An unclean sample won’t be useful in court, but it will be useful to the investigation. When that contaminated sample comes back to 4 people who don’t match the description, and two people who do, the investigation narrows to those two suspects. When they finally do find the guy, they can go back and re-interpret the rest of their evidence in 20/20 hindsight before taking it to court.

About the only way this guy isn’t going to get caught is if investigators are selectively incompetent, and/or the general public realizes it needs to “I’m Spartacus” the investigators.