A secret algorithm is transforming DNA evidence. This defendant could be the first to scrutinize it.

The Exxon clerk never got a good look at the assailants who robbed him at gunpoint in Fairfax County, so investigators hoped to bolster their case with the smallest of clues: the minuscule number of skin cells one perpetrator left behind when he grabbed the victim’s shirt.

Crime labs that have long pulled DNA from blood or semen have been pushing the frontiers of forensics by teasing genetic material from ever tinier and more challenging samples, such as sneaker sweat. But this time, the Virginia crime lab could not make a match because of a ubiquitous problem: DNA from too many people was on the shirt.

So authorities turned to advanced software that its creator promises will sort complex DNA mixtures much like a prism breaks down white light. The software doesn’t provide a direct “match” but assesses the probability that a suspect’s DNA is included in the sample. In the Fairfax case, police had a tip about one suspected robber’s name, and the software backed it up.

TrueAllele is reshaping DNA analysis, providing key evidence in thousands of homicides...