Identifying latent fingerprints; a basic look at how police match fingerprints from a crime scene.

Identifying fingerprints is done almost at the touch of a button – at least that’s how it seems from watching TV.

In real life, fingerprint comparisons can be a complex process, and quite difficult at times. The ability to accurately identify recovered fingerprints is critical in law enforcement, as a matter of fact, it’s invaluable.

No two individual fingerprints have ever been found to be identical, and that is a big deal when you consider that there have been several million fingerprints examined over the world since the practice began. Uniqueness, and the permanency of fingerprints are the fundamental reasons why they are so relied upon in investigations. Scars, creases, warts, etc. will affect the appearance of a print, but otherwise, you are born with the same prints you will die with.

A very simple definition of a fingerprint impression is any impression of, or left by, the friction ridge skin of a finger or thumb. Friction ridges are the ridges and furrows that are on the pads of the fingers and thumbs. These ridges are there to provide grip, hence the name friction ridges. A palm print is the same thing, it’s just from the palm instead of the pads of the finger.

So what’s a ridge and what’s a furrow? Think of a row crop; the ridge is the elevated section, whereas a furrow is the area between the ridges. In an inked impression, the ridges show up as black, and the white between them is the furrow.

Inked (rolled) fingerprint impression

Every fingerprint will have characteristics that make it unique, and the characteristics are termed minutiae. Minutiae, are the major features of fingerprints that are used in matching one print to another. Minutiae are broken down by their form, but they basically fall into two main categories; ending ridges, and bifurcations. An ending ridge is simply the point where the ridge ends, a bifurcation is when a ridge branches out, or bifurcates, making two ridges instead of just one.

There are subcategories, but they really are just variants of these two. These subcategories consist of, in part, dots, islands, and spurs. (Dots – very short ridges; Islands – the middle space between two diverging ridges; Spurs – are notches protruding from a ridge.)

Put simply, to conclude that two separate fingerprint impressions belong to the same individual, the examiner has to match the minutia points in both prints to one another; the minutiae must have corresponding orientation, they must be present in sufficient numbers, and the must be distinct enough to distinguish and show agreement between the two impressions.

The examiner analyzes, compares, and evaluates the details of the prints by examining the configuration and layout of the ridges, called ridge flow, by examining the length of the ridges. The examiner also looks at the ridge edges, and the presence and placement of sweat pores on a ridge, when the clarity of the impression allows it.

In comparing prints, you will find challenges because the two fingerprint images, although made from the same finger, will present slightly different from each other because of factors like the amount of pressure used in depositing the impressions, the orientation of the finger in relation to the surface when the impression was deposited, movement of the finger as it left the impression, and the clarity of the impression.

There will always be slight variations and distortions seen between the impressions. The examiner must have experience in studying prints to be able to understand the distortions, and they must understand how the differences in appearances of impressions affect the examination.

The actual examination of prints begins with the examiner evaluating the prints, making a determination, based upon training, and experience, whether the questioned print is sufficient for comparison against a known print. If the print is determined to be insufficient for comparison, the examination is concluded there. The conclusion in this case would be that the print is insufficient for comparison.

If the first step shows sufficient detail, then you proceed to step two, a side-by-side comparison of the prints. Here, the examiner is looking to see if the minutiae points are in agreement in both prints, based upon similarity of type, that they fall into the same sequence in both impressions, and that they are in the same location on both impressions.

The third step is the evaluation phase. Here, the examiner makes the final determination as to whether the two impressions were made by the same individual, known as agreement, (match) a finding of disagreement (no match), or a finding that is insufficient (inconclusive.)

As with any scientific examination, there must verification of the finding before before it is valid; the verification phase is the last step. Verification involves the unbiased examination by another qualified examiner, that results in the same conclusion.

The verification phase is mandatory on all verifications, it’s optional for exclusions or in inconclusive findings.

Let’s look at the how the prints are actually examined. A fingerprint is broken down into three basic levels of detail. Level 1, 2, and 3, friction ridge details.

The first is Level 1, and it includes the general ridge flow and pattern configuration. There isn’t sufficient detail here for an individualization, but you can used it for exclusion; if you clearly have an Arch pattern in the questioned print, and clearly had a whirl patter in the known print, you wouldn’t proceed, because there is no agreement in the level 1 detail.

Level 1; ridge flow and pattern are clearly different. The evaluation here is that the two prints aren’t in agreement,

Next, level two looks at the individual friction ridge paths, and the minutiae: ending ridges, bifurcations, dots, etc. Minutia are often referred to as Galton points. Minutia or Galton points are often referred to in court, because they represent a simple way to explain identification methods in court, but they are just part of the puzzle.

Individual friction ridge paths, and minutiae are sufficient for comparison

Level 3 detail is the third level, and includes all other attributes of the print – the ridge width, ridge shape, sweat pores, creases, scars, and incipient ridges (a ridge that is thinner and shallower than the surrounding ridges).

Level 3 detail; the white “dots” in the black ridges are sweat glands.

The final identification decision is subjective and is reached when sufficient quality (clarity) and quantity of corresponding Level 1, 2 and 3 friction ridge details are present. Some individual examiners may have a self-imposed or agency imposed “point rule” say 8-points of minutia to constitute an identification, but there is not a recognized threshold in terms of simply finding points to make a match.

Inked print left, recovered print on right
Minutia points marked on both prints
Match !

As apposed to a match, If the examiner is unable to explain the variations of appearances, distortions, discrepancies, differences, or disagreement between the two prints, the inconclusive determination is the conclusion.

According to the Scientific Working Group on Friction Ridge Analysis, an expert conducts an examination based upon knowledge and beliefs from training, experience, understanding, and the demonstration of repeatable determinations with verification.

2 thoughts on “Identifying latent fingerprints; a basic look at how police match fingerprints from a crime scene.

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