By Robert Bazell Chief science and health correspondent
NBC News
updated 2/12/2008 10:30:48 AM ET 2008-02-12T15:30:48

Last week sheriff’s deputies in Chickasaw County, Miss., arrested Justin Albert Johnson for the 1992 rape and murder of a 3-year-old girl.

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What makes the case noteworthy is that another man, Kennedy Brewer, was convicted and sentenced to death for the same crime. Brewer spent 12 years in various prisons and jails, including death row, at the Mississippi State Penitentiary in Parcham.

DNA evidence exonerated Brewer in 2001. But because certain prosecutors were reluctant to admit they made a mistake, Brewer remained imprisoned until last August. Charges against him are still technically pending, but they will likely be dropped soon.

Brewer’s case is yet another victory for the Innocence Project, a non-profit group that used DNA evidence to overturn 212 cases since 1992. Fifteen of the accused had been sentenced to death.

Each of these wrongful convictions raises a profound question: What evidence was used to reach a guilty verdict in the first place? As part of a series "Who We Are: The Truth About DNA," airing on "NBC Nightly News" this week, on Wednesday I will explore how DNA testing is shaking up the world of forensic science.

‘Misleading results’
Courts believe DNA evidence because it is scientifically proven. It originated in the worlds of science, with molecular biology and clinical medicine. But in criminology different rules apply. With the number of DNA acquittals rising, many defense attorneys and prosecutors say it's time to take a hard look at current forensic techniques.

The Truth About DNA: A Nightly News seriesFor example, at Brewer’s trial there were bite marks on the child. A local expert testified they matched Brewer’s teeth. Brewer’s attorneys now contend animals bit the child after her body was thrown in a pond.

In another case, Wilton Dedge spent 22 years in a Florida prison, convicted of the rape and slashing of a 17-year-old girl, in part because an expert testified that hairs found on the victim matched his. DNA analysis of the same hairs were the proof of Dedge’s innocence.

In a laboratory where he teaches forensics at John Jay College of Criminal Justice in New York, Dr. Lawrence Koblinsky showed me two hairs under a microscope that certainly looked identical, but were not.

“We learned that often microscopic analysis of hairs can give you misleading results," he says. “In other words, what looks like a perfect match turns out not always to be a perfect match, and that we know through DNA testing.”

Most forensic techniques “started because some officer had a case he had to solve, and they had to come up with some new angle on it,” Peter J. Neufeld, founder of the Innocence Project with his law partner Barry C. Scheck, told me. "So, they invented a technique. And those techniques, whether they're bite marks, or hair or fiber, never went through the kind of robust research that DNA was subjected to.”

Congress has asked the National Academy of Sciences to study the validity of current forensic techniques. The report, due out this summer, could shake up the field even more.

Not like on TV
One issue nowadays is the false impression the public often gets from crime shows like “Law and Order” and “CSI: Crime Scene Investigation.” Video: Genealogy for sale

“The truth is, 80 percent of the stuff you see on TV hasn't been invented yet," Neufeld says. “It's good TV, but it's not realistically used in any crime laboratory in America or in the world, for that matter.”

But Neufeld and many others argue that is a tiny concern compared to what real crime labs really can and cannot do.

People are often convicted by eyewitnesses, an informant, or someone with a grudge. Even witnesses with the best of intentions can be totally wrong, psychology tests show.

Physical evidence will always be important. Unfortunately, there is no DNA to be found at the  scenes of many serious crimes. Forensic techniques need to be a good as possible and courts and juries need to understand the limits.

It is not just a question of freeing those wrongfully convicted, but catching the real perpetrators of the crimes.

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