There was just enough spit on the back of the 9-cent stamp to piece together the identity of the person who licked it. Everything except for his name.

The anonymous person had mailed two threatening and racially tinged letters to Dane County Circuit Court Judge Juan Colás, the Wisconsin county's first Hispanic judge, after one of the most controversial decisions of his career. It was early October 2012, and Colás had just struck down a law that gutted collective bargaining rights for public employees, resulting in praise from unions and piles of hate mail from others — including death threats from this anonymous mailer. The judge's decision was later overturned. But the pursuit of the anonymous letter-writer continued.

The first threat on the judge's life came enclosed with a Los Angeles Times newspaper clipping that detailed the murders of two Mexican politicians, suggesting Colás may be next. The second came enclosed with an advertisement for dentures, suggesting Colás may be losing his teeth soon — or perhaps worse.

"Sometimes radical steps are required to repair our laws + our idiots sitting on a bench supposedly dispensing justice," read the second letter. It called him "expendable."

The sender has never been caught. They don't know his name. But he put a 9-cent stamp on one of the envelopes he mailed — never mind that the last time that stamp was sold by the Postal Service was about 1975. And forensic analysts took the spit left on the back of the stamp and ran it through the lab to obtain a DNA profile, which, as it happened, matched the DNA found on anonymous threatening letters mailed to three other public officials in Wisconsin.

But they still had no name. So on Oct. 8, the night before the statute of limitations would have made an indictment impossible, the Wisconsin Department of Justice filed felony charges, in a case now known as State of Wisconsin vs. John Doe, Unknown Male With Matching Deoxyribonucleic Acid (DNA) Profile at Genetic Locations D3S1358 (15, 18), etc.

"Your complainant believes that the sender of the envelope and, therefore, the sender of the threats, is the person matching the above DNA profile," a Wisconsin Department of Justice special agent wrote in the complaint.

And while many defense attorneys question this way of skirting the statute of limitations, the indictment of the DNA profile itself is entirely legal in Wisconsin.

The approach, used rarely by prosecutors, is designed for occasions when no suspect can be identified but their genetic makeup can. The state of Wisconsin, prosecutors said, actually pioneered the tactic.

Called a "John Doe warrant," or in this case, a "DNA profile indictment," it originated in the late 1990s as a way to file charges against unidentified rapists before it became too late, leaving rape victims without any prospect of justice.

As states across the country have changed their laws to remove or extend time limits on filing sexual assault charges, the tactic has become less necessary in those cases but more useful in other crimes — such as sending death threats through the mail, prosecutors said, or kidnappings and burglaries.

"We use this for the type of crimes that send a chill across the public," said Greg Totten, district attorney in Ventura County, Calif., and vice president of the National District Attorneys Association. "As a law enforcement official, you want to do everything you can to bring the perpetrator to justice, and if the statute of limitations is expiring, you want to find a way to toll [extend] that statute. The John Doe warrant approach provided a great mechanism to be able to do that."

John Doe warrants took off in 1999, when then-Milwaukee County Assistant District Attorney Norman Gahn became one of the first prosecutors in the nation to file one. The idea came to him out of frustration, he said. The six-year statute of limitations was about to expire on a brutal 1993 serial rapist case, in which three young women had been kidnapped at knife point while walking alone in Milwaukee. The semen collected at each crime scene pointed to the same perpetrator — and that's when the idea hit Gahn: Why not just indict that DNA profile?

"I thought we were on firm, solid ground," he said. "The way I looked at it, it was as if the victim, before she got raped, for some reason was able to take a photograph of the guy. And then when she came to report it she'd say: 'Look, I have no idea who this person is. He's a stranger. I don't know anything about him. But here's his picture.' I'd take that picture and staple it to the criminal complaint. And I thought this was the same thing. Here's his DNA profile from the semen. That's the guy."

The strategy simultaneously boomed in other states, including California and New York. But it was never universally accepted, particularly among defense attorneys who argue it violates defendants' Sixth Amendment rights to a speedy trial and to due process. It could be many years before a DNA profile from a crime scene is matched with one in the national DNA database. In 2004, the National Association of Criminal Defense Lawyers passed a formal resolution opposing John Doe warrants. Drew Findling, the organization's current president, said its position remains the same today.

"The main constitutional concern is that the passage of time just makes cases more and more difficult to defend," Findling said. "It is just such an infringement upon an individual's right to fairly defend themselves as guaranteed by the Constitution. And you lose the right to a speedy trial, really, when you basically say that you can extend an accusation ad infinitum."