This is perhaps the most important criminal procedure case that this Court has heard in decades," Justice Samuel Alito said at yesterday's oral argument in Maryland v. King, which will determine whether the government can seize DNA samples from people at the time they're arrested. "This is what is at stake: lots of murders, lots of rapes that can be solved using this new technology that involves a very minimal intrusion on personal privacy." Other justices, however, recognized that the potential privacy invasions are far from minimal: In light of the enormous amount of personal information that can be gleaned from DNA, Justice Sonia Sotomayor noted that "there is something inherently dangerous about DNA collection that is not the same as fingerprinting."

The privacy stakes in the case are obvious from a recent New York Times report that scientists at the University of Connecticut will study the DNA of the Sandy Hook shooter Adam Lanza to see if he had a genetic predisposition to extreme violence. Although the state and federal government repeatedly claimed during the Supreme Court argument that they are currently examining DNA samples only for identification purposes, the Court recognized that they might change their minds in the future. After seizing DNA on arrest, for example, the government might choose to profile a suspect's entire genome more broadly, looking for a predisposition to violence, for example, that could be used to deny people bail or even health insurance.

Given these stakes, Kannon Shanmugam, the lawyer for Alonzo King, a Maryland man whose DNA was seized on his arrest for assault and then was connected to an unsolved rape, argued that the Court should require a warrant before allowing the government to seize the DNA of arrestees. He stressed that the Court has never before accepted the proposition that people who have been arrested have a constitutionally diminished expectation of privacy the way that convicted criminals on probation or parole do.

The Supreme Court has said that warrantless searches are permitted when the government has a "special need" unrelated to the goal of ordinary criminal law enforcement. But the primary purpose of the federal and state DNA databases at the moment is clearly focused on law enforcement—the government wants to search the database to see whether the person who has been arrested is wanted for other unsolved crimes. Yesterday, the government tried to escape from this legal bind by arguing that DNA searches are necessary for pretrial supervision, to see whether someone who has been arrested deserves to be let out on bail. The justices met the claim with skepticism: Justice Elena Kagan noted that in this case, it took four months to get results back from the DNA database about the fact that King was also suspected of rape.

In her rebuttal argument, Katherine Winfree, Maryland's chief deputy attorney general, said the technology would soon change. "On the question of rapid DNA," she said, "the FBI estimates that we're about 18 to 24 months away from that world," where devices will be able to analyze and produce DNA identification within ninety minutes rather than four months. But the figure appears nowhere in the Supreme Court briefs and seems to have been pulled out of thin air. The Global Alliance for Rapid DNA Testing, which manufactures rapid DNA testing machines filed a brief that said nothing about how quickly the technology will develop, and the FBI's discussion of rapid DNA testing on its website says that "At this time, it is not known when law enforcement agencies will be able to search profiles developed by a Rapid DNA instrument in CODIS."

Might five justices strike down Maryland's DNA collection statute, which would require a redrafting of the laws of 29 states, as well as the regulations governing the federal DNA database? It’s possible, given the skeptical questions of Justice Antonin Scalia as well as Chief Justice Roberts and Justice Anthony Kennedy. Winfree said that since 2009, when it began testing the DNA of people arrested for violent crimes, Maryland had obtained "225 matches, 75 prosecutions and 42 convictions." Scalia responded: "Well, that's really good. I'll bet you if you conducted a lot of unreasonable searches and seizures, you'd get more convictions, too. That proves absolutely nothing."

Scalia took a similar position during the oral arguments of a recently decided case U.S v. Baily, where he joined Justice Kennedy’s 6-3 decision holding that the government couldn't detain a suspect nearly a mile from his house while preparing to execute a search warrant on his house. The fact that a warrantless search may be useful to the government, Scalia suggested, doesn’t make it constitutional. And in fact, even the usefulness of warrantless DNA searches is open to question: A recent AP report found that 29 of the 43 convictions Maryland claims were aided by DNA searches would have happened even if the state hadn’t extended its law to arrestees, since these suspects were ultimately convicted of the offense for which they were arrested, rather than for an unrelated crime. Under the old law, Maryland could have tested the DNA of these people anyway; it just would have had to wait until they were convicted.

A similarly bipartisan majority of justices might rule in favor of restricting the warrantless collection of DNA from arrestees. But that would hardly put an end to the FBI's Combined DNA Index System, or CODIS database, which is the national DNA database into which the state databases feed. Instead, the government could go back to the drawing board and restructure the CODIS database so it's used primarily to identify suspects (by collecting abandoned DNA at crime scenes and then plugging into CODIS to generate a name) rather than checking whether a particular suspect is also wanted for an unsolved crime. The government could, for example, add a suspect’s DNA profile to the existing national fingerprint database, known as the Integrated Automatic Fingerprint Identification System, or IAFUS. If generating cold hits on unrelated crimes was an incidental purpose of the federal database, rather than its primary purpose, the Court might be more likely to uphold its constitutionality.

But this restructuring of the federal database would only increase the privacy concerns: As the government moves to integrate all the identifying information it holds on citizens into a single national database, the incentives to test an individual’s entire genome for identifying information will grow exponentially. The government is free at the moment to collect DNA that people have unwittingly abandoned on coffee cups or at a crime scene. And the next generation of genetic testing will allow it to use this abandoned DNA to identify the racial and ethnic background of a suspect with unsettling precision. A DNA test for "biogeographic ancestry" traits, for example, has been able to identify, from abandoned DNA samples, not only the sex and appearance of suspects, but also the precise ethnic background of their parents. These techniques could indeed be useful in solving crime, but they also threaten to lead to the kind of genetic profiling that Connecticut is attempting in the wake of the Sandy Hook shooting.

The Court can't solve these future challenges on its own, but by ruling narrowly for Alonzo King, it might encourage Congress and the states to begin the difficult process of passing new laws allowing the use of DNA for solving existing crimes, but prohibiting its use for the prediction of future crimes. As Justice Kagan succinctly put it yesterday, "it seems as though the technology is not the same as the fingerprint technology; and because the technology is different, it is used differently."