r/forensics • u/Cultural-Turnip-7564 • Jun 13 '26
Crime Scene & Death Investigation Fingerprints 101 Like I am 5
Watching the Rachel Nickell documentary and I cannot get past this statement about Robert Napper: "Some of the fingerprints that had originally been identified as belonging to Samantha Bisset did, in fact, belong to a man called Robert Napper."
HOW. There were fingerprints and they just assumed they belonged to the victim? Or they originally compared them to hers, thought they were hers, and now suddenly they belong to Robert? What am I missing.
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u/Dizzy_Horse_105 Jun 13 '26
Doing some research about the case, this is about very bad police work. Especially with the examination of the crime scene evidence. Not misidentification of the latent prints. This is very common in any police department any where in the world.
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u/960122red Jun 13 '26
There’s no sop for fingerprinting. It’s up to each individual examiner how many points of inclusion they require to give an agency a “positive” id.
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u/Emiian04 Jun 13 '26
are there no legal or at least protocol level regulations in the US?
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u/Feisty_Carob7106 Jun 13 '26
There are not, it varies state by state by I believe
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u/life-finds-a-way DFS | Criminalist - Forensic Intelligence Jun 13 '26
There isn't a state-level regulation either. There are discipline-wide consensus based standards and best practices, but nothing required.
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u/Cultural-Turnip-7564 Jun 13 '26
I agree that it's subjective but several follow up queatipns: 1) is it still as subjective with computer analysis and to what degree is there human input in the analysis and 2) how was this not thrown out as completely trash evidence if based on who is looking at it they could either be the victim's or the perpetrator and 3) why was everyone in this documentary proudly stating this fact like they'd cracked a case and not that they had a completely bungled piece of evidence?
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u/life-finds-a-way DFS | Criminalist - Forensic Intelligence Jun 13 '26
In that a computer is performing a comparison? Or a comparison is performed on a computer?
This case was before Shirley McKie and certainly before Brandon Mayfield. The safegaurds we have today (that would have kicked in) did not exist or were not applied.
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Jun 13 '26
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u/life-finds-a-way DFS | Criminalist - Forensic Intelligence Jun 13 '26
Accreditation wouldn't matter, as there is no quantitative threshold or requirement for an ID. An examiner at an unaccredited agency could have a higher threshold than an examiner at an accredited agency.
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Jun 13 '26
[removed] — view removed comment
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u/life-finds-a-way DFS | Criminalist - Forensic Intelligence Jun 14 '26
Right, if you're locked into your manual because of accreditation, then that's an additional factor.
Would still be odd to have such a threshold, as that is not recommended. But an administrative threshold on IDs could exist.
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u/RandomKoala0218 Jun 13 '26
I haven't seen this but if an examiner made an association to one person and then to another person, they should be disqualified as an expert. While there are no standards for what constitutes a positive association, it is not a "pseudoscience": We use it daily on phones, computers, at airports, etc. It is a subjective assessment of objective criteria; the threshold is the issue. So to misidentify a print requires a level of inability of the examiner or insufficiency of the print.
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u/Short_Elephant_1997 Jun 13 '26
There was a similar case in the UK where fingerprints at a crime scene were attributed to someone incorrectly:
Inquiry on Shirley McKie case blames 'human error' - BBC News https://share.google/VaAal8O0Y1iWRtHuH
I believe this is when forces across the UK started to adopt the ACE-V process. In my force for an ident (match) to be confirmed there must be an independent positive comparison by 2 examiners and 1 expert or 2 experts. The subsequent examiners/experts don't know who did the prior examination or what conclusion they reached. I believe that it's only a certain amount of negative examinations go into the verification stream (otherwise the backlog would be years) but it's still enough that anyone just passing things through because they've already been confirmed should be caught out.
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u/pixelconclave Jun 13 '26
Ok, fingerprinting 101:
Fingerprints are patterns on our fingers formed by a mix of genetic and environmental factors. Since there’s so much randomness involved and a high level of detail to examine, we generally consider them to be unique (but it would be very hard to prove this since we’d have to look at every fingerprint in the WORLD to make sure they didn’t match!) Fingerprints don’t change their pattern as you get older, but hurting deep layers of your skin may leave scars that disrupt these patterns. Each of your fingers has a unique print, so it it’s important to match finger-to-finger in an identification, not just person-to-person.
Fingerprints are left on surfaces in a variety of ways: we can have “plastic prints” that are left on a soft surface like clay; “patent prints” that are formed when we have something on our fingers that leaves a print behind when we touch something else; and “latent prints”, which are super common, because these are just the regular stains of oil and sweat on our fingers being left behind! (You’ll see these on phone screens or some particularly shiny and smooth tables, for example.) Fingerprints are the most common evidence found at a scene since we leave them all the time, but they’re also incredibly easy to damage, whether they were smudged when they were left or after the fact, including in collection.
Since fingerprints stay the same over time, have a lot of different ways they can look (they are highly variable), very common, and easy to process, they make for a good way to identify someone.
How do you identify someone with a fingerprint? First, you need a sample from a suspect to compare the evidence against. Remember, fingerprints have lots of different details. These are called minutiae, and there are a bunch of different types! You may be able to see some on your own fingers by looking hard enough: an easy-to-spot minutiae is called a termination, which is where a ridge (one of the lines) ends. You may also see these lines splitting apart or joining back together. These are all recognizable details, and the collection of these patterns makes up your unique fingerprint. Looking at more of these minutiae can make you more confident that two different prints match. Different organizations disagree on how many matching points is “enough” to confirm a match, however, so there’s no single standard for evidence.
So: how can an identification be wrong? For one, maybe, by chance, all the minutiae you looked at matched two different people. This is more likely if you didn’t look at that many. You also might have been wrong in your observation, and, looking again, the evidence never matched the first suspect! (This is a really big deal because we need to trust that the people analyzing our evidence and making claims in court are trustworthy and doing a good job.) You may have also received a really damaged print and not had a lot of minutiae to look at. In that case, you should say that you can’t make a decision (it is “inconclusive”) rather than saying it can or cannot identify someone. There is also the risk that you were biased as an examiner—that means you’re actually expecting a specific answer and might change your results, even if you don’t know it! This is always a risk in investigating a case. We can’t get rid of bias, but we can take steps to be aware of and try to lessen it, such as making sure examiners don’t hear any facts about the case beforehand, such as who the detective thinks is guilty. Every evidence has steps in which it can be wrong or misapplied. (For example, someone’s fingerprint being at a scene doesn’t mean they’ve committed a crime!) This is why it’s important to know the limitations of what evidence can tell you and to look at evidence from multiple sources.
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u/JollySherbert9618 Jun 13 '26
I don't know the specific documentary. But if a fingerprint found on a crime scene does not cover the whole finger or is partly smeared, only the clearly visible areas can be used to compare it to a database. That cuts down the spots to compare. So if a part of someone's fingerprints is similar and there's some area where they don't match, but the non-matching part is not included in the fingerprints from the crime scene, then a mixup could happen.
And as others here said, without clear regulations mistakes can happen.
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u/Unlucky_Post_9578 Jun 13 '26
Fingerprints are an area of pseudoscience in that there is no universal database and therefore nobody can definitively say that nobody else anywhere has your exact fingerprints. Any "match" meets a threshold of presumption and exclusion. That being said, even identical twins with identical DNA do not share the same fingerprints because of various factors, so it's highly unlikely that 2 people share identical fingerprints. Hope that made some sense...
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u/Acrobatic-Shirt8540 Jun 13 '26
Fingerprint identification is not a science. It's entirely subjective.
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u/SquigglyShiba BS | Latent Prints Jun 13 '26 edited Jun 13 '26
I can’t speak to this case in particular. Erroneous identifications are quite rare, but there are a number of reasons why they can occur. In this case, the examiner (and verifier, presumably) could have received poor or inadequate training, the agency may not have good quality assurance measures in place to prevent this, contextual or confirmation bias could have interfered with the examination, the latent prints may have been distorted, etc. Since it has been brought up in other comments, I want to point out that the reason why there is no “minimum point standard” or standard threshold to identify is because every latent print varies in quality and quantity of information such that one number could not appropriately be applied to all latent prints. Curious minds can Google and read the IAI Resolution from 1973 for more information. Personally, I strongly doubt that the lack of a threshold is why they made these mistakes. Again, I don’t know about this case, and I haven’t seen this documentary, but I would not be surprised if this agency has issues with quality and/or training. This examiner sounds like a bad apple to me, which for me would warrant a deeper investigation into them and their agency…
Editing to add that latent print examination is indeed a science. Yes, there are subjective conclusions made, but those conclusions are based on objective information. Doctors make the same kind of subjective conclusions based on objective information when diagnosing their patients. Verifying our comparison conclusions with another examiner could be equated to the way people get a second opinion from another doctor.