Police have been given a sweeping new identification tool by the Supreme Court. In a 5-to-4 decision in the case of Maryland v. King, the Court ruled that law enforcement officers can take DNA samples of suspects upon arrest for "serious offenses." In his dissent, Justice Scalia warns of the consequences.

Under a 2008 Maryland law, Alonzo King had his cheek swabbed for DNA sample when he was arrested for assault in that state in 2009. That sample matched an unsolved rape from 2003. King's attorneys wanted the sample suppressed, arguing that the swabbing constituted an unreasonable search of their client. In the majority opinion (which, along with the dissent, can be read below), Justice Kennedy summarized why the Court disagreed.

[T]he Court concludes that DNA identification of arrestees is a reasonable search that can be considered part of a routine booking procedure. When officers make an arrest supported by probable cause to hold for a serious offense and they bring the suspect to the station to be detained in custody, 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.

That "serious offense" qualification stems from the case under consideration. Maryland's law established clear rules around when such samples can be used, as noted in the majority opinion.

Once taken, a DNA sample may not be processed or placed in a database before the individual is arraigned (unless the individual consents). It is at this point that a judicial officer ensures that there is probable cause to detain the arrestee on a qualifying serious offense. If “all qualifying criminal charges are determined to be unsupported by probable cause ... the DNA sample shall be immediately destroyed.” DNA samples are also destroyed if “a criminal action begun against the individual . . . does not result in a conviction,” “the conviction is finally reversed or vacated and no new trial is permitted,” or “the individual is granted an unconditional pardon.”

The five Justices who supported the ruling — conservatives Roberts, Thomas, and Alito, along with Kennedy and the typically liberal Breyer — determined that such a system was akin to fingerprinting, with one exception. "The only difference between DNA analysis and fingerprint databases is the unparalleled accuracy DNA provides." Taking samples from those in custody, then, didn't comprise a suspect's Constitutional rights.

The dissent — representing the unusual coalition of Justices Scalia, Ginsburg, Sotomayor, and Kagan — strongly disagreed. "The Court’s assertion that DNA is being taken, not to solve crimes, but to identify those in the State’s custody, taxes the credulity of the credulous." And:

[T]he Court’s comparison of Maryland’s DNA searches to other techniques, such as fingerprinting, can seem apt only to those who know no more than today’s opinion has chosen to tell them about how those DNA searches actually work.

In other words, first, the dissent argues that this is not an identification tool but rather an investigative one, with which the four Justices take issue. Second, it argues that the complexity and involvement of DNA analysis makes it qualitatively different than fingerprinting.

Scalia's dissent ends with an admonition that may prove telling (and will certainly provide fodder to privacy advocates). Suggesting that the "serious offenses" distinction is a poor guidepost, he writes:

As an entirely predictable consequence of today’s decision, your DNA can be taken and entered into a national DNA database if you are ever arrested, rightly or wrongly, and for whatever reason.

If you're wondering, the FBI has maintained a database of fingerprints since 1924.

Read both opinions below.