Trace Evidence Trace evidence results from the transfer of material from one place to another. Examples include: fibers glass fragments paint hair
Trace Evidence Locard s principle: Every contact leaves a trace. When a crime is committed, material will be mutually exchanged between the perpetrator and the crime scene. It is up to the investigator to identify materials that are seemingly foreign to the location.
Trace Evidence The amount of trace evidence transferred depends on the nature and duration of contact, as well as the type of contacting surfaces. Trace evidence transfer is more likely to be found in brutal crimes occurring over a long period of time than in the case of less forceful encounters.
Trace Evidence Investigators must use caution when entering a crime scene to avoid: introducing new trace evidence destroying existing evidence As time passes after a crime more evidence is lost. Elimination standards are often collected from crime scene personnel to exclude them as the source of evidence.
Hair and Fiber Trace Evidence The two most common types of trace evidence are hair and fibers. The type, condition, and number of hairs found at a scene all contribute to their value in an investigation.
Hair Evidence Hairs are comprised of the protein keratin and grow outward from follicles in the skin of mammals. Hair undergoes two life stages: anagen phase: active growth telogen phase: dormant, easily shed Most of the hair collected at a crime scene is in the telogen phase.
Hair Evidence Anagen and telogen hairs can be distinguished by examining the root sheath. Anagen hairs show damage and stretching of the root area due to the force required to remove them from the follicle. Telogen hairs have clubshaped roots.
Structure of Hair The root sheath is the base from which the hair shaft grows. DNA can be obtained from the hair root. Hair shafts contain 3 layers: medulla (inner) cortex (middle) cuticle (outer)
Structure of Hair The medulla is the central core of the hair shaft. Pattern defined as: Absent Continuous Discontinuous
A human medulla is poorly defined, broken, or even absent. Hair Evience Animal medullas are well-defined and continuous. The medullary index can be used to determine if the hair is of human or animal origin.
Structure of Hair Human hairs have a medullary index of about 0.3 Animals have a medullary index of 0.5 or greater.
Structure of Hair The cortex layer of the shaft is the main body of the hair. It is made from elongated spindle-shaped cells. Additional identifiers in the cortex are: pigment granules (small, dark, solid structures) ovoid bodies (solid, sphericle structures) irregular air spaces
The cuticle is the outer layer scaly and translucent. Structure of Hair The scale pattern can be used to determine the species of mammal that shed the hair.
The three scale patterns are: coronal (crown-like) spinous (petal-like) imbricate (flattened) Structure of Hair Human hairs are comprised almost entirely of the imbricate scale pattern. Hair with strong coronal or spinous patterns is not likely human.
In humans and animals the hair s structure is influenced by where it grows on the body. The primary types of human hair used in forensic investigations come from either the scalp or pubic regions. Determining Hair Location
Scalp hairs are longer with a moderate shaft diameter and broken up medulla. Pubic hairs are more coarse and wiry. They have broad, continuous medullas throughout. Determining Hair Location
Determining Hair Location The structure of hair depends on its function: Guard hairs form an outer coat, providing protection Inner fur or wool hairs provide insulation Tactile hairs or whiskers are sensory devices
Determining Hair Location Scalp hairs show characteristics from grooming: coloring bleaching split or cut ends Human hair grows at about ½ inch per month, giving an investigator the ability to measure the duration between a crime and an event such as coloring the hair.
Determining Hair Ethnicity Current FBI standards suggest human hairs can be classified into 3 broad categories: Caucasoid (of European origin) Negroid (of African origin) Mongoloid (of Asian or Native American origin) Note that this designation is a purely microscopic analysis. Suspects may or may not identify with these racial groups.
Determining Hair Ethnicity Caucasoid class hairs have: moderate shaft diameters (80µm) with minimal variation pigment granules at even intervals either loosely or densely packed oval cross-sections
Negroid class hairs have: Determining Hair Ethnicity a wide range of shaft diameters prominent twisting and curling shafts pigment granules that are densely distributed throughout (opaque under microscope) flattened cross-sections
Determining Hair Ethnicity Mongoloid class hairs have: large shaft diameters with little variation broad medullas thick cuticles densely distributed pigment granules in patchy areas or streaks round cross-sections
Analysis Conclusions An examiner can form one of 3 conclusions from hair comparisons: 1. Hairs from the known source have the same characteristics as the evidence. 2. Hairs from the evidence are different from the known source. 3. Hairs from the evidence have some characteristics that match and some that do not match the known source, therefore no conclusion can be drawn as to whether the samples are from the same source.
Identify Scale Patterns A cast of the scale pattern on a hair is made and examined under a microscope at 40 to 400X magnification. If the scale pattern is difficult to observe, a cast may be made. A thin coat of nail polish is painted on a microscope slide. The hair is placed on the slide and allowed to dry. The hair is removed and the pattern remains in the cured nail polish.
Fibers can be from natural or synthetic sources. Fibers Fibers may support or refute statements from both witnesses and suspects. Fibers found on the accused present a compelling argument that the individual was at the crime scene.
Fiber evidence properties: color variation, staining, color additives material thickness length degree of twist location number of matching fibers found Fibers
Natural Fibers Wool and cotton are so common that the twist, color, and weave are necessary to match them. Less common natural fibers: flax jute hemp alpaca camel cashmere mohair
Synthetic Fibers polyester nylon acrylic rayon acetates
Fibers tend to exhibit different cross-sections depending on their use. Fiber Cross-Sections Carpet is usually tri-lobal.
Matching Fibers Fibers are very common, therefore trace evidence analysts will use the phrase consistent with rather than matching when comparing a known and unknown fiber sample. There are no standards for two samples being consistent with each other. It is a subjective judgment call from the trace evidence analyst.
Matching Fibers Finding multiple types of fibers on the suspect and scene increases the likelihood that this did not occur by chance alone. Cross-transfer of fibers (the suspect both leaves and takes fibers from the scene) provides more compelling evidence in a case.