WASHINGTON » The Salisbury, Md. police department thought they had finally caught a break.

A man wearing a hat and scarf and brandishing a gun had raped and robbed a 53-year-old woman in her home and then vanished into the night. Almost six years later, Alonzo King was arrested in a nearby county and charged with felony second-degree assault. Taking advantage of a Maryland law that allowed DNA tests following felony arrests, police took a cheek swab of King’s DNA, which matched a sample from the 2003 Salisbury rape. King was convicted of rape and sentenced to life in prison.

But then a Maryland court said it had to let him go.

King was never convicted of the crime for which he was arrested and swabbed. Instead, he pled guilty to the lesser charge of misdemeanor assault, a crime for which Maryland cannot take DNA samples. The courts said it violated King’s rights for the state to take his DNA based on an arrest alone. The state Court of Appeals said King had "a sufficiently weighty and reasonable expectation of privacy against warrantless, suspicionless searches."

The Supreme Court today will try to balance the rights of Americans who have not been convicted of a major crime to keep their DNA out of the government’s hands against the government’s interest in closing cold cases and the rights of crime victims to finally see justice done.

If the justices rule for King, more than 1 million DNA profiles that have been stored in a federal database for matching with future crime scene evidence may have to be purged and others will never be collected, leading some repeat offenders to go free, advocates say.

"The early collection of DNA prevents crime," said William C. Sammons of the Maryland Coalition Against Sexual Assault. "Had the recidivists been identified early in their career through arrestee collection, they would not have been able to commit the bulk of their crimes."

But privacy activists see letting police use DNA information without a warrant or a conviction as another loss for American privacy, with Americans’ genetic information held by the government eventually being used for other purposes, just as Social Security numbers were originally not intended to be used for identification.

"Regardless of what the government does with the DNA sample and the limits it places on the sample’s use, all the highly personal data in it is in the government’s possession, and outside the individual’s control," said Jennifer Lynch, lawyer for the Electronic Frontier Foundation.

Getting DNA swabs from criminals is common. All 50 states and the federal government take cheek swabs from convicted criminals to check against federal and state databanks, with the court’s blessing. But now 28 states and the federal government now also take samples from people who have been arrested for various crimes, long before their guilt or innocence has been proven. According to court documents, the FBI’s Combined DNA Index System or CODIS — a coordinated system of federal, state and local databases of DNA profiles — contains more than 10 million criminal profiles and 1.1 million arrestee profiles.

Victims’ rights groups argue that the earlier the DNA test, the earlier repeat criminals are put in jail. And since arrestees already have to tell police their names and give them their fingerprints and any identifying documents they’re carrying, they have no right to hide genetic information that could help solve cold cases, they say.

"Arrestees have no greater interest in withholding the identifying information used in DNA fingerprinting than in withholding traditional fingerprints. The only difference is that it is a better means of identification that is more effective in protecting the public from recidivists like Alonzo King," said Jonathan S. Franklin, a lawyer for DNA Saves and other victims’ rights groups.

The Obama administration added that the government’s interest in solving crimes outweighs the right to keep personal genetic information secret. "The government — and society at large — has an overwhelming interest in solving crimes," which not only helps victims but also exonerates the innocent, Solicitor General Donald B. Verrilli Jr. wrote in court papers.

Groups like the American Civil Liberties Union see DNA evidence as a slippery slope, however.

"In less than 25 years CODIS has expanded from including samples only from persons convicted of serious felonies, to the now-routine collection of DNA from persons convicted of any felony, to samples from persons who have not been convicted of anything but have merely been arrested for minor offenses," said Michael T. Risher, lawyer for the American Civil Liberties Union Foundation of Northern California.

He said that expansion is best seen in California, which seizes and searches the DNA of everyone arrested for any felony — leading it to have the third-largest DNA database in the world, after the United States and the United Kingdom.

"The brightest, most fundamental line in our criminal justice system is the one that separates those who have been convicted of a crime from those who are presumed innocent," Risher said. If the government can cross that line to collect DNA, the database can grow without limit, he said.

Governments rarely get rid of the samples once they have them. Only nine states that collect DNA from arrestees automatically expunge samples from individuals who are not eventually convicted, court papers said. "The other states and the federal government retain these samples even when the subject has never been convicted, or even charged, of any crime," he said.

The Supreme Court is expected to make a final decision before summer.