“Law enforcement officials are vastly expanding their collection of DNA to include millions more people who have been arrested or detained but not yet convicted. The move, intended to help solve more crimes, is raising concerns about the privacy of petty offenders and people who are presumed innocent….”

In fact, mathematicians call it the “prosecutor’s fallacy.” That is, the probability that a given DNA sample matches a given person (and only that person) are not the same as the probability that that person committed the crime. That has to be demonstrated by other means. Merely having been at a crime scene is not, by itself, proof of guilt, though it certainly is something a defendant must have a pretty darn good explanation for.