WASHINGTON (AP) — The Supreme Court on Tuesday struggled with what one of the justices called its most important criminal procedure case in decades, whether to let police take DNA from those arrested, but not convicted, in hopes of using it to solve old cases.

Justices seemed conflicted over whether police have a right to take genetic information from people who have only been arrested but not yet convicted of a crime without getting a judge's approval first, or if the government's interest in solving cold cases trumped the immediate privacy rights of those under police suspicion of other crimes.

One justice seemed to make clear what he thought, with Justice Samuel Alito, a former prosecutor calling the arguments the court's "most important criminal procedure case" in decades.

Alito compared DNA swabbing to fingerprinting, saying police can use the genetic information of suspects in the same way they do the inked impression of suspects' fingers — to identify the person and match that person to older crimes that haven't yet been solved. "Why isn't this fingerprinting of the 21st century?" he said.

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.

In the case before the court, a 53-year-old woman was raped and robbed but no one was arrested. Almost six years later, Alonzo King was arrested and charged with felony second-degree assault. Taking advantage of a Maryland law that allowed DNA tests following some 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.

King eventually pled guilty to a lesser charge of misdemeanor assault, a crime for which Maryland cannot take DNA samples. The state 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 is reviewing that decision.

King's lawyer, Kannon K. Shanmugam, said governments are using the "Just Trust Us" defense by saying that they will only use the information provided in DNA swabs for identification and cold-case matching, and that people who have not been convicted of a crime deserve the ability to keep their genetic information private. But Chief Justice John Roberts noted that people leave behind DNA behind all the time.

"You disclose all of this intimate private information when you take a drink of water" at a police station, Roberts noted.

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.

The final decision may not fall along the usual ideological lines.

Justice Antonin Scalia cut off Maryland Chief Deputy Attorney General Katherine Winfree when she started listing the number of cases DNA testing helped the state solve. Unreasonable searches and seizures would also help solve cases, said Scalia, usually a reliable conservative vote.

"That proves absolutely nothing," he said. Scalia noted later that "the Fourth Amendment sometimes stands in the way" of crime solving.

On the other side, the usually liberal voting Justice Stephen Breyer seemed to think that DNA cheek swabs were no more intrusive than fingerprinting. "It's hard to say it's more," he said at one point.

Winfree also argued that technology will soon give the police the ability to get results back from DNA swabs within minutes, instead of the days it takes now, just like fingerprints. But "how can I base a decision today on what you say is going to happen in two years?" Roberts said.

The justices will make a decision later this year.