Except for identical twins, triplets, etc., no two people have the same DNA. For this reason, DNA is an ideal tool to identify criminals and exonerate the wrongly accused in cases where the perpetrator's DNA is unquestionably linked to the crime. It is extremely helpful for the criminal justice system to have access to a database containing DNA profiles of people who have been arrested for serious crimes.

Luckily, we have such a database. The FBI's Combined DNA Index System (CODIS) contains profiles from all 50 states and the federal government. When DNA specimens are taken from arrestees and catalogued in CODIS, investigators in other cases may be able to match the DNA donor to crime scene evidence in an unsolved case.

The Fourth Amendment to the U.S. Constitution guards against unreasonable searches and seizures and establishes the probable cause requirement in issuing warrants. Under what circumstances would the Fourth Amendment allow routine collection of DNA samples upon arrest and booking? A recent Supreme Court decision addressed this issue.

Maryland v. King

In 2003, a masked and armed intruder raped a woman in her home in Salisbury, Md. DNA evidence was collected from the victim, but it did not match anyone already in the CODIS database. The case went cold for six years.

In 2009, Alonzo King was arrested in Wicomico County, Md., for a felony assault with a shotgun. Under a state statute authorizing collection of DNA specimens from certain classes of offenders, booking officers took a buccal swab (pronounced "buckle") from inside King's cheek. Subsequent comparisons matched King's sample to the DNA in the unsolved rape.

On the basis of the DNA match (a random match probability is approximately 1 in 100 trillion, assuming unrelated individuals), King was successfully prosecuted for rape and given a life sentence. He appealed, arguing that the buccal swab was an unreasonable search and seizure of evidence which should have been suppressed. The Maryland Court of Appeals agreed with King and reversed his conviction. The state appealed to the U.S. Supreme Court. In a 5 to 4 ruling, the Supreme Court reversed the Maryland appellate court and ordered King's conviction reinstated.

The court began its discussion of the Fourth Amendment issue by saying, "The advent of DNA technology is one of the most significant scientific advancements of our era. It has the potential to significantly improve the criminal justice system and police investigative practices." Given the court's view of the importance of DNA evidence to help ensure conviction of the guilty and exoneration of the wrongly accused, a ruling upholding routine collection of arrestee samples was almost a foregone conclusion.

It was undisputed, said the court, that taking a buccal swab is a Fourth Amendment search. The question then becomes whether such a search is reasonable, without a warrant or some recognized exception. Answering this question requires balancing "the promotion of legitimate governmental interests against the degree to which the search intrudes upon an individual's privacy."

The governmental interests served by DNA collection are much the same as those involved in fingerprinting and photographing arrestees at booking—namely, identifying the person who is in custody, including his criminal history; ensuring the safety of officers and other detainees; making sure the suspect appears at trial; setting appropriate bail; and freeing others who might have been wrongly arrested for the same crime. In the court's words, "In this respect, the only difference between DNA analysis and the accepted use of fingerprint databases is the unparalleled accuracy DNA provides."

As for the intrusion into the arrestee's mouth to take the swab, the court noted that this is a quick, painless procedure that in many cases can be completed more quickly than taking fingerprints. Said the court, "This additional intrusion upon the arrestee's privacy beyond that associated with fingerprinting is not significant."

The balance, therefore, weighs in favor of allowing the swabs to occur.

Not in Every Case

The court's holding is a limited one. Unlike mugging and printing, which routinely occur at booking for any category of offense, the bodily intrusion to obtain a DNA sample is only reasonable as specified in the precise ruling of the majority decision in King, where the court said the following:

"The court concludes that DNA identification of arrestees is a reasonable search that can be considered 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."

These are the three key requirements:

1. Arrest based on probable cause

2. A serious offense

3. Custodial detention

Therefore, an arrest found lacking in probable cause would taint the search for DNA; the procedure is restricted to crimes that may qualify as "serious" offenses; and it does not apply to cases where the suspect will simply be cited and released, rather than booked into a holding facility or jail.

As the dissent noted, the majority in King did not undertake to define "serious offense." Also, the ruling does not say "felony offense," presumably because some misdemeanors may be considered "serious" (domestic assaults or driving while impaired, perhaps), while some felonies might not be regarded as "serious" (writing bad checks or underpaying income taxes, possibly). It remains to be seen how lower state and federal courts will decide which crimes are serious offenses, and which are not. In the meantime, it's probably safe to collect DNA samples at felony bookings, unless and until your local courts decide otherwise.

Not just like Maryland's law?

The opinion in King noted that all 50 states and the federal government have statutes authorizing collection of DNA samples from those convicted of felonies. Also, the federal government and 28 states allow DNA collection from some or all arrestees. In upholding the admissibility of DNA evidence taken from King under the particular statute that applied in Maryland, the court did not invalidate other states' differing laws, nor undertake to require other jurisdictions to pattern their laws after Maryland statutes.

The King decision stressed that the rule it announced is not limited to the State of Maryland or to states that have identical statutory schemes. Referring to the body of various laws, the court said this: "Although those statutes vary in their particulars, their similarity means that this case implicates more than the specific Maryland law."

And in a 2001 decision, the Supreme Court cautioned lower courts against making the mistake of assuming that every search or seizure has to be identical to one that the Supreme Court has approved. The court called this "dubious logic—that an opinion upholding the constitutionality of a particular search implicitly holds unconstitutional any search that is not like it." (U.S. v. Knights)

This means that other states need not enact a DNA collection scheme that is just like Maryland's. It also means that specimens taken under existing statutes that vary in some particulars from the Maryland law are still admissible, provided they are based on a custodial arrest supported by probable cause, for a serious offense.

The U.S. Supreme Court decision in Maryland v. King provides an excellent opportunity for law enforcement agencies to review their DNA collection practices.

Devallis Rutledge is a former police officer and veteran prosecutor who currently serves as special counsel to the Los Angeles County district attorney. He is the author of 12 books, including "Investigative Constitutional Law."