FBI TRACE EVIDENCE AND DNA ANALYSIS
Who did it? Where’d they do it? Not to mention when why and how? These all are questions FBI investigators must try to answer during a criminal investigation. Sometimes the clues are obvious. But more often than not, solving a crime requires agents to take a closer look – a much closer look.
      Even the most cunning criminals typically leave behind evidence at the crime scene without realizing it. FBI agents have the tedious task of searching for this evidence by collecting even the most minute particles at a crime scene and studying them in a laboratory.
      The Trace Evidence Unit, one of many FBI units that helps with criminal investigations, identifies and compares specific types of trace particles found at a crime scene. These clues might include materials such as human hair, animal hair, textile fibers and fabric, rope, feathers and wood.

Hairs and Fibers
In the early 1900s, Dr. Edmond Locard proposed that whenever two objects come into contact, a transfer of material will occur. Today, that’s known as Locard’s Exchange Principle. Physical contact between a suspect and victim often results in the transfer of trace evidence, such as hair and fibers. This evidence can be used to associate objects, individuals or locations. For example, if a strand of hair that matches the suspect’s is found on a homicide victim, it could indicate that the two had contact with each other. The greater the number of matching hairs found, the more likely it is that contact occurred.
      By examining hairs under a microscope, experts can gain a great deal of information, including whether the hair came from a human or animal. Hair specimens also can reveal a person’s race, what part of the body it came from, whether it fell out naturally or was removed with force and whether the person had a disease or used certain drugs. Plus, experts can tell if the hair was dyed or bleached, whether it was cut with scissors or a razor and whether it was burned.
      To examine the characteristics of a hair sample, FBI personnel often use a compound microscope, which magnifies a sample up to four hundred times. To compare hairs, a comparison microscope can be used, which is simply two compound microscopes bridged together, allowing the examiner to see two hairs side by side at the same time.

Hair Characteristics
Experts look at a variety of different physical characteristics of hair, including the shaft diameter, pigment granules and cross-sectional shape. They also examine a hair’s cuticle, the translucent outer layer of the hair shaft consisting of scales, and the medulla, a central core of cells present in some hair types.
      It is very unusual to find two people with the same microscopic physical characteristics. By comparing a questioned hair (Q) found during a criminal investigation with a known hair (K), such as a hair from the suspect, investigators can determine whether the two hairs are a match. In other words, it’s highly likely that the two hairs came from the same source. But a physical match alone is not usually considered an absolute identification. Other than looking at a hair’s physical make-up, DNA testing of hair samples sometimes can provide additional information. DNA analysis will be discussed later in this article.

Fibers
Like hair, fibers also can suggest contact between a suspect and victim. A fiber is the smallest unit of a textile. Some fibers occur naturally and come from plant or animal sources, such as cotton or wool. But more than half the fibers currently used in the production of textiles are man-made, such as polyester and nylon. Fibers sometimes are transferred directly, such as, from one person’s clothing to another’s. Or, fibers can be transferred indirectly — when a fiber that has already been transferred is transferred again. For instance, a transfer is said to be indirect when a fiber from a bedspread clings to someone’s shirt and is then transferred to a second person’s clothing.
      Trace particles of fiber evidence differ in their value as evidence. In general, the more rare the fiber, the more significance it will hold in court. For example, plain white cotton fiber, such as T-shirt material, is typically not very useful as evidence because it is too common. If a fiber originating from a suspect matches a fiber found at a crime scene, experts must try to show that it is more than a coincidence. Fortunately for investigators, textiles today have more variety than ever.
      Different companies often use different processes to produce fabric. Therefore, analyzing the shape of a fiber’s cross section gives experts one way to compare various specimens. If the cross section is unusual, it can add value to the fiber as evidence. Many different dyeing processes are also used by textile producers to give fiber its color. Sometimes color is also added to the surface of fabric, such as printed fabrics. How a particular fiber is dyed and colored provides another important characteristic to examine.

Hair and Fiber Collection
At a crime scene, the utmost care is taken to make sure trace evidence is not lost. Clothing and other materials are carefully packaged in sealed paper bags and sent to a lab. For extraction, the evidence is taken into a processing room, which provides a clean, sealed environment that is temperature- and humidity-controlled
       Hair and fibers are removed through a combination of scraping, picking, lifting, vacuum sweeping, combing or clipping. Often, the evidence is hung on a rack and a clean sheet of paper is placed below. The paper is changed after each piece of evidence to avoid contamination. A technician then scrapes the item inside and out using a metal spatula. Any hairs, fibers or other debris that fall to the paper are collected and forwarded to the examiner. There, they are placed on glass microscopic slides for identification and comparison.

Paint Chips & Glass Fragments            
Hairs and fibers aren’t the only tiny pieces of the puzzle that can be used as evidence. Paint chips and glass fragments are two other examples. In an automobile accident, paint can be transferred from one vehicle to another. Or, it can be transferred from a vehicle to a person, or to another object. By examing the paint chip, experts can often determine a vehicle’s make and model. They do this by examining various physical characteristics of the paint, including color, layer sequence and thickness, surface and layer features, contaminants and weathering. They also look at chemical characteristics such as pigments, polymers and additives. Then, they compare their findings with a database of known paint colors and processes used by different manufacturers.
       Two different specimens of glass fragments also can be examined to determine if they came from the same source. This is useful not only in hit and run accidents, but home break-ins. When a suspect breaks a glass window or door, glass fragments tend to get on that person’s clothing and the tool used to break in. By comparing glass fragments from the clothing or tool and the glass at the crime scene, experts can determine if they have a common origin.

DNA Analysis
Looking at clues from a physical standpoint is just one way the FBI examines evidence. Analyzing DNA, deoxyribonucleic acid, from body fluids, stains and other biological tissues recovered from a crime scene can also help connect a specific person to a crime.
       Simply put, DNA is an extremely complex chemical that consists of four bases: Guanine, adenine, thymine and cytosine. These bases combine in various proportions
to form molecules called nucleotides. The nucleus of every human cell contains
over three billion nucleotides, linked in a chainlike sequence in a variety of
arrangements that are almost infinite. Each human has a different arrangement, except
for identical twins, that is.
       A small segment of the DNA chain can be isolated and printed on photographic paper. This “print out” of a person’s genetic code can then be used as evidence to connect a suspect to a crime, or maybe to prove his or her innocence.
       DNA is found in the nucleus of our cells, which often are carried by our body fluids. To collect liquid fluids from a crime scene, a cloth or cotton swab is used. For example, if liquid blood is being collected, a portion of it will be absorbed onto a clean cotton cloth or swab. A portion of the swab or cloth also will be left unstained to use as a control. After the blood has air-dried it is packed in a clean paper envelope with sealed corners. The exact same process is used to collect dried body fluids, only the cloth or swab is first moistened with distilled water. DNA evidence might also be obtained from other sources such as cigarette butts, chewing gum and envelopes and stamps that have been licked.
       Whether it’s a barely visible fiber, a single strand of hair, or a small chip of paint, traces of evidence are almost always left behind. And no matter how much a criminal tries to cover up a crime, the truth is usually there somewhere just waiting to be discovered.

GLOSSARY
Locard’s Exchange Principle — whenever two objects come into contact, a transfer of material will occur; proposed by Dr. Edmond Locard in 1910.
fiber — the smallest unit of a textile.
comparison microscope — two microscopes joined together, allowing an examiner to see two hairs side by side at the same time.
hair cuticle — the translucent outer layer of the hair shaft consisting of overlapping scales.
hair medulla — the central portion of hair, the core area.

SOURCES & SITES
Deedrick, Douglas W. “Hairs, Fibers, Crime & Evidence: Part 2: Fiber Evidence.” Forensic Science Communications. FBI. July 2000. www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric3.htm

Deedrick, Douglas W. and Koch, Sandra L. “Microscopy of Hair Part 1: A Practical Guide and Manual for Human Hairs. Forensic Science Communications. January 2004. FBI. www.fbi.gov/hq/lab/fsc/current/research/2004_01_research01b.htm

Deedrick, Douglas W. “Hairs, Fibers, Crime & Evidence: Part 3: Crime and Evidence.” Forensic Science Communications. FBI. July 2000. www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric4.htm

“DNA Examinations.” 2003. FBI. www.fbi.gov/hq/lab/handbook/intro6.htm

FBI Web site: www.fbi.gov

Fisher, David. Hard Evidence. Simon & Schuster. 1995.

“Forensic Paint Analysis and Comparison Guidelines.” Forensic Science Communications. July 1999. FBI. www.fbi.gov/hq/lab/fsc/backissu/july1999/painta.htm

“Trace Evidence Unit.” 2000. FBI. www.fbi.gov/hq/lab/org/teu.htm

“Trace Evidence Recovery Guidelines.” Forensic Science Communications. October 1999. FBI. www.fbi.gov/hq/lab/fsc/backissu/oct1999/trace.htm.

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