Wednesday, July 29, 2009

Trace: It's the Little Things

by Andrea Campbell

Remember Locard's Exchange Principle, that whenever two objects come into contact a transfer of material will occur? Trace evidence—fibers, fabric, pet hair, paint chips and a thousand other items and artifacts can help to place the accused at a crime scene. While matching a tiny piece of evidence with its source may not be enough to link a suspect to a crime, when the pieces start to pile up it becomes more than coincidental and begins to become “conclusive” evidence. The more relevant the fibers found, the more convincing the evidence.

It is natural then, that fabrics, which are millions of fibers woven together, may provide the most compelling evidence of all. Fabric matches are much like jig-saw puzzles, the pieces must be found and reconstructed with exactness and certitude.

Hair and fiber evidence is useful in:

1. Helping to establish the scope of the crime scene
2. Placing a perpetrator at a scene
3. Connecting a suspect with a weapon
4. Supporting witness statements
5. Connecting crime scene areas (abduction, vehicle used, dump site)

Sources of Fibers

Fibers are divided into classes. There are four major types: animal fibers such as wool, vegetable fibers like cotton, asbestos is the only mineral fiber, and manmade or synthetic fibers such as polyester. Synthetic fibers make up about 75 percent of all textile fibers—there are over 1,000 different types in the United States and, consequently, they are the most common fiber investigated in the crime lab.

Tiny Details

As with other types of evidence, trace evidence is kept separate in a file or box, documented with the date and time, the name of person collecting it, a description, a case number for filing, and a sketch or photograph of where it came from. The chain of custody—a list of every person who touched it—is maintained for each item, no matter how small. Since evidence in this category can be wee stuff, preventing contamination and loss is a must, and trace evidence is usually collected before other examinations.

Finding the objects begins with a general look-see, but different types of alternate light sources such as UV, laser, and high intensity lamps may be needed.

The Little Things

A single thread is a fiber made up of tiny filaments. When fibers rub together, they leave fragments of themselves on each other and everything around it. As evidence, we could call hairs and fibers evidentiary minutiae because some particles can best be seen through a microscope.

Where Fibers are Found

Hairs and fibers can be found in the most remarkable of places but generally come from clothing, drapery, wigs, carpeting, furniture and blankets. Because fibers are transferred in such large numbers, they are much more likely to be found at a crime scene and are more likely to have a common origin. For example, a wool thread caught on a window sill at a crime scene could be matched to a pulled thread on the sweater found at the suspect’s house. However, it is actually quite common to find them in the barrel of guns! They get in there through a phenomenon called "blowback". The blast of a gun produces a vacuum in the wake of a wave of high-velocity gases. This negative phase creates a vacuum, which sucks up things in close proximity such as hair or fibers.

In 1982, a girl named Kristen Lea Harrison was abducted from a ball field in Ohio and her body was found six days later thirty miles away. She had been raped and strangled. Investigators found orange fibers in her hair that looked similar to those found on a twelve-year old murder victim eight months earlier in the same county. Some time later, a 28-year old woman was abducted and held prisoner in a man’s home. He tortured her and she was afraid for her life. After he left, she escaped and reported him. Police found a van with orange carpeting and scientists traced it back to Robert Anthony Buell. Other evidence helped to establish a more solid link and Buell was eventually convicted.

Fiber and Hair Collection

Fibers are collected in several different ways.

Picking. If visible to the eye, items can be picked up with tweezers or a section can be cut out and put into paper packets or petri dishes.

Lifting. Sometimes trace evidence is collected by taking wide cellophane tape and rolling it over the suspected area. The collected lifts are typically placed on a transparent backing such as clear plastic sheeting, easy for viewing under a microscope.

Vacuuming. Often, car seats, floorboards, and trunk space are vacuumed for fiber particles when there is a suspected abduction or when probable cause indicates the transportation of a murder victim or some illegal product. (*Note: some jurisdictions no longer use a vacuum.)

Scraping. Trace evidence scraped from clothing onto clean paper with a spatula or similar tool is used to dislodge certain items. This technique is most often conducted within the laboratory in a controlled environment that reduces the risk of contamination.

Combing. A clean comb or brush is used to recover trace evidence from the hair of an individual. The combing device and collected debris from the hair should be packaged together.

Clipping. Trace evidence can be recovered from fingernails by nail clipping, scraping or both.
Commonly, fingernails from the right and left hands are packaged separately.

Pieces of fabric found at the scene can be examined in a manner similar to fibers to determine color, type of cloth and fiber, thread count, direction of fiber twist and dye. When fabrics are torn or cut apart, the ends can be matched physically to another piece. In hit-and-run cases, pieces of the victim’s clothing are often found on the grill, the car’s fender or door handle.

Analysis of Fibers

Experienced examiners must have a good eye for comparison. At times they look at literally hundreds to thousands of hairs and fibers under a comparison microscope and from these weed out the few matches. Special microscopes found in some labs can magnify particles found at a crime scene by as much as 200,000 times. Sophisticated machines and new procedures help scientists identify chemicals found in complex mixtures.

To begin, fibers are first determined to be natural, manufactured, or a mix of both. Then analysts compare shape, dye content, size chemical composition and microscopic appearances. A phase-contrast microscope reveals some of the structure of a fiber, while various electron microscopes either pass beams through samples to provide a highly magnified image, or reflect electrons off the sample’s surface.


As any fiber analyst knows, color in a product is produced by mixing dyes. There are few "pure" colors. In a chemical laboratory, the color blue is never purely blue. The different ways in which manufacturers create their colors can aid in fiber identification and comparison. An instrument called a microspectrophotometer is used to create a "spectral fingerprint" of the color. When different dye analysis shows two color fingerprints as being identical, it is very strong evidence for proving that two fibers found in different places are from the same source.


Wendy Roberts said...

Awesome post! I've already copied and pasted this info into a research file :)

cheryl said...

Andrea's posts are always excellent and informative. Little room for argument when it comes down to scientific evidence.

Andrea Campbell said...

Thank you Wendy and Cheryl, the check is in the mail (no, honestly, they are not on my payroll).