In the Montgomery County police lab, technicians have used the unique genetic human identifier contained in DNA to convict killers and rapists.
When the bars on those slides match up, it's a scientific certainty that an identity has been confirmed.
So since 2009, police across Maryland have collected 33,000 DNA samples from those arrested for violent crimes whether they were convicted or not. The growing data base has led to 43 convictions in crimes that had been unsolved.
Now, Maryland's attorney general is asking the U.S. Supreme Court not to take that tool away
"DNA is the 21st century fingerprint so we need to be able to solve cases," says Maryland Attorney General Doug Gansler. "Cold cases we otherwise wouldn't be able to solve."
At issue is the case of Alonzo Jay King. When he was arrested in Salisbury for waving a shotgun at a crowd in 2009, police took a DNA sample. His DNA linked him to a rape.
He was convicted, but then his lawyers argued that taking his DNA before conviction on the unrelated assault charge DNA was a 4th Amendment violation, an unreasonable search and seizure.
On Tuesday, the case landed before the U.S. Supreme Court with both sides trying to sway the justices.
"The only difference between DNA and fingerprints or a photograph is that they actually have to open your mouth to touch swab to the inside of your cheek," said Kannon Shanmugam, the attorney for King. "We believe that fingerprinting is really fundamentally different because of the profound information contained in an individual's DNA."
Maryland continues to take DNA samples at arrest, as do 27 other states. A decision in this case is due by the end of June.