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.

“Mixture cases are their own little nightmare,” said William Vosburgh, director of the D.C. police’s crime lab. “It gets really tricky in a hurry.”

Technical advances allow bits as tiny as several skin cells to be swabbed from items that a person who committed a crime may have touched — from a waistband to a windowsill, a gun grip to a ball-peen hammer — but not left visible marks on.

But the world is a dirty place, and a criminal may not be the only one who left behind the DNA traces crowded into that very small sample.

‘Statistical sleight of hand’

DNA is rarely the only piece of evidence in a case and is not always the most compelling one.

But any adjustment in how DNA is handled ignites debate over whether the process alone gives either side in a court case an advantage.

Bowman’s attorney, James G. Connell, is among several defense lawyers pressing for more details about the new analytical approach.

Connell objected to what he called “statistical sleight of hand” in the May 2010 test on DNA lifted from the wrapper. The test concluded that Bowman could not be eliminated as a contributor to the DNA mix.

But that calculation factored in readings at only the 12 comparison points — of a possible 15 sites — where Bowman’s DNA showed up, Connell said in court papers that included a copy of the results.

Connell’s review of the previous tests in the Bowman case heightened his skepticism about the new analytical approach the state plans to use.

“I want them to release the details,” he said. “It’s important that we have the science determine the suspect, rather than we determine the suspect and make the science fit.”

Loudoun County Commonwealth’s Attorney James E. Plowman said he could not comment on the Bowman case because it is pending.

In February, one of Bowman’s co-defendants, Jaime Ayala, 19, pleaded guilty to second-degree murder in Bennett’s death.

Plowman said he has been told that there are “less than a handful” of his office’s cases that will be among those recalculated by the state. “At the end of the day, I don’t expect much to change,” he said.

The issue also has come up in central Virginia, where a May hearing in the capital murder case against Matthew F. Brady, 27, was delayed until October to allow the contractor for the state lab to run the new calculations on DNA evidence.

Brady is accused of fatally beating Joseph and Evelyn Bland, who were both in their 80s, during a robbery in January at their home in Colonial Heights, about 25 miles south of Richmond.

William Bray, the commonwealth’s attorney for Colonial Heights who had asked for the delay in Brady’s case, said DNA samples submitted to the state lab “were analyzed a few months ago, but the new best practices for the statistical analysis came into effect before a report was issued to us.”

“I want the most correct analysis possible under the revised criteria and believe that is what will be received” from the state’s outside contractor, Bray said.

‘Proactive approach’ lauded

Kermit Channell, executive director of the Arkansas State Crime Lab who also works on DNA issues as an official with the American Society of Crime Laboratory Directors, said DNA mixtures are among the most difficult to analyze. “I give Virginia kudos for its proactive approach,” he said.

As technology enables more DNA to be collected, it drives the search for the best way to present it, Channell and several other lab directors said.

Ray Wickenheiser, lab director of the Montgomery County police crime laboratory, said, “We start with a basic: How do you determine when DNA is DNA?

“On TV . . . Poof! You get DNA matches. In reality, you have a continuum that moves from there is no DNA present, to I see some there, to it’s either the DNA of this guy or his evil identical twin.”

“What we all are trying to do is find the right way to reveal what is there in the evidence and put the right amount of weight on it when it is presented to a jury,” Wickenheiser said.