From Scott:

After a much-needed hiatus, I have returned to revisit the topic of fingerprints. If you recall from my earlier entries on the subject, we discussed what makes up a fingerprint, how they form, biologically speaking, on your skin, and the various methods we have for developing them. In this entry, we’ll get more into the technical side, showing how a fingerprint identification is actually made. If you’re writing a crime novel, or any novel involving fingerprints, a basic understanding of these principles will give your book a greater sense of realism.

As I mentioned in the first entry in this series, fingerprints are divided into three primary groups: loops, whorls, and arches. Each of those groups has its own subsets, but for the most part the subsets have fallen into disuse. They were important up until the last decade or two, because they helped eliminate prints from consideration. At that time, fingerprints were named based upon the type, the subset, and a numeric identification that had to do with the number of friction ridges between certain identifying features (deltas, cores, and so on). But now, with the advent of computerized fingerprint comparisons, the importance of the subsets has faded.

The comparison of fingerprints is a four-step process, whereby a known fingerprint (from a suspect’s fingerprint card, usually) is compared to an unknown print. The unknown latent typically comes from a crime scene, preferably in an area that strengthens the case for law enforcement. This is one factor that I have to explain repeatedly, both to police officers and to victims. If I have a car burglary, or a stolen car, and I find a fingerprint on the outside of the vehicle, it has no evidentiary value. All it means is that the person who left that fingerprint touched the outside of the vehicle. Touching it on the outside is not a crime. Now, if we can turn up a fingerprint inside the car, we have something. But even then, all we can prove is that the person was inside the car. It doesn’t mean the suspect actually committed any other crime.

Other factors figure into our ability to recover fingerprints. The environment plays a huge roll in whether or not a person actually leaves a fingerprint behind, and if so, how long that print will stay. If you touch a glass or painted surface on a car, you will usually leave a fingerprint. But if you touch that same surface in a rainstorm, you won’t. And in favorable weather, if you do leave a print behind but then it rains afterward, that print will likely be obliterated. Extreme heat can also play a factor. If the surface holding the fingerprint is exposed to summer heat, or even worse, direct sunlight, the moisture that our powders look for will evaporate.

The surface that was touched, known as the “substrate,” plays an enormous roll in whether or not a fingerprint is left behind. Modern appliances, along with most of the interior of today’s cars, are made to repel fingerprints, for the sake of appearances. This is great for the owner, but awful for the police. Other than on the glass, obtaining a usable fingerprint on the inside of a car is nearly impossible.

Once we have recovered a fingerprint from the crime scene, we have two routes to identification. If we have no idea who the suspect is, and the print is of high enough quality, we send it to the state police lab for inclusion in the AFIS (Automated Fingerprint Identification System) database. If that person has been arrested before, their prints will be in the database, and we should get a “hit” from the computer. Contrary to what you see on television, if the computer finds what it thinks is a match, a human must still make the comparison and identification, as described below. The computer isn’t perfect, especially with latent prints. The fingerprints recovered from crime scenes are typically messy and difficult to read, leading to mistakes by the computer.

If the computer can’t find a match, it will store that print in its memory. The benefit there is that if that same person commits another crime (highly likely), and the police find another print, the computer can link those two crimes together. Or, if the suspect is arrested on a different charge, the fingerprint card taken at his booking will be entered into AFIS, resulting in an identification.

In many cases, the detectives will have an idea of who committed a crime. They might have one suspect, or several, but this will give the examiner a place to start. The first step is an analysis of the latent fingerprint. Are there enough features (called “minutiae”) to make an identification? What type of fingerprint is it? The answers to these questions will allow the examiner to move to the next step: a comparison.

Let’s say the print being examined is a whorl. This is a fairly uncommon fingerprint class, so the examiner could fly through the fingerprint cards of the list of suspects, eliminating anyone who has no whorl-classed fingerprints. So if, for example, there are five suspects, we have a pool of fifty fingerprints to choose from. Roughly two-thirds of those prints would be eliminated, simply because they are the wrong pattern classification. so we might have only fifteen or twenty prints to consider. Next, we can look at such identifiers as the number of ridges that pass between main features, such as the core and one of the deltas (because, as you remember, whorls have two deltas!). We can also look at the features of the ridgelines in the center of the core. By this type of comparison, we can eliminate most of the remaining candidates.

Next, the examiner will take a closer look at the latent print, finding as many points of minutiae as possible. For those who don’t remember the earlier blog entry, here is the minutiae hunt in a nutshell. We look for anyplace where a ridge begins or ends, or where it splits in two, or short ridges (“islands”). Although that’s not an all-encompassing list, it gives you the idea. The reason the latent print is examined first, and not the known print, is simple. If you were to look at a ridge ending on the known print, and then try to find that same ridge ending in a blurry mess on the latent print, your mind could very easily trick itself into believing that ridge ending is there, when it really isn’t. We call that “pushing points,” and it can lead to a false identification.

With the minutiae identified on the latent print, the same process occurs with the known print. Then the two prints are compared. Naturally, since the known print is rolled from one edge of the friction skin to the other and is very clean, there will be minutiae visible on the known print that can’t be found on the latent print. But by comparing the ones that can be found in both, a case for identification can be built. If, however, a bit of minutiae is plainly visible in one print, and just as plainly isn’t there in the other, you have proof positive that these prints were not left by the same person.

The third step is an evaluation. Was there even a single point of minutiae in one print that should be visible in the other, but isn’t? Then this is a negative identification. Can a sufficient number of minutiae be identified in both prints, depending upon that agency’s requirements? Then it’s a positive identification. However, if there is an insufficient number of minutiae matches, yet there is nothing conflicting to say it’s a negative identification, then the case is listed as undetermined.

Finally comes the most important step: verification. Regardless of the training, experience, and knowledge of the examiner, another certified fingerprint examiner must verify the identification before it can be called official. If the second examiner disagrees, the identification cannot be made. This is done to protect the examiners from making a false identification, as well as to add strength to the claim when it goes to court. This whole process–Assess, Compare, Evaluate, Verify– is known as ACE-V.

So there you have it. From start to finish, the science and process of fingerprint examination. As with any scientific process, your novel will be more respected by readers if the content is correct. Follow the information from this three-part series, and your readers will be impressed!