Jewel Samad / AFP - Getty Images file The Supreme Court is weighing whether police have the right to take a DNA sample immediately after an arrest.

The Supreme Court is about to decide what one justice says may be its most important criminal procedure case in decades — whether the police have the right to take a DNA sample after they make an arrest.

The question before the justices is whether taking DNA, often with the quick swab of a cheek, is the latter-day equivalent of fingerprinting or violates the Fourth Amendment protection against unreasonable searches.

“This is what’s at stake,” Justice Samuel Alito said during an oral argument Feb. 26. “Lots of murders, lots of rapes that can be — that can be solved using this new technology that involves a very minimal intrusion on personal privacy.”

The case arises from the arrest of a 26-year-old Maryland man, Alonzo King, in 2009 on a charge of second-degree assault. The police took a swab of DNA from his cheek, ran it through a database and matched it to an unsolved rape from six years earlier.

King was convicted of rape and sentenced to life in prison. He pleaded guilty to a misdemeanor for the 2009 assault. The Maryland Court of Appeals later reversed the rape conviction on the grounds that the DNA sample was an unreasonable search.

The question before the court has vast implications: 28 states and the federal government take DNA swabs from people under arrest before they can be judged innocent or guilty. In Maryland alone, DNA samples during arrests have led to 75 prosecutions and 42 convictions since 2009, Katherine Winfree, the state’s chief deputy attorney general, told the justices.

Maryland law restricts DNA swabbing to people arrested for certain violent crimes. But Chief Justice John Roberts, worried about the reach of similar laws, wondered during the oral argument why they couldn’t be applied to simple traffic stops.

Tim Sloan / AFP - Getty Images Chief Justice John Roberts indicated concern about the reach of state laws that allow DNA sampling during arrests.

“There’s no reason you couldn’t, right?” he asked Winfree. “I gather it’s not that hard. Police officers who give Breathalyzer tests, they can also take a Q-tip or whatever and get a DNA sample, right?”

Michael Dreeben, a lawyer for the federal government, which supports the Maryland law, told the justices that people under arrest “are no longer like free citizens who are wandering around on the streets” with full Fourth Amendment rights.

They can be subjected to a strip search, for example, or given a medical screening when they are thrown in jail, he said. While he conceded that law enforcement officers must get a warrant before searching a home, he said DNA was “not of that character.”

“It is far more like taking a fingerprint,” he said.

Kannon Shanmugam, a lawyer for King, argued that the two were different, partly because fingerprinting is mostly used for identification, not to solve cold cases, and is much more invasive.

“An individual’s DNA contains far more information and far more personal information than an individual’s fingerprints,” he said.

Prosecutors around the country will be watching the court’s ruling closely. If the justices decide that DNA swabbing during arrest is unconstitutional, untold numbers of cold-case convictions could be appealed.

Mindful of the implications, the court could narrowly tailor its ruling, said Jeffrey Urdangen, director of the Center for Criminal Defense at the Northwestern University School of Law.

“The repercussions of this are enormous,” he said.

For victims of violent crime, as for the justices themselves, the question presents a difficult balancing act — how to weigh the crime-solving power of forensic advances against the rights of the accused.

Mai Fernandez, executive director of the National Center for Victims of Crime, acknowledged that the issue is tough, but she said the center supports DNA sampling at the time of arrest, partly because it could prevent future crime.

She likened it to vaccination: Patients have to grapple with side effects, she said, but that pales next to the potential for good.

Of the DNA sampling, she said: “It’s a tool that can save many, many, many lives, and we should take hold of it. It doesn’t mean that we don’t remain a good country. It’s not the end of democracy. It’s just a new tool that we need to learn how to handle.”

Justice Antonin Scalia was less welcoming. At the oral argument, he cut off Winfree, the Maryland state lawyer, immediately after she mentioned the 75 prosecutions and 42 convictions.

“Well, that’s really good,” he said. “I’ll bet you if you conducted a lot of unreasonable searches and seizures, you’d get more convictions, too.”

Alito, the justice who called the sampling issue “perhaps the most important criminal procedure case that this court has heard in decades,” appeared to lean toward classifying it with fingerprinting.

He and Scalia, two of the court’s conservatives, generally come down on the same side of rulings. But they appeared to differ on DNA sampling, an indication of the trickiness of the issue.

Matching the DNA against databases now takes two to three weeks. Two years from now, it could be almost instant, the Maryland lawyer said, meaning the judges could use it to make determinations about bail.

Scalia was unmoved.

“You just can’t demonstrate that now,” he said. “Maybe you can in two years. The purpose now is — is the purpose you began your presentation with, to catch the bad guys, which is a good thing.”

“But you know,” he continued, “the Fourth Amendment sometimes stands in the way.”

The Associated Press contributed to this report.

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