The Supreme Court on Monday ruled in a 5-4 decision that police can take DNA samples from people arrested on suspicion of committing a crime.

The practice, the court said, was essentially the same as fingerprinting and photographing, in that it allows officers to identify suspects. Writing for the majority, Justice Anthony Kennedy said simple DNA swabs were "no different than matching an arrestee's face to a wanted poster of a previously unidentified suspect; or matching tattoos to known gang members to reveal a criminal affiliation; or matching the arrestee's fingerprints to those recovered from a crime scene."

Every state except one, Idaho, requires DNA samples to be taken from all felony convicts. Some states go even further, like California, which requires DNA samples from all people convicted of felonies and misdemeanors.

The practice of swabbing those who have not yet been convicted, however, is less widespread and far more legally contentious. Twenty-eight states and the federal government allow officers to take DNA samples from people who have been arrested, but not found guilty of any crime.

In this case, Maryland v. King, Alonzo King was arrested on assault charges. A subsequent swab linked him to an unsolved rape, and he was then charged and sentenced to life for that crime.

That, critics of the practice claim, is precisely the problem with pre-conviction testing. Rather than serving as a tool to verify someone's identity, they argue that it's really a backdoor to circumvent the Fourth Amendment's protection against "unreasonable searches." Police had no reason to suspect King had been involved in that rape, yet they used his arrest and DNA to charge him for an unrelated crime.

The case did not break on the usual ideological lines. In a withering dissent, Justice Antonin Scalia, joined by three liberals on the bench, said it "taxes the credulity of the credulous" to suggest DNA testing is really about determining someone's identity:

The Fourth Amendment forbids searching a person for evidence of a crime when there is no basis for believing the person is guilty of the crime or is in possession of incriminating evidence. That prohibition is categorical and without exception; it lies at the very heart of the Fourth Amendment. Whenever this Court has allowed a suspicionless search, it has insisted upon a justifying motive apart from the investigation of crime. [SCOTUSBlog]

The court did not rule on specific limits for conducting pre-conviction sampling, which is another major point of contention. The court said it can only be done in the case of "serious" crimes. Yet that term is subjective, a point Scalia lambasted while arguing that the majority had "disguise[d] the vast (and scary) scope of its holding by promising a limitation it cannot deliver."

"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," he said.

The American Civil Liberties Union, which filed an amicus brief in the case, added that expanded testing could lead to even more problematic privacy issues, like an arrestee's DNA being used as a tool to probe family members. ACLU Legal Director Steven Shapiro blasted the ruling for creating a "gaping new exception" to reasonable searches, saying it ran counter to the long-held belief that the Constitution provided for searches only in cases of "individualized suspicion."