At a crime scene, detectives mark items of possible evidentiary value, which are photographed and collected by technicians. Detectives may later decide that certain items are not relevant to the investigation, but they are tested for DNA nonetheless, Clifford said.
"Very often those crime scenes are enormous, sometimes covering entire city blocks," Clifford said. "You can imagine that a great deal of material gets picked up that's later determined to be too old or just not connected to the crime where it was picked up."
Because the items were deemed irrelevant, the presence of a criminals' DNA was not considered important, he said. Still, Col. John Bevilacqua, the head of the criminal investigations division, has ordered a change in policy since the issue arose.
"In a city where close to 100,000 people were arrested in one single year, the chances that you're going to turn up evidence of somebody with a criminal history being at or near a crime scene are pretty good," he said.
Jessamy said it was up to her office, not the Police Department, to determine whether the unreported hits are relevant to the cases.
"We make that decision," she said.
Such evidence must be offered to defendants and their attorneys during the discovery phase of their trials so that it is available to use in their defense. Prosecutors are examining the cases to determine "what, if anything" should be done, Jessamy said.
Defense attorney Donald Daneman said one of his recent cases might be among those in which DNA hits were not pursued. He said he was informed by prosecutors that DNA lifted from a beer can picked up two blocks from the scene of a shooting matched someone besides the defendant.
Though a jury found his client guilty of attempted first-degree murder on Sept. 10, Daneman said the beer can evidence would not have been a factor. He said he was more concerned about the broader problem of evidence disclosure.
"It wasn't a question of my client drinking a beer," Daneman said.
Michele M. Nethercott, who runs the public defender's Innocence Project in Maryland, expressed frustration that evidence-collection issues continue to emerge. She criticized Koch, the former lab director, and said his lack of "fastidiousness" combined with the high volume of cases caused the problem.
"This is coming out in dribs and drabs," Nethercott said. "First they tell us they fired the lab director, and it's no problem. Then they tell us the contamination is limited to crime scenes, then we find out it's in the lab. Now this."
