About halfway through a Supreme Court argument on Tuesday over whether the police may take DNA samples from people they arrest, Justice Samuel A. Alito Jr. reflected on just how momentous the issue was.
But the value of such evidence to law enforcement was only one side of the equation, Justice Antonin Scalia said after hearing that Maryland had obtained 42 convictions based on DNA from people arrested there.
The case arose from the collection of DNA in 2009 from Alonzo Jay King Jr. after his arrest on assault charges in Wicomico County, Md. His DNA profile, obtained by swabbing his cheek, matched evidence from a 2003 rape, and he was convicted of that crime. Last April, the Maryland Court of Appeals ruled that a state law authorizing DNA collection from people arrested but not yet convicted violated the Fourth Amendment.
In July, before the Supreme Court agreed to hear the case, Chief Justice John G. Roberts Jr. issued a stay of the lower court decision, saying that collecting DNA from people accused of serious crimes is “an important feature of day-to-day law enforcement practice in approximately half the states and the federal government.”
Collecting DNA from people convicted of crimes was not at issue in the case argued Tuesday, Maryland v. King, No. 12-207. The question was, rather, whether the Fourth Amendment allowed collecting it from people who have merely been arrested and so are presumed innocent.
The chief justice seemed wary of going too far, too fast. The Maryland law, he said, is limited to people arrested for serious crimes. But other laws are broader, and the state’s argument did not have an obvious stopping point.
“Under your theory, there’s no reason you couldn’t undertake this procedure with respect to anybody pulled over for a traffic violation?” Chief Justice Roberts asked Katherine Winfree, the state’s chief deputy attorney general. She said drivers might have a reasonable expectation of privacy that people arrested for serious crimes do not.
She added that people under arrest lose an array of rights. Last year, the court ruled that they may be subjected to strip searches if admitted to a jail’s general population.
Justice Elena Kagan said there must be limits, saying an arrest would not justify the search of an individual’s home for possible evidence of an unrelated crime. She added that under the state’s theory, the law enforcement interest in solving crimes could be used to justify obtaining a DNA sample in many settings.
Much of the argument concerned whether DNA is like fingerprint evidence. Kannon K. Shanmugam, a lawyer for Mr. King, said the two were different, as fingerprints are generally used to identify suspects. DNA, on the other hand, he said, is used for a purpose unrelated to the arrest: to solve cold cases, he said.
Several justices seemed interested in a third way DNA could be used: to assist judges in making bail determinations. For now, they were told, turnaround times are too long to make that practicable.
But Michael R. Dreeben, a lawyer for the federal government, which supported Maryland, said the day would soon arrive when DNA could be analyzed in 90 minutes. Ms. Winfree agreed. “This is not science fiction,” she said. “We are very, very close to that.”
Chief Justice Roberts said that left the court in a difficult position. “How can I base a decision today on what you tell me is going to happen in two years?” he asked.
For now, Justice Scalia said, the law’s purpose is “to catch the bad guys, which is a good thing.” But, he added, “the Fourth Amendment sometimes stands in the way.