It's not very often a Supreme Court Justice says something like this at arguments: "I think this is perhaps the most important criminal procedure case that this court has heard in decades."

But that was Justice Samuel Alito today, talking about Maryland v. King, a case concerning a Maryland law that allows officials-without a warrant- to take the DNA of someone who has been arrested but not convicted of a serious crime. The federal government and 28 other states have similar laws.

Alonzo Jay King Jr. is challenging the law. In 2009 he was arrested for assault. But when, pursuant to the Maryland DNA Collection Act, officials took his DNA, they were able to eventually produce a match with a previously unsolved rape case from 2003. King argued that the DNA draw violated his constitutional rights, but he lost and was sentenced to life in prison for the 2003 case. In April, an appeals court ruled in favor of King. The Court said that King's rights to be free from unreasonable warrantless searches had been violated.

Read more about the DNA case here.

Today, lawyers for the State of Maryland and the Department of Justice asked the Supreme Court to rule in favor of so-called Arrestee DNA laws.

During extremely animated arguments the Justices seemed divided on some key issues.

Alito teed up a central question :"So this is what is at stake: Lots of murders, lots of rapes that can be solved using this new technology that involves a very minimal intrusion on personal privacy. But why isn't this the fingerprinting of the 21st Century?"

That is a big question.

Kannon K. Shanmugam, King's lawyer, argued emphatically that the taking of DNA is distinguishable from the taking of fingerprints because DNA contains far more information, the search is physically intrusive, and law enforcement's primary purpose to take the DNA is not for identification purposes.

On the question of the law's purpose, Shanmugam stressed the fact that the government was using the DNA to solve cold cases. He said the intrusive swab of his client's cheek constituted a search.

"Maryland searched my client without a warrant in order to investigate crimes for which there is no suspicion," Shanmugam argued "It is settled law that warrantless, suspicionless searches are presumptively unconstitutional."

Justice Elena Kagan posed critical questions regarding the law. She asked Katherine Winfree, the Chief Deputy Attorney General of Maryland, that if the purpose of the law is not so much for identification, but to solve cold cases, "then it's just like searching your house, to see what's in your house that could help solve cold cases." A search of the home- unless there is an emergency- requires a warrant.

"Just because you've been arrested, doesn't mean that you lose the privacy expectations and things you have that aren't related to the offense that you've been arrested for, " Kagan said.

Winfree responded that there was a "very real distinction" between the police " generally rummaging in your home" and "swabbing the inside of an arrestee's cheek to determine his DNA profile." She said that the DNA numbers revealed "tell us nothing about that individual. "

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The United States government favors the law, and argues that arrestees lose some of their privacy by virtue of the fact they've been arrested, "Arrestees are in a unique category, they are on the gateway into the criminal justice system. They are no longer like free citizens who are wandering around on the streets retaining full impact Fourth Amendment rights." Deputy Solicitor General Michael R. Dreeben noted that when someone is taken into jail he is subject to a visual strip search. If he's admitted into the prison population a TB test and a thorough medical screen.

Dreeben also said that the government's analysis of the DNA is restricted and that it is not like going into a house and exposing "a substantial number of highly private things" . He said it is much more like taking a fingerprint.

The fingerprint comparison brought up another concern of the Justices. A fingerprint analysis is almost immediate while DNA can take days or sometimes weeks to analyze depending upon the available technology and back log. How can something that can take days to acquire have a primary purpose for identification?

Dreeban said that soon this distinction won't matter. "The future is very close to where there will be "rapid DNA analyzers" that are devices that can analyze and produce the identification material in the DNA within 90 minutes."

Winfree followed up on the point. She said that "rapid DNA" is developing so quickly that the FBI estimates that within about two years it will be more of a reality.

"This is the fingerprinting of the 21st century, but it's better," she said echoing Justice Alito.

But Chief Justice John Roberts and Justice Antonin Scalia noted that if , at the end of the day, the case revolves around the primary purpose of the law, and if that is for identification purposes, then today, DNA is not readily available for that purpose.

" How can I base a decision today on what you tell me is going to happen in 2 years? You say in 2 years we will have this " rapid DNA" available, but we don't now. Don't I have to base a decision on what we have today?"

And Scalia said: "If we believe that the purpose of it has much to do with whether it's legitimate or not, you can't demonstrate that the purpose is immediate identification of the people coming into custody."

"The purpose now," he continued is "to catch the bad guys, which is a good thing. But you know, the Fourth Amendment sometimes stand in the way."