Hennepin County Attorney Mike Freeman relied on DNA evidence when he declined to charge two Minneapolis police officers involved in the fatal shooting of Jamar Clark. At one point, he described the DNA findings as "truth serum."
Experts, however, say DNA results are not as certain as they may appear to be and that the DNA evidence in the Clark shooting would have likely been challenged in court.
Freeman spent hours studying Minnesota Bureau of Criminal Apprehension reports that found the DNA of at least four individuals on the grip of Officer Mark Ringgenberg's gun. Ringgenberg took down Clark in the confrontation before Officer Dustin Schwarze shot him last November.
While experts acknowledge DNA kits have become more precise than in the past, experts won't testify they're certain a particular person touched a particular item, said Patrick Sullivan, senior attorney in charge of forensic science litigation for the Hennepin County public defender's office.
• The Jamar Clark shooting, aftermath: Timeline | Full coverage
"DNA is, as they say, the gold standard in forensics. But it's not a truth serum," Sullivan told MPR News on Wednesday. "It provides you information that you can use to help you make a decision."
According to the BCA report,
Sullivan says those skin cell tests are sensitive and almost always provide a mixture of results, making it difficult to be 100 percent certain.
"If you have a single source DNA sample from a blood stain for example," he explained, "then you can produce a pattern that might match somebody, so that can provide you a lot of terrific information."
But in this case and many others, a DNA test is not the gold standard. Sometimes people shed skin and leave skin cells behind. Other times, people's DNA can be indirectly transferred. The January issue of the American Academy of Forensic Sciences journal cautioned against assuming that a direct transfer happens based on the results of a DNA test.
"I might shake your hand and touch your gun and you never touched the gun but your DNA might end up on the gun," Sullivan said.
Because of that, he added, the end result is almost always inconclusive.
"With guns, it's very difficult because I can only recall a couple of cases out of the hundreds I've looked at that weren't mixtures," he said. "And this test isn't able to pluck somebody out of a mixture and say that they're there."