Let’s assume that a lab has come back with a report that a person’s DNA was found at a crime scene, and let’s assume that the technician at the lab hasn’t committed some kind of fraud. For instance, let’s assume that the technician hasn’t falsified the results. What could explain a person’s DNA at a crime scene if that person was really and truly not the person who committed the crime.
Contamination. If police suspect that a person is involved in a crime and want to test that person’s DNA against DNA found at the crime scene, contamination may occur which could seem to show the person was at the crime scene who he or she was no where near the crime. Police will typically collect “knowns” from suspects either through that person’s consent or by getting a search warrant. They will use a buccal swab of the person’s inner cheek to collect DNA, which they will try to match against evidence collected from the crime scene. If, however, in the process of doing this match, the lab technician inadvertently or mistakenly mixes the “buccal swab” with the unknown DNA on the evidence, the person’s DNA could end up on the evidence, having been mistakenly been put there by the lab technician. Then further testing would show that the person’s DNA was present on the evidence which would obviously be a result produced by contamination, and not accurate testing of the evidence.
Contact DNA. DNA can be transferred from person to person. Let’s assume that the real criminal knows the innocent person and that the two of them, for instance, shook hands. Or maybe they’re close friends or family and share clothes. It would be possible for DNA to be transferred from the innocent person, to the criminal, and then later left on evidence at the scene of the crime. Or the innocent person could know one of the victims, and have gotten DNA, through a hug, or handshake, to the victim who was later victimized in the crime.
Residual DNA. Let’s assume the person visited the home, business, car, or other place where the crime occurred. Perhaps this visit happened months ago. Maybe the person visited several times, or only once. Whatever the case, it’s possible that the person left behind DNA from a purely innocent visit. Later when the crime occurred and police conducted evidence collection, they collected items that have the person’s DNA on it even though the person was not involved in the crime at all.
Fake DNA. DNA could purposefully be planted. For instance, a criminal, hoping to blame someone else for the crime, might intentionally collect DNA from someone who is entirely innocent of the crime. The real criminal could commit the crime, and then leave DNA behind to try to get the innocent person accused and arrested of the crime. This is obviously rare.
Mixed DNA. Many DNA samples collected from a crime scene contain multiple DNA profiles from several people. Some of those people may be victims, perpetrators, or even investigators or police officers. It’s possible to distinguish a mixed DNA sample so that two partial profiles may be produced. This can be done if one of the contributors to the DNA is identified. For instance, if police obtain a swab from one of the victims and match it against DNA obtained from evidence, they will be able to get a partial profile for the other person (assuming only two DNA profiles are mixed on the evidence). Mixed DNA samples make the mathematical computation of the final statistics somewhat less certain. But even mixed DNA samples can yield results that would appear to be very conclusive to a potential jury.
These are some of the possible explanations for why an innocent person’s DNA might end up at a crime scene.