Roger Reece Kibbe: The I-5 Strangler
Soon a trace-evidence specialist, Faye Springer, got involved, and with her painstaking examination she found blue trilobal carpet fibers from a car, the same color and type as the fiber in one of Kibbe's cars. They had even been dyed with the same chemicals. This type of evidence is known as "class" evidence, or just one of many specimens of that class, and thus does not definitely identify someone, it contributes to the totality of circumstances needed to persuade a jury beyond a reasonable doubt. The more precise the analysis, the more convincing, which involves both finely-honed technical expertise and high-powered microscopic analysis. This case received both.
Trace evidence analysis relies on Locard's Exchange Principle: "Objects or surfaces which come into contact always exchange trace evidence. Every contact leaves a trace." In other words, when someone commits a crime, that person leaves something behind at the crime scene and also takes something away. Cross transfers of fiber often occur in cases in which there is person-to-person contact, and investigators always hope that fiber traceable back to the offender will be found at the crime scene, and that fiber or trace evidence from the scene will be found on the offender as well. Success in solving the crime often hinges on the ability to identify the source of trace fibers on either the victim or suspect.
Unlike fingerprints or DNA, however, fibers cannot pinpoint an offender in any definitive manner. There must be other factors involved, such as something unique to the fibers, that sets them apart. Fibers are gathered at a crime scene with tweezers, adhesive lift tape, or an evidence vacuum. They generally come from clothing, drapery, wigs, carpeting, furniture, and blankets. For analysis, they are first determined to be natural, manufactured, or a mix of both.
Natural fibers come from plants (cotton) or animals (wool). Manufactured fibers are synthetics like rayon, acetate, and polyester, which are made from long chains of molecules called polymers. Generally, the analyst gets only a limited number of fibers to work with—sometimes only one. Whatever has been gathered from the crime scene is then compared against fibers from a suspect source, such as a car or home, and the samples are laid side by side for microscopic inspection.
A compound microscope uses light reflected from the surface of a fiber and magnified through a series of lenses, while the comparison microscope (two compound microscopes joined by an optical bridge) is used for more precise identification. A phase-contrast microscope can reveal the structure of a fiber, while various electron microscopes either pass electron beams through samples to provide a highly magnified image or reflect electrons off the sample's surface. A scanning electron microscope converts the emitted electrons into a photographic image for courtroom display. It also provides the elemental composition.
Another useful instrument is the spectrometer, which separates light into component wavelengths. In 1859, two German scientists discovered that the spectrum of every organic specimen is unique determined by its constituent parts. By passing light through a sample to produce a spectrum, the analyst can read the resulting lines, called "absorption lines." That is, the specific wavelengths that are selectively absorbed into the substance are characteristic of its component molecules. Then a spectrophotometer measures the light intensities, which yields a way to identify different types of substances.
A combination of these instruments for the most effective forensic analysis is the microspectrophotometer. The microscope locates minute traces or shows how light interacts with the material under analysis. Linking this instrument to a computerized spectrophotometer increases the accuracy. The scientist can get both a magnified visual and an infrared pattern, which increases the number of identifying characteristics of any given material.
The first step in fiber analysis is to compare color and diameter. If there is agreement, then the analysis can go to another stage. Dyes can also be further analyzed with chromatography, which uses solvents to separate chemical constituents. Under a microscope, the analyst looks for lengthwise striations or pits on a fiber's surface, or unusual shapes—-as with the one short and two long arms of the trilobal fibers in the Kibbe investigation.
Yet closer inspection of the parachute cord also revealed something interesting.