A case before the Supreme Court Tuesday could change when and how police gather DNA evidence.
The high court is considering whether states can analyze DNA from people arrested for a serious crime before they are convicted.
The case centers around a Maryland man who says his Fourth Amendment right to protection from unreasonable searches was violated by the practice.
The FBI stores more than one million DNA samples obtained from individuals arrested but not convicted of a crime.
In 2009 Alonzo King, Jr. became one of them.
Police in Maryland arrested King for felony assault, took his DNA and and linked it to a separate rape case.
King was convicted.
Now his attorney argues the state violated King's right to privacy by conducting a warrantless search.
"The government conducted the search in this case for the invalid purpose of investigating other crimes for which the government lacked individualized suspicion," attorney Kannon Shanmugam says.
All 50 states currently obtain DNA samples from convicted criminals.
28 states also gather DNA from individuals arrested but not yet convicted for various crimes.
Jayann Sepich pushed New Mexico to mandate DNA sampling for felony arrestees after her daughter Katie's murder.
"Ultimately, this is about the truth and DNA is truth," Sepich said on the steps of the Supreme Court after Wednesday's arguments.
Attorneys for Maryland argued the information has helped officers solve hundreds of cold cases not related to the initial arrest.
"This is one of the most important criminal procedure cases they've had in decades because DNA is the 21st century fingerprint," says Maryland Attorney General Doug Gansler.
In court Justice Stephen Breyer compared DNA to a fingerprint calling it "no more intrusive" and "much more accurate", but Justice Sonia Sotomayor argued "there is something inherently dangerous that is not the same as fingerprinting."
The Supreme Court is expected to make a final decision before summer.
If justices rule in favor of King more than one million DNA samples in the federal database may have to be destroyed and others will never be collected.