Baltimore Public Defender Elizabeth Julian said her office is researching the cases directly affected by the problems and trying to learn more about the potential overall impact.
Kent said that the number of cases of known DNA contamination matters less than the fact that there has been contamination at all. He said defense attorneys have good cause to wonder if DNA collected from suspects has been transferred to samples from crime scene evidence.
"Contamination of any sort shows that there has, in fact, been a failure of lab practices," he said. "Any suggestion that is not a systemic problem simply shows a lack of basic understanding of how a lab should operate."
Dean Wideman, a forensic expert in San Antonio, Texas, said the contamination reflects poorly on the individual analysts and on the lab itself.
"It comes down to technique and carelessness - either way, it shouldn't happen that often," Wideman said. "It's bad for that analyst as well as the credibility of the lab. It's a reflection of the kind of work being done and the way they process samples in general."
Koch said that contamination does occur but called criticism a "smoke screen" and an attempt to "taint a jury pool by making accusations."
A 2004 report in the Seattle Post-Intelligencer found that forensic scientists at the Washington State Patrol laboratory had contaminated tests or made other mistakes while handling DNA evidence in at least 23 cases involving major crimes over a three-year period, including eight instances in which analysts contaminated samples with their own DNA.
And in 2003 the Houston Police Department suspended DNA testing and disciplined nine crime laboratory employees after an audit revealed that thousands of cases had to be retested because of errors in DNA analysis and possible contamination of samples. Evidence of DNA mishandling there resulted in a handful of convictions being overturned.
Practices in Baltimore's crime lab have been called into question before. Three years ago, Kent's forensics division launched a campaign against the crime lab's methods of analyzing gunshot residue, tiny particles left behind when a gun is fired. Police practices and disorganization at the lab led to contamination and unreliable gunshot residue test results, Kent said. He said his office is still sifting through years of cases to check for potentially false gunshot residue tests.
Kent said the city's lab has been "consistently unable to produce accurate scientific analysis" and Koch's dismissal is "years overdue." Koch was paid about $105,000 last year.
Sharon Talmadge will serve as acting director of the lab, which has 114 employees, including 22 in the DNA section.
julie.bykowicz@baltsun.com
justin.fenton@baltsun.com
Sun reporter Annie Linskey contributed to this article.
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