A basic understanding of statistics is sufficient to understand how this happens. Authorities have become obsessed with the idea of assembling giant DNA databanks of entire populations and then doing "cold hits" for unsolved crimes against the entire database.

The problem with this is that DNA testing is not 100% accurate, and when you test an entire population you're going to start getting false positives. The larger the population, the more false positives.

But then a jury may be told "there was a 100% DNA match" or that "There is a less than 1 in a billion chance that he is not the killer." And the wrong guy goes to prison.

Check out this other recent cold hit case. They took DNA samples from Occupy Wall Street protesters and then ran them against unsolved crimes files with no connection to the protesters just to see what they could get.

http://www.kens5.com/news/DNA-match-in-cold-case-murder-call...

So a protestor is arrested and charged for a 2004 murder based on this. Later found to be an error, but getting to that point assumes you can afford a good lawyer. There are lots of people getting charged for unsolved crimes they have no connection to. Not all of them can afford a lawyer and expert witnesses.

New York recently passed a law requiring DNA samples from all persons convicted of any crime, even misdemeanors such as loitering.

http://www.npr.org/2012/03/15/148692189/n-y-passes-dna-requi...

There is a nationwide push for mass DNA collections. Other jurisdictions are pushing to take DNA of people who are merely arrested, regardless of whether there is a later conviction.

http://chronicle.augusta.com/news/crime-courts/2012-09-01/so...

> South Carolina’s law enforcement agency will soon collect DNA samples from people when they’re arrested for a felony – rather than post-conviction – four years after legislators passed a law requiring the state’s DNA database to expand.

This push to states even comes from the federal level:

http://www.acluofnorthcarolina.org/?q=congress-votes-expand-...

http://thomas.loc.gov/cgi-bin/query/z?c111:H.R.4614:

> In a 357-32 vote, the House voted to offer cash incentives to states that start taking DNA upon arrest for certain crimes.

It even has it's own lobbying group: http://www.dnasaves.org/

> Every day innocent people needlessly become victims of violent crimes. Most of these are committed by repeat offenders. By passing state legislation that enables law enforcement to collect DNA from felony arrestees, at the same time as fingerprints, your state can catch criminals sooner

There you can see that 26 states now mandate collection on arrest. More than half of the 50 states.

Supreme Court Chief Justice John Roberts has ruled that DNA sampling on arrest, regardless of conviction, is permissible:

http://www.huffingtonpost.com/2012/07/18/dna-collection-mary...

The US has a nationwide database now that they do cold hits on, which is a statistically questionable process.

http://www.bioforensics.com/news/relatives_6-05.html

> Since the mid-1990s, the USA and the United Kingdom have maintained databases that use a series of such alleles to match DNA from unsolved crimes to known or suspected offenders. In the USA, states and the federal government keep DNA indexes of suspects and unsolved crimes, and share information through a computer system maintained by the FBI.

"Guilt by the Numbers: How fuzzy is the math that makes DNA evidence look so compelling to jurors?" by Edward Humes was an article that covered the problems here pretty well.

Original is gone, but here it is in cache:
http://webcache.googleusercontent.com/search?q=cache:www.cal...

More information about the case discussed in that article and the problems with the cold hit methodology: http://articles.latimes.com/2008/may/04/local/me-dna4

There are also various articles by DNA cold-hit advocates who claim it is invalid and cold-hits are a good practice, for example this article:

http://lawprofessors.typepad.com/science_law/2009/04/taking-...

Doing blind searches of samples on large DNA databases in order to find cold hits gives you a very high probability of a false match. A match that the falsely accused defendant will be quite difficult to challenge since there is a general belief by the public that DNA matches on a few markers are irrefutable proof of identity.

DNA can more strongly connect a suspect who is already known through other information. Doing blind searches isn't science and it isn't justice.

http://www.bioforensics.com/articles/Legally%20Scientific%20...

> For instance, if a DNA test capable of distinguishing between 'unrelated' people with one million to one confidence was used to create a database of two million personal profiles from a population of 20 million potential suspects you could be pretty certain that most crime stain profiles run against it would produce at least one cold hit.

> You could also be more than 90% certain that it would be the wrong cold hit. You could further expect that around 80% of the personal profiles on the database would match at least one other on record from a different person.

> Problems like these had led the 1996 National Research Council publication "The Evaluation of Forensic Evidence" (NRC-II) to recommend that "When the suspect is found by a search of a DNA database, the random match probability should be multiplied by N, the number of persons in the database".

On one side we have an argument that there are severe statistical problems with this. On the other side we have those who claim that is false and that the chance of any DNA match being wrong are so small as to be virtually impossible. Yet every year the number of false accusations and convictions from cold hits grows, as we see in this latest case.