When a latent print is obtained from evidence of a crime it carries with it the potential to be compared to it’s possible source. This may be a victim, suspect, or another individual who came into contact with the object or scene itself whether it was during the crime or at some point in the past. Currently there are three broad conclusions that the latent print examiner will choose from as a result of the comparison: identification (the latent print was made by the subject being compared), exclusion (the subject compared is not the source), and inconclusive (the subject could neither be attributed as the source or excluded as the source). This last category seems to exist in a gray sea of indecisiveness however it not necessarily the case and often the inconclusive decision can easily be resolved with additional resources.
For now we’ll leave the undecided category for another time and focus on the quite common use of the inconclusive decision. Often, a latent print is deposited from a part of the finger or palm that is not sufficiently recorded on the exemplar (known fingerprint card recorded during an arrest or perhaps as is required for an application for a pistol permit; see above image) that the fingerprint examiner is asked to compare the latent print to. Sometimes even if the desired region is recorded on the exemplar/standard, it is of such poor quality that it cannot be used as needed for the comparison to the latent print. Unfortunately, it is not rare to have to rely upon exemplars that are of poorer quality than the latent print.
Take for example the three latent prints above that were produced by the tips of the fingers. These types of latent impressions are quite common when you consider just how often the tips of our fingers are used throughout the day. Chances are you’re using them right now to scroll through this on your smartphone. A typical fingerprint card has recordings of fingers that are rolled laterally from one side to the other and in doing so it is not likely to include the tip region of the fingers. Therefore, it’s not at all uncommon to come across latent prints that are of value for comparison but cannot be properly compared to the subject’s fingerprint card. Specially recorded prints including major case prints, which are an attempt to record all of the friction ridge skin (the skin responsible for leaving fingerprints behind) of the fingers and/or palms (see below) are required in order to be able to render a conclusive decision of either exclusion or identification. It may seem tedious to have to obtain all of these samples but it can go a long way in understanding whether an individual is the source (or not) of a particular latent print. So, it’s not necessarily “I don’t know” but more like, “I don’t know yet.”
SWGFAST (Scientific Working Group on Friction Ridge Analysis, Study, and Technology)
Document #10 Standards for Examining Friction Ridge Impressions and Resulting Conclusions (Latent/Tenprint)
An inconclusive conclusion resulting from a suitability decision as described in approach #1 in section 184.108.40.206.1 occurs when an examiner is unable to individualize or exclude due to an absence of complete and legible known prints (e.g., poor quality fingerprints and lack of comparable areas). In such an instance, the inconclusive conclusion means that the impression needs to be reexamined using clearly and completely recorded known impressions.
The fourth letter our alphabet (“D” if you were wondering) traces its roots to the fourth letter Daleth (top left image) from the Phoenician alphabet, represented by a three sided triangle and translated into “door”. Depicted as ℸ (image second from left) in Hebrew, as 𝚫 (image third from left) in Greek, and “4” in our numerical system. Usage of the word “delta” includes the description of alluvial deposits at the mouth of a river (image fourth from left) where the river meets a larger body of water (which can take on the triangular appearance of the Greek letter). In mathematics it can be understood to mean the difference between two values. Its meaning and use in regards to fingerprints (and latent prints) is based on its visual appearance similar to that of the delta of a river. The text, The Science of Fingerprints defines it as “that point on a ridge at or in front of and nearest the center of the divergence of the type lines” (images fifth and sixth from left). The delta of a fingerprint is a key focal point useful in attributing a pattern type (loop, whorl, arch). For fingerprint classification it is a locatable point in space upon a ridge but the more general appearance of the delta area is three sided (like the Greek letter) and is simply the convergence of three systems of ridges which during fetal development formed in separate locations and eventually spread across the finger to the same point. This type of ridge convergence also occurs on the palmar surfaces of the hands and the plantar surfaces of the feet which also are characterized by the friction ridge skin.
Federal Bureau of Investigation, The Science of Fingerprints: Classification and Uses, U.S. Department of Justice, U.S. Government Printing Office: Washington DC, 2006.