Fingerprints are used to identify an unknown victim, witness, or suspect, verify records, and most importantly, serve as links and matches between a suspect and a crime. Even without a suspect, fingerprints can reveal clues and sometimes provide hints about the criminal’s size, gender, and occupation. Small fingerprints tend to belong to smaller individuals, and prints on a wall can indicate a suspect’s height.
Construction workers often have rough hands, and musicians tend to develop calluses on their fingertips. It’s important not to rely too heavily on these clues, as they are not facts. Fingerprints can corroborate or refute a victim or witness’s story by locating their prints where they said they were. Even the absence of prints can be a key factor. Suicide scenes, for example, should never show any attempts to erase fingerprints.
Occasionally, a palm or barefoot print is found. These are usually processed using the same methods as fingerprints. Several months before a baby is born, their fingers and thumbs develop ridges. These ridges are arranged in more or less regular patterns. For classification purposes, experts divide these ridge patterns into three basic classes: arches, loops, and whorls.
Each class can be further divided into numerous subcategories. Regarding arches, loops, and whorls, there are slight racial variations. People of African descent tend to have many arches; people of European origin frequently have loops; and Asians/Easterners have a relatively high frequency of whorls.
History of Fingerprinting
Around 1750 BC, Babylonians used fingerprints to sign their identity on clay tablets. Around the year 220, the Chinese were the first to use ink impressions, but then fingerprints were forgotten for several hundred years. In 1686, a man named Malpighius wrote describing the ridges of fingerprints, but stopped there.
Later, in 1823, J.E. Purkynie discovered that it was possible to classify fingerprints, and from there the use of fingerprinting began. In 1858, Sir William Herschel required workers to sign contracts with their fingerprints in India, and later, in 1877, suggested taking prisoners’ fingerprints. In 1880, Dr. Henry Faulds published an article about the use of fingerprints in England, and two years later Gilbert Thompson used fingerprints on checks to prevent fraud in the United States.
Between 1901 and 1910, many countries began using fingerprints, and soon, in 1924 in America, the FBI’s Identification Division started.
Methods for Detecting and Collecting Latent Prints
Today, fingerprinting is widely used worldwide for many purposes. One of the main uses of fingerprinting is criminal investigation. One routine activity at crime scenes is fingerprinting, through various methods.
Fingerprints can be found and collected at the crime scene, and later used to link suspects to the crime scene. Sometimes, fingerprints can be seen by themselves, like impressions on glass. But often, they are not visible to the naked eye, so methods are used to make them visible. There are several methods for detecting latent prints.
Powder and Adhesive Tape
Probably the most well-known method for detecting latent prints is dusting them. Various powders are used, many of which contain aluminum or carbon. This finely ground powder is gently applied to a surface, and the tiny powder particles adhere to the fingerprint residue, making it visible to the human eye. Then, these prints are lifted with adhesive tape. For dusting to work, the surface to be dusted must be completely dry and relatively free from other contaminants.
Magna Brush
This is a magnetic wand that attracts iron. It is dipped in iron powder, and the particles stick to it. It is used in the same way as carbon or aluminum powder. The Magna Brush is also less messy, as the leftover iron particles can easily be collected with the Magna Brush. A wide variety of fluorescent colors are available, and some of these powders reflect alternative light sources like UV and laser.
Cyanocrylate Fuming Method
The cyanocrylate fuming method, also known as the superglue fuming method, has proven to be another very useful way to detect latent prints. This method was first used by the Criminal Identification Division of the National Police Agency of Japan in 1978. Shortly after, it was adopted in the U.S., and now is a commonly used method for detecting prints. Most superglues are methyl cyanoacrylate or ethyl cyanoacrylate. This reacts with the amino acids, fatty acids, and proteins in fingerprints, as well as with the moisture in the air, making them visible.
Ninhydrin
Another common method for detecting fingerprints is the use of ninhydrin. It is sprayed, rubbed, or dripped onto the surface. Ninhydrin reacts with the amino acids in the prints, forming a purple or pink compound.
Iodine Fuming
Iodine crystals are placed in a glass tube known as a fumer. Then, the examiner blows into the fumer, causing the transformation from solid to gas. The iodine vapors are emitted from the other end, and if the tube is directed at a latent print, it will become visible for a brief period.
Silver Nitrate
Silver nitrate is a less toxic way to detect prints on paper. Silver chloride turns black in light, and one of the components of sweat is sodium chloride. Silver nitrate is mixed with distilled water and applied to the paper. The paper is exposed to light, and any print will turn black.
Amido Black
Amido Black is a chemical used to reveal fingerprints in blood. First, a fixing agent is applied to the bloodstain, and then Amido Black is used. There are other methods for detecting latent prints, most similar to the techniques mentioned above. Some methods even include laser technology. Different surfaces will require different techniques to reveal latent prints. For example, when revealing a print on paper, ninhydrin should be used, though powders can work but not as effectively.
When revealing prints in blood, Amido Black should be used, and powder works best on non-porous surfaces. When taking fingerprints from people, ink is rolled onto the fingers, and then the fingers are pressed onto papers or cards. However, when it comes to taking fingerprints from the dead, it is not so straightforward. If we take fingerprints from a recently deceased corpse, there is no problem. However, if it is a decomposing or mummified corpse, where the skin has hardened and contracted, a special method is required.
Normally, the fingers are immersed in a solution of glycol, lactic acid, and distilled water, which softens the finger tissues. If the skin has wrinkled due to moisture, the fingers can be printed using a hypodermic syringe, gently manipulating the fingertips by hand, or by completely removing the skin from the fingers and mounting it for printing. It is very rare that fingerprints found at the crime scene are completely intact, meaning the quality of the print is lower and may be more difficult to positively match.
When comparing several sets of prints, a certain number of characteristic points must match. Worldwide, there is no fixed number of points that must match for acceptance as a match, but it differs in some countries. For example, in the Netherlands, 12 points are required, while in Africa only 7 points are needed. There are also differences within the same country. In Paris, 17 points are required, while in the rest of France only 12 are needed.
Other Uses
Fingerprints are not only used to link criminals to crime scenes. When human remains are found, fingerprinting is a very common way to identify them. Fingerprints are also used for security purposes, such as on certain identity documents and as entry control in important buildings.
Without Prints
In very rare cases, some people are born without fingerprints on their fingers, palms, or feet. Although it can be a condition one is born with, some people’s ridges degenerate over their lifetime.
Fingerprint Databases
The Automated Fingerprint Identification System, or AFIS, is a database of fingerprints taken and stored in the United States, although other countries like Canada and the United Kingdom also have their own AFIS. Fingerprints may have been taken for various reasons, and the system is used for various purposes, such as criminal identification, benefits reception, background checks, and credentialing.
The machine used to scan fingerprints is called a LiveScan Device. AFIS allows screening huge amounts of fingerprints every second, and the computer marks all the characteristic points (individual features) it recognizes. The technician reviews these points marked by the computer, submits the characteristic points to a one-to-many search, and the computer gives the results with a percentage of matching characteristic points. Then, a latent print examiner checks the possible matches to determine the most likely one.
The FBI-NCIC classification system and other techniques based on the HENRY SYSTEM assign numerical values to the general patterns of a complete set of ten prints. This allows coding and filing millions of prints in an orderly manner. Tens of millions of prints are filed. In addition, police departments often maintain a file of UNIDENTIFIED PRINTS from open or unresolved cases.
If matches are found at a later crime scene, it proves the same person was involved in both cases. Moreover, as suspects are arrested and fingerprinted, their prints are compared with those in the file. The most common prints are plastic, which are impressions left in soft material like wax, paint, or putty; visible, made by blood, dirt, ink, or grease; and latent, which are usually invisible and must be revealed before they can be seen and photographed.
Printed Evidence is Fragile
It is possible to obtain prints from snow or mud, and small objects containing traces of evidence also often contain fingerprints. Cars are a frequent source of prints. The most common places are the door, trunk, hood handles, exterior mirrors, license plates, trunk unlock, emergency brake unlock, seat adjustment levers, seatbelt buckle, and rearview mirror. Prints are difficult to remove from carpets and furniture.
When photographing fingerprint evidence, a complete record of all technical data about the camera, lens, film, shutter speed, lens aperture, lighting, camera position, distance to the object, and angle is also kept. This protects the police department from accusations that it is the photograph that makes it appear to be a match. Additionally, for the sake of good public relations, household furniture is usually protected with a cloth while the police search for prints. With the photos, three different exposures are usually taken: a normal exposure, an underexposure, and an overexposure. The most popular development technique is DUSTING.
The Operating Principle of Powder is Simple
The fingers of most people carry a layer of sweat and oil. When fingers come into contact with any relatively smooth surface, friction releases the oil from between the ridges. For this reason, fingerprinting is sometimes called friction ridge pattern analysis. When powder is applied to the surface, it adheres to the oil and brings out the pattern. Powder is ideal on wood, metal, glass, plastics, formica, and tiles. It is not so much on paper, cardboard, and leather. Powders vary in color, stickiness, and photographic and magnetic qualities.
The most common colors are black, white, gray, aluminum, red, and gold. The best color is the one that contrasts with the surface color. For example, a white or gray powder works best on a dark surface, and a black powder works best on a whiter surface. In multicolor situations (like a magazine cover or a pack of cigarettes), it is best to use a FLUORESCENT powder. When the dusted object is exposed to ultraviolet light, the powder will glow, making the impression appear regardless of the background color.
Powder is never poured directly from a jar. Instead, a little is poured onto a piece of paper and used as a palette. The tip of the brush is dipped into the powder and gently tapped to remove the excess. Brushing is done lightly, with short, fairly quick, and even strokes. An expert will try to follow the general direction of the ridges. The next step is called lifting the print. Lifting involves using some adhesive material to remove the powdered impression from the surface.
The three most common lifting materials are hinge lifters, rubber lifters, and cellophane tape. If using adhesive tape, a clear, high-quality transparent tape is best, not a shiny or opaque magic tape. The tape is slowly unrolled and bent a little to use as a tab for manipulation. It is important that the handler does not smudge the tape with their own fingerprints. The handler will pull the roll of tape so that the rest of the exposed tape remains slightly taut and will cover the print area about two inches beyond in the other direction.
They will take care to ensure no air bubbles are left under the tape. Bubbles destroy the value of the print. The tape is gently rubbed over the print. Once the tape is well placed, the print is lifted by pulling the roll smoothly and evenly away from the surface. Then, the tape is quickly applied to a card or piece of paper. Next, the excess tape will be cut.
