Meet John Doe

In Chicago, a serial killer is on the loose, methodically murdering women in shuttered buildings on the city's south side. Police have no one in custody, reports CBS News Correspondent Carol Marin.

"I don't know his name. I don't know his face. I don't have his fingerprints," says Commander Thomas Cronin, Chicago Police Department. "But I know it's him."

He knows it's him because one person's DNA has been found at each of seven crime scenes.

Just 90 miles north of Chicago, in the case of three unsolved sexual assaults, Milwaukee authorities believe they have their man too.

"I know his genetic code. I know who he is, based upon his genetic code," says prosecutor Norman Gahn. "I just don't know who he is yet."

But even though they don't know his name, Gahn has taken the unusual step of charging his suspect anyway, based on his DNA alone. Time, after all, was running out -- the statute of limitations was going to expire in this case.

The warrant identifies the assailant as "John Doe, unknown male," with matching DNA at "genetic locations D1S7, D2S44, D5S110, D10S28 and D17S79."

An individual's DNA -- shorthand for the genetic material deoxyribonucleic acid -- is similar to a fingerprint in proving one's identity. DNA testing can be done on blood and sperm.

But is science trumping the Constitution here? Can you use DNA to stop the clock on the statute of limitations?

Some legal experts see problems in dragging cases out so long that other evidence and witnesses have disappeared.

Barry Scheck, who runs the Innocence Project at Cardozo Law School in New York, says, "The DNA is being used on the one hand to prove identity. On the other hand somebody who's going to be charged years later really would be unfairly prejudiced, unable to find alibi witnesses."

But in Milwaukee, prosecutors emphasize the victims.

"I think it's good for our victims of sexual assault to know we haven't forgotten them," says Gahn. "We'll use every bit of technology to go after these attackers and assailants."

CBS News Legal Consultant Andrew Cohen says, "If the DNA matches the suspect and the suspect actually has committed the crime, why not use the technology to perhaps prosecute and convict more criminals?"

Though the constitutionality of a DNA indictment will have to be decided by the courts, on the street, DNA is quickly becoming a cop's best friend.