Fingerprints: The Infallible Means of Identification

Imagine walking into a stranger’s house, ducking under yellow “CRIME SCENE” tape, to find blood-spattered walls and a corpse on the living room floor. The blood and small hairs found in the room belong solely to the victim. How are you going to find this murderer? By finding the unchangeable means of identification that every individual has: fingerprints. Detecting, collecting, and identifying the different characteristics of various fingerprints found at a crime scene can bring you one step closer to catching a criminal.

There are three kinds of fingerprints that can be found a crime scene. Visible prints, plastic prints, and latent prints. Visible prints come from a finger that was covered with some sort of colored material, such as paint or blood. Plastic prints are prints left on a soft material, such as wax or putty. Latent prints, also known as invisible prints, are caused by body perspiration or oils that were on the fingertip and transferred to a surface.

Locating prints at a crime scene can be both easy and difficult, depending on the kind of print it is. Visible and plastic prints are relatively easy to find, because the prints are visible to the eye. Latent prints are much harder to find, due to the fact that they are invisible. There are different techniques to detect these prints, and the technique used depends on the type of surface being examined.

For hard and nonabsorbent surfaces, powders will work to develop fingerprints. There are many fingerprint powders of various colors and composition. It is important that the color of the powder used contrasts with the color of the surface being dusted. Testing a similar area with different powders is one of the best ways to know which powder will work best. As far as brushes to use, camel-hair will work for most surfaces.

In their textbook, “Criminal Investigation,” Kӓren M. Hess and Christine Hess Orthmann give these four steps to dust for fingerprints:
1. Make sure the brush is clean. Roll the hands of the brush between your palms to separate the bristles.
2. Shake the powder can to loosen the powder. Apply the powder lightly to the print, following the contour lines of the ridges to bring out details.
3. Remove all excess powder.
4. Photograph.

Although every police department and crime lab has their own procedure, this provides a good outline for a proper way to dust for fingerprints.

Chemicals are generally used for prints on soft and porous surfaces. Three of the commonly used chemical methods are iodine, ninhydrin, and silver-nitrate (also known as Physical Developer). When using iodine, the object suspected of having fingerprints is placed in a cabinet with iodine crystals. The crystals are then heated, which produces a violet vapor that is absorbed by the oils in the fingerprints, thus revealing the print. Ninhydrin, available in spray cans or in a powder, will react with amino acids in sweat to reveal the fingerprint. This method, at room temperature, may take from as little to an hour to as long as twenty-four to forty-eight hours to develop prints depending on their strength. However, heat tends to speed up the development. The silver-nitrate method, also known as Physical Developer, turns sodium chloride in fingerprints to silver chloride. The material is briefly immersed in the solution and hung to dry. However, if combining these three chemicals, the order should be iodine, ninhydrin, and then the Physical Developer. The chemical reactions that take place in one test may destroy chemicals in the fingerprint used to react in another test.

After the fingerprints have been detected, they must be collected to examine in the lab, and later on during the case. Before any lifting or moving is done, photographs must be taken of all prints. Then, one of two procedures is followed. If the object with the fingerprint is small enough, it is to be covered with cellophane and transported back to the lab. If the object is too large or immovable, the print is lifted. The common way of lifting is to take wide adhesive tape, place it over the print, work out any bubbles, lift the tape, and place it onto a labeled card.

When it comes to the identification of fingerprints, there are three fundamental principles. The first fundamental principle says that fingerprints are individual characteristics. What this means is that no two people, and more specifically no two fingers, have identical fingerprints. There are many small details, known as ridge characteristics or minutiae, in every fingerprint. These exact characteristics will only match in fingerprints that came from that finger. This is what makes matches of fingerprints from crime scenes to those of suspects or victims important for solving crimes. Any fingerprint found will only match perfectly with the fingerprint of one individual.

The second principle says that fingerprints remain unchanged. Every individual is born with their fingerprints. Skin is made of layers of cells. Cells closest to the surface make up the epidermis, and beneath that layer is the dermis. It is the boundary of these two layers, known as the dermal papillae, that determines the fingerprint patterns. Therefore, once this area develops in a fetus, it will never change. According to Saferstein, many attempts to alter or destroy fingerprints have been made, including that of the “notorious gangster John Dillinger, who tried to destroy his own fingerprints by applying a corrosive acid to them.” It was later determined that his attempt failed.

The third principle of fingerprints says fingerprints can be classified based on general ridge patterns. The three classes are loops, whorls, and arches. Sixty to sixty-five percent of the population have loops, thirty to thirty-five percent have whorls, and about five percent have arches.

In general, a loop pattern has one or more ridge lines that enter and leave on the same side of the print. Loop patterns will usually have one core and one delta. The core is the center of the loop. A delta is a triangulation or dividing of ridge lines. There are two subdivisions of loops. Ulnar loops have loops that open towards the little finger, and radial loops have loops that open towards the thumb.

A whorl pattern usually has ridge lines that form a circular pattern. This fingerprint will have two or more deltas. Whorls are subdivided into four categories: plain, central pocket loop, double loop, and accidental. Plain whorls and central pocket loops are classified as such by having at least one ridge making a complete circle. A double loop has a combination of two loops and an accidental either combines patterns or does not fit into any category.

Arch patterns have ridge lines that enter on one side and leave on the other, while rising in the middle. This type of fingerprint has no core or delta. There are two subdivisions of arches: plain and tented. A plain arch rises smoothly in the middle, while a tented arch rises sharply.

Think back to the opening scenario. After detecting fingerprints and taking valuable lifts, you analyze and identify the general patterns and tiny details of the prints you’ve found. You take this information and compare it to the prints of all suspected murderers. You eliminate all possible killers to find the individual responsible. Of course! It’s the butler! It’s always the butler. Good detection, collection, and identification of fingerprints help to catch another criminal and solidify the usefulness of this infallible means of identification.