Hanging by a Thread

By Linda Maria Frank
Hairs and fibers are everywhere. In forensic science, they are considered trace evidence. But how significant is a hair or a fiber found at a crime scene?
One of the basic concepts considered in the investigation of a criminal act is Locard’s Principle, which states that when a crime is committed there is a transfer of material from the perpetrator to the victim and the scene. The crime scene technicians use Locard’s principal as a rationale for what they collect as evidence. Of course, they must consider what the capabilities of the lab are for prizing information from objects found at “the scene”.
There are two kinds of evidence, class and individual. Class evidence belongs to a group in a population. For example, Type O blood belongs to almost 50% of the population. Individual evidence can be traced to only one person or source. Fingerprints are individual evidence. Even identical twins’ fingerprints can be differentiated due to accidental marks made in uteri, or after birth.

Hairs are considered class evidence, unless you are lucky enough to have hairs with roots attached. This will provide the forensic scientist with DNA. When hairs are found at the crime scene, there must be control hairs to compare the evidentiary hair to. This means that if someone is murdered in their bed, we expect to find hairs there belonging to the victim, family members, and frequent visitors. Hairs will be collected from these folks and a comparison will be made. If there is an inconsistency in the hair evidence, it is hoped that suspect will be found who is a match. Since hair is class evidence, the microscopic comparison of the hairs must have no inconsistency. Obvious inconsistencies would be characteristics based on race, color patterns, and environmental factors affecting the hair. Even one inconsistency eliminates the suspect as the source of the hair.

Fibers are a more interesting, and sometimes incredibly helpful item of trace evidence. Natural fibers are not as good as synthetic fibers as evidence. Most cotton fibers are alike. Dye can be helpful, if it is an unusual color or has a distinct chemical composition. Synthetic fibers are a whole different story. The reason lies in the manufacturing. Synthetic fibers are made from a chemical goop that is cooked up from a petroleum by-product. How’s that for a scientific explanation? The goop is then forced through a spinneret. Think of a spider making spider silk. The silk is made in the spider’s special gland, and is forced out through holes in a structure on its abdomen called a spinneret. The key here is that each manufacturer of synthetic fibers, for example Dupont, have their own specially designed spinnerets, based on what they want that fiber to do, and on the patent they have for making that fiber. So, if you look at a cross-section (think of slicing a carrot and viewing the cut piece) with an electron microscope, the fiber will have a distinct structure. Based on the cross-section the technician can tell if it’s a rug fiber for a home, or a rug fiber for a car. This is truly handy if the clothing of a kidnapping victim, for example, contains rug fibers. If those fibers are from a car, he can tell the make of the car from a combination of the cross-sectional appearance and the dye used to color the fiber. Usually it will be a number of makes and models, but still very helpful. Especially if there is a suspect who has a car that falls into that range of makes and models.

The dyes used to color synthetic fibers are also great evidence, because they too, are chemically individual to the manufacturers.

Okay, you ask. When does this help catch the criminal?

Consider the Wayne Williams case, known as The Atlanta Child Murders. Wayne Williams was convicted of the murder of two of the victims of these murders. The total number of victims was thirty. The DA decided to try Williams for the murders of the two victims possessing the most compelling evidence. The victims were dumped in the Chattahoochee River, many times without clothing. After careful post-mortem examination, the victims were found to have something in common, stuck among their head hairs. They possessed hairs and fibers not consistent with their personal environments. The investigators found that most of them possessed a green rug fiber that was traced to a carpet manufactured by West Point Pepperell called “Luxaire”. Other fibers were violet synthetic fibers, blue rayon fibers, dog hairs, car rug fibers, among others.

Wayne Williams was stopped by a police stake-out immediately after one of the victims was tossed from a bridge that spanned the Chattahoochee River in Atlanta. This gave the police probable cause to search his home and car. All of the fibers matched some item in William’s environment. The hairs were consistent with the family dog. The car rug fibers matched his station wagon. The most important fiber, the green “Luxaire” fiber matched a rug in his home. The purple fiber matched his bedspread. The blue rayon fiber matched a throw that was found in the trunk of his car.

At the trial the prosecuting attorney had to prove that the chance of these “class evidence” fibers all showing up on the two victims, and not coming from a source other than Williams, was very small. By doing a statistical study of the amount of “Luxaire” carpet sold in the Atlanta area, the forensic scientist came up with a chance of 1 in 7,792. The probability for the carpet in William’s 1970 Chevy was 1 in 3,828. With statistical probability, the calculation is based on multiplying these fractions. What happens is the number on top stays at one, but the number on the bottom becomes larger and larger, making the chances of two victims have multiple fibers that were found in Wayne Williams environment, but not in the victims’, a very small one indeed.

There are countless cases where fibers transferred by Locard’s principle account for compelling evidence to convict the alleged criminal. I suggest you look up Wayne William’s trial. The Atlanta Child Murders was made into a movie by the same name. This case is a fine example of forensics at work.

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