Human Anatomy

Forensic scientists have used fingerprints in criminal investigations as a means of identification for centuries. Fingerprint identification is one of the most important criminal investigation tools due to two features: their persistence and their uniqueness. A person’s fingerprints do not change over time. The friction ridges which create fingerprints are formed while inside the womb and grow proportionally as the baby grows. Permanent scarring is the only way a fingerprint can change. In addition, fingerprints are unique to an individual. Even identical twins have different fingerprints.

Types of Prints
In general, the purpose of collecting fingerprints is to identify an individual. This person may be the suspect, a victim, or a witness. There are three types of fingerprints that can be found: latent, patent, and plastic. Latent fingerprints are made of the sweat and oil on the skin’s surface. This type of fingerprint is invisible to the naked eye and requires additional processing in order to be seen. This processing can include basic powder techniques or the use of chemicals. Patent fingerprints can be made by blood, grease, ink, or dirt. This type of fingerprint is easily visible to the human eye. Plastic fingerprints are three-dimensional impressions and can be made by pressing your fingers in fresh paint, wax, soap, or tar. Like patent fingerprints, plastic fingerprints are easily seen by the human eye and do not require additional processing for visibility purposes.

Surface Characteristics and Collection Methods
Characteristics of the surface in which the print is found are important in deciding which collection methods should be employed on scene. The general characteristics of the surface are: porous, non-porous smooth and non-porous rough. The distinction between porous and non-porous surfaces is their ability to absorb liquids. Liquids sink in when dropped onto a porous surface, while they sit on top of a non-porous surface. Porous surfaces include paper, cardboard, and untreated wood. Non-porous smooth surfaces include varnished or painted surfaces, plastics, and glass. Non-porous rough surfaces include vinyl, leather, and other textured surfaces. For porous surfaces, scientists sprinkle chemicals such as ninhydrin over the prints and then take photographs of the developing fingerprints. For non-porous smooth surfaces, experts use powder-and-brush techniques, followed by lifting tape. For rough surfaces, the same powdering process is used, but instead of using regular lifting tape for these prints, scientists use something that will get into the grooves of the surface such as a gel-lifter or Mikrosil (a silicone casting material).

Analysis of Collected Prints
Once a print is collected, analysis can begin. During analysis, examiners determine whether there is enough information present in the print to be used for identification. This includes determining class and individual characteristics for the unknown print. Class characteristics are the characteristics that narrow the print down to a group but not an individual. The three fingerprint class types are arches, loops, and whorls. Arches are the least common type of fingerprint, occurring only about 5% of the time. This pattern is characterized by ridges that enter on one side of the print, go up, and exit on the opposite side. Loops are the most common, occurring 60-65% of the time. This pattern is characterized by ridges that enter on one side of the print, loop around, and then exit on the same side. Whorls present a circular type of ridge flow and occur 30-35% of the time. Individual characteristics are those characteristics that are unique to an individual. They are tiny irregularities that appear within the friction ridges and are referred to as Galton’s details. The most common types of Galton’s details are bifurcation, ridge endings, and dots or islands.

Comparison of Prints
After analysis, unknown prints are compared alongside the known prints. The unknown print is the print found at the crime scene, and the known print is the print of a possible suspect. First, the class characteristics are compared. If the class characteristics of the two prints are not in agreement, then the first print is automatically eliminated. If this is the case, another known print may be compared to the unknown print. If the class characteristics appear to match, the examiner then focuses on the individual characteristics. They look at each individual characteristic point by point until they have found a possible match.

Evaluation of Comparison
After the examiner completes the comparison, they can make a proper evaluation. If there are any unexplained differences between the unknown and known fingerprints, then they can exclude the known fingerprint as the source. This means that if the class characteristics are in disagreement, then the conclusion would be exclusion. However, if the class characteristics as well as the individual characteristics are in agreement and if there are no unexplained differences between the prints, the conclusion would be identification. In some cases, neither of these conclusions is possible. There may not be a sufficient quality or quantity of ridge detail to effectively make a comparison, making it impossible to determine whether or not the two prints came from the same source. In these instances, no conclusion can be made and the report will read “inconclusive.” The three possible results that can be made from a fingerprint examination are therefore exclusion, identification, or inconclusive.

Verification of the Evaluation
After the first examiner reaches one of the three conclusions, another examiner must verify the results. During this verification process, the entire exam is repeated. The second examiner does the repeated exam independently from the first exam, and for an identification conclusion, both examiners must agree. If they agree, the fingerprint evidence becomes a much stronger piece of evidence if and when it goes to court.

Databases such as AFIS (Automated Fingerprint Identification System) have been created as ways of assisting the fingerprint examiners during these examinations. These databases help provide a quicker way to sort through unlikely matches. This leads to quicker identification of unknown prints and allows fingerprints to be as widely used as they are in criminal investigations.

Types of Fingerprints

Over the past sixty years, television has always been a reflection of our society. The programs are an indication of the norms, values and interests that society holds dear at any given time. Crime shows, detective stories and police tales have historically been a staple of the country's television viewing habit, and the country has always had a fascination with these kinds of shows. Today's programs, such as the most popular TV program over the last five years, C.S.I., bring a level of sophistication to the viewing audience that producers of the early crime shows, such as "Peter Gunn" and "Seventy-Seven Sunset Strip," never dreamed was possible. America has always had a fascination with the solving of crimes, and fingerprints are one of the most common types of evidence that investigators search for at crime scenes.

One of the main tasks of the crime scene investigator is to recover fingerprint impressions in order that a positive identification can be ascertained. Since no two individuals have the same fingerprint pattern and these remain unaltered during the course of a person's lifetime, the main type of physical evidence that can be extracted from a crime scene are fingerprints.

There are three distinct types of fingerprint impressions that can be recovered from a crime scene or a scene of interest for investigators looking for some clues as to a missing person, or for other identification purposes. These categories are as follows:

PATENT PRINTS - are visible prints that occur when a foreign substance on the skin of a finger comes in contact with the smooth surface of another object. These prints leave a distinct ridge impression that is visible with the naked eye without technological enhancement of any kind. The tried and true "blood on his hands" evidence is an example of patent prints recovered from a crime scene or scene of interest to investigators. These foreign substances contain dust particles which adhere to the ridges of the fingers and are easily identifiable when left on an object.

PLASTIC PRINTS - are visible, impressed prints that occur when a finger touches a soft, malleable surface resulting in an indentation. Some surfaces that may contain this type of fingerprint are those that are freshly painted or coated, or those that contain wax, gum, blood or any other substance that will soften when hand held and then retain the finger ridge impressions. These prints require no enhancement in order to be viewed, because they are impressed onto an object and are easily observable.

LATENT PRINTS - are fingerprint impressions secreted in a surface or an object and are usually invisible to the naked eye. These prints are the result of perspiration which is derived from sweat pores found in the ridges of fingers. When fingers touch other body parts, moisture, oil and grease adhere to the ridges so that when the fingers touch an object, such as a lamp, a film of these substances may be transferred to that object. The impression left on the object leaves a distinct outline of the ridges of that finger. These fingerprints must be enhanced upon collection and, because they serve as a means of identifying the source of the print, they have proven to be extremely valuable over the years in the identification of its source.

Now that we have categorized the various types of fingerprints, let's determine if we were crime scene investigators, could we differentiate among the fingerprint types? If you were a crime scene investigator or an investigator of a scene of interest, what type of fingerprints would you have discovered in these cases?

A Hershey's chocolate bar
A bloody print on a knife
A baseball helmet

The correct answers are:

Plastic prints because the chocolate bar softens when held and the ridges of the finger are present and visible to the naked eye.
Patent prints because a foreign substance, namely blood, has left a visible impression on an object, namely the knife, which is visible to the naked eye.
Latent prints because the helmet must be examined and the surface of the helmet technologically enhanced in order for the fingerprints to be viewed. Some techniques available to provide for identification of these types of prints are lasers, powders and various light sources.

The knowledge of the different types of fingerprints is invaluable to investigators in their quest to identify the source of the fingerprints, and the science of fingerprints is fascinating to the lay person. For investigators, fingerprints can provide invaluable clues as they serve as a means of identifying the source of the print. Because the seasoned investigator has a thorough knowledge of the different types of fingerprints, he is able to recover them for use as evidence or for other purposes.

Developing Latent Prints With Superglue

In the late 1970s, members of the US Army CID Lab in Japan brought home a novel method for developing latent prints. The forensic scientists in the Tokyo National Crime Lab had discovered that the fumes from cyanoacrylate adhesives (CA) or superglue, reacted with the moisture of latent fingerprints in such a way that a latent print on a non-porous surface was covered with a hard coating that encased the delicate ridge structure of the latent print.

Today, at least here in the U.S., superglue fuming is a basic step in the crime-solving playbook for a majority of crime labs. Back in the early days of superglue research, fuming chambers ranged from large plastic bags to converted fish tanks. Many new, more functional appliances are now available.

The pros and cons of employing superglue fuming are many: On the PRO side—it protects fragile latent prints by resisting accidental bumps and scuffs that would damage the ridges, it encapsulates the moisture content of a latent thus preventing vaporization and it is a means of processing large quantities of evidence at one time in all shapes and sizes.

On the CON side; superglue fuming produces white latent prints that often require the addition of a contrasting color to enable better viewing and photography of the latents, and it leaves a white residue on many of the surfaces the fumes contact. Although some early literature indicated that the fumes may be toxic, this theory has been disproven. The fumes are noxious but not dangerous.

Over the years many innovative advances have grown out of basic research employing superglue. Among some of the best ideas is vacuum fuming. Researchers discovered that superglue is a top performer when evidence and a few drops of superglue are sealed into a vacuum chamber. Current Lab-Type Fuming Cabinet users of the vacuum fuming method report that is possible to fill a chamber with numerous articles, including evidence such as large plastic trash bags, and cramming in all that will fit seems to have little or no effect on getting excellent results.

During the first few years of superglue experimentation, researchers sought a means of accelerating the development. Faster is better—right? Not necessarily. One of the first acceleration methods was impregnating cotton pads with a sodium hydroxide solution. This did indeed produces near instant clouds of white smoke. But over time such use fell into disfavor due to health-related concerns.

Two of today's most popular methods for superglue acceleration are:

Apply superglue to untreated cotton pads. This is quite safe and provides much faster fuming than just placing a few drops of glue inside a fuming chamber.
The application of heat is quite effective in speeding up the development process. This is accomplished by using a small coffee warmer. More sophisticated fuming chambers have built-in heaters.

A number of crime labs are encouraging crime scene investigators to fume fragile evidence prior to packaging for transmittal to the crime lab. This step has probably saved countless numbers of latent prints from being accidently destroyed during shipment to the crime lab.

Cyanoacrylate fuming is the number one choice as a first step in locating latent prints on non-porous surfaces. Until something better comes along, the crime scene technician should consider adding this process to the crime scene equipment kit — if this hasn't been done already.