Sometimes the smallest piece of evidence found at a crime scene can be the thing that provides a major clue or becomes a major factor in winning a conviction. Hairs found on a victim routinely point to a suspect and because DNA analysis can often be done, this provides a definitive identification of the perpetrator. Likewise, fibers obtained from victims clothes or in the area of the crime scene can guide investigators in the direction of a particular suspect. Fiber analysis by the forensic laboratory has become increasing important in many cases. A caution, however, is that fiber evidence unlike DNA is not unique. It can not definitively identify a suspect but it can serve to narrow the field of potential suspects.
Color Me Busted -Sometimes fiber analysis can prove to be very strong evidence in a case. This was particularly true in the case of the abduction and murder of Kristen Harrison in Ohio in 1982. The young girl was raped, murdered and left in a field about thirty miles from her home. The forensic investigators on the scene found orange fibers in her hair. These fibers were polyester and had a distinctive shape; a shape similar to fibers used in carpet. These fibers were relatively unique because of their color and shape. They were also similar to orange fibers found on a twelve year old girl found murdered eight months prior in the same general area.
Initially these orange fibers did not lead investigators anywhere. A few months later a young woman was abducted by a man and held hostage in his home where he tortured her and indicated that he planned to kill her. She was able to escape and went to the police. On investigation it was observed that he drove a van similar to the one a witness had reported Kristen Harrison had been forced into. The van had unique orange carpeting in the back. On analysis the fibers matched those found on Harrison’s hair. The color of this fiber was so unique that the lab was able to identify the dye used and trace it to a specific carpet manufacturer. Less than seventy-five yards of this carpeting had been shipped to this particular area of Ohio. This greatly limited the number of potential suspects. Eventually, in conjunction with other evidence that provided a strong link to the owner of this van he was convicted for the murder of the eleven year old girl.
Fiber Type and Transfer - While fibers are important pieces of trace evidence, we can’t say that a specific fiber came from a specific perpetrator or location. As such, fiber evidence is considered circumstantial evidence and strengthens any additional circumstantial or solid evidence against a suspect. When two objects come in contact during the commission of a crime there is transfer of material from one individual to another. Usually small fabric fibers are transferred from the perpetrator to the victim. This is direct or primary transfer. If a fiber is transferred to a victim from the rug in a car trunk or from a blanket this is considered indirect or secondary transfer. It is also possible for fiber materials to be transferred from the victim to the suspect clothing but this is a less frequent source of evidence.
At the crime scene fibers are collected with tweezers or a gloved hand and placed in small plastic evidence collection bags. The bags are labeled appropriately and sent to the lab for analysis. Fibers found on the victims clothes or hair are removed and sent for analysis in a similar manner. Fibers may be classified as Natural which includes, cotton, wool, lama, alpaca, or other animals. Natural fibers can also come from plant materials like flax, hemp or kapok among others. A third class of fibers is man-made fibers like nylon, polyester, rayon and acrylics.
Fiber Properties – Fibers, whether from a rug, towel, or clothing, all have certain physical characteristics. These include: texture (smooth or rough), color, crimp (straight or wavy), pliability (pliable if bends, non-pliable if breaks), and resilience (does it spring back when crushed or not). A number of properties can be determined by placing fibers in or near a flame. Does the material burn or melt? Does the flame turn color? Does the material smoke on burning? If so, what does it smell like? However, in most abduction, rape or murder cases the amount of fiber material found is very small. In these cases destructive testing like flame analysis is not possible because of the limited material.
The number of fibers of a specific type found on the victim is important and is determined by the CSI investigator. The larger the number of fibers present the greater the degree of contact between the victim and the attacker. Where fibers are found on the victim’s clothes or on the victim’s body is also very important. This is especially true in cases of abduction, rape or murder.
Microscopic Analysis -Fiber evidence is viewed microscopically under a number of types of microscopes. In addition to regular light or compound microscope, the fibers are compared under a phase contrast microscope which allows some of the internal structure of the fiber to be revealed. Colored fibers can be viewed by fluorescence microscopy to distinguish dyes with fluorescent properties. Tying this to a florescence spectrometer allows the absorption (Color) profile of the dye to be developed. Matching these profiles to known dyes can identify the dyes used and in some cases the specific dye lot number of a batch of fiber. Viewing the manner in which dye is adsorbed and applied along the length of a fiber may also provide a lead to the manner in which the fibers were dyed. Viewing a cross section of man made fibers can, for some fibers, identify the manufacturer of the fiber. If sufficient fiber material is present some may be extracted and detailed chemical analysis by gas chromatography or other analytical methods performed to determine dyes, chemical treatments and even metals present in the fiber material. Comparison of this to the fiber composition from a suspect can provide strong supporting evidence of the fibers being from the same source.
Fiber data is developed by comparing the shape, structure and size, dye content, chemical composition, and physical and microscopic appearance of two fibers. However, in the final analysis just because two identical fibers can be matched it does not prove that the fibers are from the same location nor does it prove that the fibers were transferred to the victim at the same period in time.
