King's lawyer, Kannon K. Shanmugam, told the justices there's "legitimate expectation of privacy" in the contents of an individual's DNA. "An individual's DNA contains far more information and far more personal information than an individual's fingerprints," Shanmugam said.
But prosecutors argued that those arrested have less expectations of privacy than people not in police custody. An arrested individual "by virtue of being in that class of individuals whose conduct has led the police to arrest him on, based on probable cause, surrenders a substantial amount of liberty and privacy," said Maryland Chief Deputy Attorney General Katherine Winfree.
In addition to solving cold cases, prosecutors argued that DNA testing is needed to help identify the person in custody, just like fingerprinting — when police make ink impressions of suspects' fingers and compare them quickly to others kept by authorities and found at crime scenes.
With arrested individuals, the government has a compelling interest in "knowing who that person is, which includes knowing what the person has done. And DNA does that in a far more powerful way than fingerprints have done," Justice Department lawyer Michael R. Dreeben said.
If that comparison can be made, "I think that you would have a quite good case," Kagan said.
But Winfree said it currently takes 11-17 days to get results from a DNA swab, making it useless for instant identification. Winfree argued that technology will soon go into use give the police the ability to get results back from DNA swabs within minutes, instead of the days it takes now. But "can I base a decision today on what you say is going to happen in two years?" Roberts said.
"You can't demonstrate that the purpose is immediate identification of the people coming into custody. You just can't demonstrate that now," Scalia said to Winfree. "Maybe you can in two years. The purpose now is the purpose you began your presentation with, to catch the bad guys, which is a good thing. But you know, the Fourth Amendment sometimes stands in the way."
Justice Anthony Kennedy, usually a swing vote on the court, compared the DNA swab to police patting a person down after an arrest, something they don't need a warrant to do. "Does the justice system have an interest in knowing whether the person committed other crimes?" he asked Shanmugam several times.
"They have that interest, but if they want to investigate other crimes, they have to do what they would have to do as to an ordinary citizen. They have to have a warrant or some level of individualized suspicion," Shanmugam said.