. >> reporter: tall blood from stephen and linda riley. none from a suspect. and nonetheless they needed to focus on their son matthew. >> and they believe that there is no evidence that will be pointing to them. they let him go free until two years later when they discovered a silent witness. and the dog, the tan one on the left. the evidence, a single dog hair found in their boots. >> that's the idea for them that they most likely, you know, killed. >> and then put the boots on to make the getaway, which they were not found on the roof of the apartment building where he used to live. the dog hair tested, showing a 90% chance of belonging to winston. introduced in court and for the first time in san francisco county, animal dna was used to convict a killer. >> and in this case, their hair linked him to what he had hidden. and that is one of the reasons that he knew and that the boots, they believe that it was somehow on out there to the crime. >> they've been used for three high-profile convictions in california and the veterinary lab at uc davis that they do