Virginia reevaluates DNA evidence in 375 cases
By Mary Pat Flaherty, July 17, 2011

Just over two years ago, William and Cynthia Bennett were viciously attacked as they went for a Sunday morning stroll. William Bennett was beaten to death, his wife brutally assaulted.

Loudoun County prosecutors handling the high-profile case say DNA on a wrapper found near the crime scene points to a man they accused of capital murder and assault: Darwin Bowman. The odds, they say, of randomly finding someone that fits that same genetic profile: 1 in 620,000.

But now, Bowman’s defense attorneys are demanding more information about those numbers and will air their complaints at a hearing next month.

Bowman’s case is among the first that could be affected by the Virginia Department of Forensic Science’s decision to reevaluate DNA evidence tested since 2010 in 375 cases.

All of the cases involved a mixture of DNA, perhaps from a victim, a perpetrator and other people. But the analysis of those samples didn’t include a step — about how to factor weak results — which national experts early last year advised is crucial to producing reliable odds.

“Once we saw that guideline, we decided it was the right thing to do, to go back on these cases,” said Brad Jenkins, the department’s biology program manager.

Some of the 375 cases have already been through court; others are pending or under investigation.

The crime lab will not repeat the tests that teased out the DNA. Rather, it is changing how it concludes what the traits convey about who was at a crime scene.

Instead of reporting the odds of finding the same traits among the population, the lab will report on how much more likely it is that the mixture includes DNA from the victim and the suspect than from the victim and someone else in the general population.

State officials said they expect to get “comparable outcomes” in most of the 375 go-backs. “But it’s a case-by-case review, and in some instances, there could be a big difference. We won’t know until we do it,” Jenkins said.

The lab will switch to a different statistical approach, approved by national experts, to determine population statistics for DNA mixtures. That change will enable more of the genetic traits that surface to be included in calculations.

A private company is running the recalculations until the state lab can get its own software, standards and procedures in place, which Jenkins said he expects to be this fall.

Virginia’s state lab said in a one-page notice in April that the lab would directly contact prosecutors about cases that could be affected by the new calculations. The lab also intends to share revisions with defense lawyers handling post-conviction cases, said Stephanie Merritt, counsel to the state lab.

The recommendations from national experts are driving the reassessment at Virginia’s crime lab and others regionally and nationwide. Those experts, who meet under the guidance of the FBI, have worked since 2007 on devising guidelines for analyzing DNA mixtures.

That kind of evidence is not a large part of most labs’ work. The 375 Virginia cases came from a pool of 6,000 tested in the same period, lab records show.

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