Criminals often leave a trail of fiber behind them. Carpet fibers might remain on a suspect’s shirt after he assaults a victim or a thief might shed fibers from his clothing on a window sill where he enters a home. If correctly collected and analyzed, fibers can be strong evidence linking a criminal to a crime site.
Defining and Studying Fiber
When collected at the scene of a crime, fibers are called trace evidence. Other types of trace evidence include hair, glass, blood flecks and grass. Investigators determine whether the fiber comes from a plant, animal or synthetic process. They study the fiber to determine its color, shape, type and length, and try to match it to a possible source.
Preserving the Fibers
Investigators at a crime scene must carefully preserve evidence to prevent loss of fiber evidence. Typically, investigators store individual fibers in a plastic pillbox or similar container. Investigators also note the number of fibers transferred. Violent physical contact will produce more fiber transfers than a casual encounter, which might produce none. The persistence of fibers depends on how the material is treated. For example, fibers transferred from the clothing of a suspect to that of a rape victim could be lost if the article of clothing is washed or brushed.
Interpreting the Fibers
Just because fibers are found on the victim or suspect doesn’t necessarily mean they are meaningful clues. An investigator needs to match those fibers to something else. For example, he may want to match carpet fibers found on a suspect’s clothing with carpet fibers from inside the victim’s home. Even if such a match is made, the match doesn’t necessarily solve the case. Unlike fingerprints or bullet tracings, fibers aren’t unique. The fibers from a pair of blue jeans are pretty much the same as fibers from other blue jeans made by the same manufacturer. The evidentiary value of the fiber is greater if the fiber type is relatively uncommon or many fibers have been transferred. The presence of many fibers -- instead of a few -- suggests more meaningful contact.
Fibers collected at crime scenes sometimes yield something even more important than the fiber itself. Some fibers contain flaked off cells from body tissues, such as skin. The DNA found in these cells can be used to identify suspects and rule out others. In such instances, fibers are a vitally important forensic tool.