The Federal Bureau of Investigation identifies fiber as the smallest unit of a textile material that has a length many times greater than its diameter. Fibers can be natural (derived from plants and animals) or synthetic (man-made). Plant-based fibers include cotton, flax, jute, and hemp. The most common animal fiber is sheep wool, but this category could include fibers from other animals, including mohair, cashmere, alpaca, and camel. More than half the fibers used in the production of textiles are synthetic, and include nylon, rayon, and polyester. Identifying rare or unusual fibers at a crime scene has increased in significance, as it may place a suspect at the scene of the crime.
Fibers are gathered from a crime scene using tweezers, tape, or a vacuum. The most common sources of transferred fibers are clothing, drapes, rugs, furniture, and blankets. Once fibers are collected, they are brought to a lab and then placed under a microscope, where they are compared against fibers from a suspected source.
Several factors are taken into account when analyzing fiber evidence. These are the type of fiber, the number of fibers present, the color of the fiber, how the fiber was transferred, and the location of the fiber in the crime scene. Each category is carefully considered, as any evidence is potentially incriminating. By matching fibers at the scene of a crime to items belonging to a suspect, investigators are able to place individuals at a crime scene.
The problem with fiber evidence (particularly when discussing clothing) is that cloth items are generally mass produced. The only way determine that a fiber came from a particular piece of cloth is to know without a doubt that the item is unique, or to actually witness the transfer of the fibers. That said, the likelihood that a garment with the exact same color, texture, weave, and material would be at a crime scene is astronomically low. A case cannot be solved with fiber evidence alone, but when coupled with additional evidence, fiber is influential in solving a case.
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