Fundamentals of Criminal Investigations
Adam J. McKee, Ph.D.
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Section 4.1: Fiber Evidence
Placing a suspect at the scene of a crime is an important element in criminal investigation. This can be achieved through the location of textile fibers similar to those from the victim’s clothing or the crime scene on the clothing of the suspect, or through the discovery of fibers like those in the suspect’s clothing at the crime scene.
Textile fibers can be exchanged between two individuals, between an individual and an object, and between two objects. When fibers are matched with a specific source (fabric from the victim, suspect, and/or scene), a value is placed on that association. This value is dependent on many factors, including the type of fiber found, the color or variation of color in the fiber, the number of fibers found, the location of fibers at the crime scene or on the victim, and the number of different fibers at the crime scene or on the victim that match the clothing of the suspect.
Whether a fiber is transferred and detected is dependent on the nature and duration of contact between the suspect and the victim or crime scene, the persistence of fibers after the transfer, and the type(s) of fabric involved in contact.
A fiber is the smallest unit of a textile material that has a length many times greater than its diameter. Fibers can occur naturally as plant and animal fibers, but they can also be man-made. A fiber can be spun with other fibers to form a yarn that can be woven or knitted to form a fabric. The type and length of fiber used, the type of spinning method, and the type of fabric construction all affect the transfer of fibers and the significance of fiber associations. This becomes very important when there is a possibility of fiber transfer between a suspect and a victim during the commission of a crime.
As discussed previously, fibers are considered a form of trace evidence that can be transferred from the clothing of a suspect to the clothing of a victim during the commission of a crime. Fibers can also transfer from a fabric source such as a carpet, bed, or furniture at a crime scene. These transfers can either be direct (primary) or indirect (secondary). A primary transfer occurs when a fiber is transferred from a fabric directly onto a victim’s clothing, whereas a secondary transfer occurs when already transferred fibers on the clothing of a suspect transfer to the clothing of a victim. An understanding of the mechanics of primary and secondary transfer is important when reconstructing the events of a crime.
When two people come in contact or when contact occurs with an item from the crime scene, the possibility exists that a fiber transfer will take place. This does not mean that a fiber transfer will always take place. Certain types of fabric do not shed well (donor garments), and some fabrics do not hold fibers well (recipient garments). The construction and fiber composition of the fabric, the duration and force of contact, and the condition of the garment with regard to damage are important considerations.
An important consideration is the length of time between the actual physical contact and the collection of clothing items from the suspect or victim. If the victim is immobile, very little fiber loss will take place, whereas the suspect’s clothing will lose transferred fibers quickly. The likelihood of finding transferred fibers on the clothing of the suspect a day after the alleged contact may be remote, depending on the subsequent use or handling of that clothing.
Many different natural fibers originating from plants and animals are used in the production of fabric. Cotton fibers are the plant fibers most commonly used in textile materials, with the type of cotton, fiber length, and degree of twist contributing to the diversity of these fibers. Processing techniques and color applications also influence the value of cotton fiber identifications.
Other plant fibers used in the production of textile materials include flax (linen), ramie, sisal, jute, hemp, kapok, and coir. The identification of less common plant fibers at a crime scene or on the clothing of a suspect or victim would have increased significance.
The animal fiber most frequently used in the production of textile materials is wool, and the most common wool fibers originate from sheep. The end use of sheep’s wool often dictates the fineness or coarseness of woolen fibers: Finer woolen fibers are used in the production of clothing, whereas coarser fibers are found in carpet. Fiber diameter and degree of scale protrusion of the fibers are other important characteristics.
Although sheep’s wool is most common, woolen fibers from other animals may also be found. These include camel, alpaca, cashmere, mohair, and others. The identification of less common animal fibers at a crime scene or on the clothing of a suspect or victim would have increased significance.
Man Made Fibers
More than half of all fibers used in the production of textile materials are man-made. Some man-made fibers originate from natural materials such as cotton or wood; others originate from synthetic materials. Polyester and nylon fibers are the most commonly encountered man-made fibers, followed by acrylics, rayons, and acetates. There are also many other less common man-made fibers. The amount of production of a particular man-made fiber and its end use influence the degree of rarity of a given fiber.
The shape of a man-made fiber can determine the value placed on that fiber. The cross-section of a man-made fiber can be manufacturer-specific: Some cross sections are more common than others, and some shapes may only be produced for a short period of time. Unusual cross sections encountered through examination can add increased significance to a fiber association.
Color influences the value given to a particular fiber identification. Often several dyes are used to give a fiber a desired color. Individual fibers can be colored prior to being spun into yarns. Yarns can be dyed, and fabrics made from them can be dyed. Color can also be applied to the surface of fabric, as found in printed fabrics. How color is applied and absorbed along the length of the fiber are important comparison characteristics. Color-fading and discoloration can also lend increased value to a fiber association.
Fiber Number and Location
The number of fibers on the clothing of a victim identified as matching the clothing of a suspect is important in determining actual contact. The greater the number of fibers, the more likely that contact actually occurred between these individuals.
Where fibers are found also affects the value placed on a particular fiber association. The location of fibers on different areas of the body or on specific items at the crime scene influences the significance of the fiber association.
How a fabric is constructed affects the number and types of fibers that may be transferred during contact. Tightly woven or knitted fabrics shed less often than loosely knit or woven fabrics; fabrics composed of filament yarns shed less than fabrics composed of spun yarns. Certain types of fibers also tend to transfer more readily.
The age of a fabric also affects the degree of fiber transfers. Some newer fabrics may shed more readily because of an abundance of loosely adhering fibers on the surface of the fabric. Some worn fabrics may have damaged areas that easily shed fibers. Damage to a fabric caused during physical contact greatly increases the likelihood of fiber transfer.
When a questioned fiber is compared to fibers from a known fabric source, a determination is made as to whether this fiber could have originated from the known fabric. It is not possible to say positively that a fiber originated from a particular fabric, although the inability to positively associate a fiber with a source in no way diminishes the significance of a fiber association. The wide variety of fiber types, fiber colors, and fabric types can make fiber associations very significant because the value of a fiber association depends on the type of fiber, the color of the fiber, the number of fibers transferred, the location of the recovered fibers, and other factors.
It could be very helpful to know the frequency of occurrence of a particular fabric and fiber, or how many fabrics with a particular fiber type and color exist, as well as who owns them. Such information, however, is extremely difficult to obtain. If the manufacturer of a fabric is known, the possibility exists that the number of fabric units produced could also be obtained, but this information is not always available. How many garments like this still exist, and where they are located, are still in question.
Once a particular fiber of a certain type, shape, and color is produced and becomes part of a fabric, it occupies an extremely small portion of the fiber/fabric population. Exceptions to this would be white cotton fibers and blue cotton fibers like those comprising blue jeans. There are other fibers that are common, but the majority of fibers of a particular type and color constitute a very small percentage of the total number of fibers that exist in the world.
References and Further Reading
Modification History File Created: 05/02/2019 Last Modified: 05/16/2019
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