal end is ordinary crimesolving,” he declared. He also nodded to the law’s more sinister implications in the aggregate. “Perhaps the construction of such a genetic panopticon is wise,” Scalia concluded. “But I doubt that the proud men who wrote the charter of our liberties would have been so eager to open their mouths for royal inspection.”

Many of Scalia’s themes would resurface in another Fourth Amendment case two years later. In Riley v. California, a California police officer stopped David Riley for expired registration tags, then impounded his car for a suspended license, and finally arrested him for concealed firearms found during an inventory search of the car. What happened next triggered the eventual Supreme Court case. During the post-arrest search, officers searched Riley’s cellphone without a warrant and found photos that seemed to link Riley to other recent crimes.

Based on this cellphone search, the officers filed multiple charges against Riley, including assault with a semiautomatic rifle and attempted murder. Riley argued on appeal that the warrantless search of his cellphone wasn’t justified by the standard Fourth Amendment exceptions. In June 2014, the Supreme Court agreed, ruling unanimously in Riley v. California that police can’t search cellphones during an arrest without a warrant.

Chief Justice John Roberts, writing for the majority, grounded his opinion in the exceptional nature of cellphones. “They could just as easily be called cameras, video players, rolodexes, calendars, tape recorders, libraries, diaries, albums, televisions, maps, or newspapers,” he noted. The federal government suggested to the Court during Riley that searching cellphone data was “materially indistinguishable” from searching its physical counterparts. Roberts wasn’t persuaded. “That is like saying a ride on horseback is materially indistinguishable from a flight to the moon,” he wrote.

Roberts did not cite Scalia’s dissent in King but shared its skepticism. (Roberts did cite similar arguments by Scalia in other Fourth Amendment cases, though.) Like Scalia in King, Roberts invoked the specter of British general warrants in the Thirteen Colonies. “Opposition to such searches was in fact one of the driving forces behind the Revolution itself,” the chief justice noted. His closing paragraph is emphatic:

Modern cell phones are not just another technological convenience. With all they contain and all they may reveal, they hold for many Americans “the privacies of life.” The fact that technology now allows an individual to carry such information in his hand does not make the information any less worthy of the protection for which the Founders fought. Our answer to the question of what police must do before searching a cell phone seized incident to an arrest is accordingly simple—get a warrant.

The arrests (and King DNA swabs) of Elzie and Mckesson show why Riley is one of the Roberts Court’s most consequential decisions. Consider the role cellphone pictures and videos have played in the national conversation on police tactics since Ferguson. Had the justices deferred to the government’s arguments, law-enforcement officials would have been able to lawfully search Elzie and Mckesson’s cellphones on Monday without a warrant after their arrests. This would have given the police free access to a wealth of texts, photos, videos, social media accounts, and more from two of the most prominent Black Lives Matter activists. You don’t have to be Fox Mulder to see the potential for government abuse; indeed, nine Supreme Court justices already did.

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