The case pitted the practical benefits of modern technology against the centuries-old right to privacy.

WASHINGTON — A narrowly divided Supreme Court ruled Monday that police can collect DNA from people arrested but not yet convicted of serious crimes, a tool that more than half the states already use to help crack unsolved crimes.

The case, described by Justice Samuel Alito as “the most important criminal procedure case that this court has heard in decades,” represented a classic test between modern crime-fighting technology and centuries-old privacy rights.

In the end, the justices had to balance the benefits and the intrusion of a simple cheek swab — and the considerable benefits won out. Justice Anthony Kennedy wrote the majority’s 5-4 decision, in which one liberal justice, Stephen Breyer, concurred. The key to the ruling, Kennedy said, is “reasonableness.”

“DNA identification of arrestees is a reasonable search that can be considered part of a routine booking procedure,” Kennedy said. “Taking and analyzing a cheek swab of the arrestee’s DNA is, like fingerprinting and photographing, a legitimate police booking procedure that is reasonable under the Fourth Amendment.”

Justice Antonin Scalia wrote an angry dissent for himself and three liberal justices, charging that the decision will lead to an increased use of DNA testing in violation of the Constitution’s protection against unreasonable searches.

“Make no mistake about it: because of today’s decision, your DNA can be taken and entered into a national database if you are ever arrested, rightly or wrongly, and for whatever reason,” Scalia warned. “This will solve some extra crimes, to be sure. But so would taking your DNA whenever you fly on an airplane.”

Twenty-six states have laws on the books that allow for the collection of DNA from those arrested for felonies or other serious crimes. The information is uploaded into a national database run by the federal government in order to find matches with unsolved crimes.

That’s how Alonzo Jay King was connected to a Maryland rape case for which he ultimately was convicted. Arrested in 2009 on an assault charge, King was linked by DNA evidence to the 2003 rape.

The Maryland Court of Appeals threw out his conviction, ruling that police needed a warrant or at least reason to suspect him of another crime before swabbing his cheek. The state, backed by the federal government, brought the case to the Supreme Court.

The justices have been inundated in recent years with difficult Fourth Amendment cases as well as others involving modern technology. Last year, they held that police could not attach a GPS tracking device to a car in order to monitor a suspect’s movements. This year, they ruled that using a drug-sniffing dog with reasonable suspicion was OK — but not at the door of a private home. And they decided that executing a search warrant after a suspect had left his home was out of bounds.

Modern technology presents a problem, however, particularly for justices who try to adhere to the Constitution. The framers didn’t have GPS or DNA to contend with in the late 18th century. In February, the court grappled with the patent rights of self-replicating soybeans. In April, they debated about a breast cancer detection technology that comes from human genes.

In this case, Maryland likened DNA to fingerprinting and other tools used to identify suspects. While fingerprinting is faster, Kennedy ruled that DNA testing is more accurate and the privacy intrusion relatively minor.

But opponents have noted that police take DNA from people upon arrest to help in other investigations — a process that can lead to false hits and wrongful convictions.

During oral argument in February, Justice Sonia Sotomayor worried that DNA swabs could find their way into the nation’s schools and workplaces. Justice Elena Kagan quipped that if it works so well, “why don’t we do this for anybody who comes in for a driver’s license?”

That was the argument Scalia used in his dissent against what he called a “suspicionless” search.

“If you believe that a DNA search will identify someone arrested for bank robbery, you must believe that it will identify someone arrested for running a red light,” he said, concluding, “The proud men who wrote the charter of our liberties would not have been so eager to open their mouths for royal inspection.”

On the other side were the court’s four other conservative justices. Most convinced about the promise of DNA during oral argument was Alito, who called it “the fingerprinting of the 21st century.” He noted that the criminal justice system has “lots of murders, lots of rapes that can be solved.”

The oral argument also revealed an unlikely split among the justices: Breyer said the practice targets only those arrested for serious crimes — and for a worthy cause. In the end, his vote proved crucial to the majority.

The case received additional attention from the parents and family members of crime victims who have fought for years to expand DNA searches.

Jayann and Dave Sepich have led that effort through the organization DNA Saves. Their daughter Katie was brutally raped and strangled at the age of 22 a decade ago. By the time her killer was identified through DNA evidence, he had committed other crimes.

“This just means that so many horrible murderers and rapists are going to be taken off the streets,” and other families won’t have to go through what hers did, Jayann Sepich said after hearing of the decision. Now, she said, “the remaining 24 states that need to pass these laws can do it and feel great about it.”

Last year, Congress passed the Katie Sepich Enhanced DNA Collection Act, which President Obama signed in January. It creates a grants program to help states pay for the expanded system.

“It’s the right thing to do,” Obama said of taking DNA from arrestees in a 2010 appearance on America’s Most Wanted. “This is where the national registry becomes so important.”

On the other side of the debate are civil liberties advocates who worry that DNA is subject to contamination, misinterpretation, sample switches and fraud.

“It won’t be long now before the federal DNA database has expanded to be as comprehensive as its fingerprint database,” said Ohio State criminal law professor Ric Simmons, “So we should all get used to the idea of the government having our DNA on file somewhere.”

Police and crime victims’ groups heralded the ruling and said states without similar laws now can consider them. Nevada became the 26th state last week. Minnesota’s high court previously struck down its law, while Connecticut and Oklahoma have versions affecting a limited number of arrestees.

Philadelphia Police Commissioner Charles Ramsey noted his state has been waiting with a draft plan for the court to act. “Hopefully, this will help move things along,” he said. “This (technology) is the fingerprint of the 21st century.”

Mai Fernandez, executive director of the National Center for Victims of Crime, called the use of DNA “the gold standard for helping to solve cold cases.” But he said the 5-4 ruling reflects a “split in society” about the use of such a tool and whether it constitutes an improper intrusion.

“There are always going to be issues around privacy rights,” Fernandez said. “Although people are becoming more comfortable with the use of DNA, there still is a lot that people don’t understand.”

Contributing: Kevin Johnson.

Follow @richardjwolf on Twitter.