Trace Evidence

Locard’s Exchange Principle Edmond Locard developed the first police crime lab in Lyons, France (1910). He also developed Locard’s Principle of Exchange which states: When a criminal comes in contact with an object or person, a cross-transfer of evidence occurs. Every area of forensics science uses this fundamental principle. Linking suspects to victims.

Trace Evidence Trace evidence is a term for small, often microscopic material. So small, criminals are unaware there has been cross-transfer. Wayne Williams was convicted of murdering two adult men in 1982, because of fibers from his house were found on the victims.

Trace Evidence Trace evidence includes an endless variety of material, however most often the following is collected: hair fiber glass soil paint

Trace Evidence Hair What is hair? Hair is keratinized cells which grow out a hair follicle. It continues into a shaft and terminates at the tip end. The shaft is composed of three layers: the cuticle the cortex and the medulla.

Trace Evidence The cuticle is outermost portion of the hair shaft. It is formed by overlapping scales that always point to the tip. Human Hair

Trace Evidence 75% to 90% of hair is made up of the cortex. It is the center portion of the hair shaft which contains the pigment that make hair black, brown, blond, or red.

Trace Evidence The medulla is a row of cells like a canal running along the center of the cortex. There are three types of medulla in humans: Fragmentary (b) Interrupted (c) Continuous

Trace Evidence Is the hair human or animal hair? Investigators will look at all three characteristics of hair to determine if the hair in question is human or animal. The cuticle pattern can tell what species the hair came from. Imbricate (human) Coronal (mouse) Spinous (cat) ? cat

Trace Evidence Hair Shape There are several different possible hair shapes. The cross section of the hair shaft determines these shapes. A round cross section makes hair straight. An oval cross section makes hair curly. A crescent moon shape makes hair kinky.

Trace Evidence Race and Hair It is risky to assign racial characteristics to hair evidence, however, here are some general guidelines: Asians and American Indians have round cross sections. American and European Caucasians and people of Middle Eastern background show oval cross sections. People African heritage have crescent moon shaped cross section.

Trace Evidence Hair as Evidence Hair is what type of evidence? Class or Individual Class Unless there are cells attached to the hair (follicle) DNA cannot be extracted from the hair shaft, Yet!

Trace Evidence Glass Glass is considered class evidence unless: Fragments found at the crime scene can be matched or pieced together with pieces found with the suspect. Example: pieces of a car head light involved in an accident fit into head light of car. Matching physical properties (density, index of refraction) of glass in question.

Trace Evidence Glass Characteristics Density The density of glass can be determined by flotation. A control glass particle is immersed and suspended in a liquid. The glass chips to be tested are added and compared.

Trace Evidence Index of Refraction The immersion method is used to determine the index of refraction for glass. We can then compare the values between glass fragments.

Trace Evidence Glass Fracture Patterns Fractured glass can reveal information related to force and direction of impact. At high velocities, a projectile leaves a round, crater-shaped hole, wider on the exit side.

Trace Evidence At low velocities, a projectile leaves an irregular shaped hole, and the glass will most likely shatter.

Trace Evidence Bullet Holes When a bullet enters glass it leaves tell tale signs of Radial and Concentric Fractures. Radial fractures appear as straight lines coming from the center of the bullet hole. Concentric fractures appear as circles around the center of the bullet hole.

Trace Evidence Radial and Concentric Fractures tell us which hole formed first. All fractures will propagate through the glass and terminate when it encounters an existing fracture. So, which hole was made first? Answer: B A B

Trace Evidence Bullet Holes Bullet holes Video and Stock Footage Clips. 16 Bullet holes videos and royalty free movies available to buy from over 20 stock motion brands.

Trace Evidence Soil Because no two places on earth (literally) have precisely the same soil, soil from one area will be identifiably different from soil collected in another location. Soil consists of naturally occurring rocks, minerals, organic matter (rotting leaves, twigs, etc.), and includes asphalt, brick, and other man made materials. Soil is class evidence unless an unusual combination of rare particles is under consideration.

Trace Evidence Soil as Evidence Similar to glass, soil density is used to compare soil specimens. Soil is added to a solution which varies in density itself, different soil particles will be suspended at different portion of the solution.

Trace Evidence Paint Paint consists of a solid portion, pigment (color), and a liquid portion know as the solvent. For all intensive purposes paint exhibits class characteristics. In some cases car makes and models can be determined with paint left at crime scenes.

Trace Evidence Paint Layering of paints or piecing together fragments can make evidence more individual.