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, rapes and other crimes like the armed robbery in Virginia in which genetic material was too complex to interpret. But that comes with a major caveat: Not a single prosecutor, government crime lab or defendant has had a meaningful look at how it works.