Trace Evidence

Locard’s Exchange Principle

•Edmond Locard developed the first police crime lab in Lyons, France (1910).

•He also developed Locard’s Principleof Exchange which states:

•When a criminal comes in contactwith an object or person, a cross-transferof evidence occurs.

•Every area of forensics science uses thisfundamental principle. Linking suspects tovictims.

Trace Evidence

• Trace evidence is a term for small, often microscopic material.

• So small, criminals are unaware there has been cross-transfer.

• Forensic crime labs maintain databases of the characteristics and manufacturing details of materials found at crime scenes for comparison.

•Wayne Williams was convicted ofmurdering two adult men in 1982,because fibers from his house were found on the victims.

Trace Evidence

• Trace evidence includes an endless variety of material, howevermost often the following is collected:

hair fiber glass

soil paint

Trace Evidence

FiberThe mass production of fabrics has caused them to possess class characteristics.

It is only under unusual circumstances that fibers will provide individual characteristics.

Trace Evidence

Types of FibersNatural Fibers – derived from animal or plant. Ex.

Wool, silk and cotton

Man-made Fibers – can be regenerated or synthetic.• Regenerated – made from natural raw materials like cotton or wood pulp. Ex. Rayon• Synthetic – produced solely from chemicals. Ex. Nylon, polyester

Trace Evidence

• Removal of fiber evidence is best done at the lab. If this is not possible, it can be removed with forceps, tape of a vacuum.

Trace Evidence

• Classes of fibers can be identified by their properties.

• First and most important step is a microscopic comparison for color and diameter. The tool used is a comparison microscope.

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 containsthe pigment that make hair black, brown, blond, or red.

Trace Evidence

• The medulla is a row of cells like a canal running alongthe center of the cortex.

• There are three types of medullain humans:

(a) Fragmentary

(b) Interrupted

(c) Continuous

Trace Evidence

Is the hair human or animal hair?

• Investigators will look at all three characteristics of hairto 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 theseshapes.

• 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 MiddleEastern background show oval cross sections.

• People African heritage have crescent moon shaped crosssection.

Trace Evidence

Hair as Evidence

• Hair is what type of evidence? Class or Individual

Class

• Unless there are cells attached to the hair (follicle) nuclear DNAcannot be extracted from the hair shaft, Yet!

Trace Evidence

Glass

• Glass is considered class evidence unless:

• Fragments found at the crime scene can be matchedor pieced together with pieces found with the suspect.

Example: pieces of a car head light involved in anaccident fit into head light of car.

• Matching physical properties (density, index of refraction)of glass in question.

Trace Evidence

Glass Characteristics

• The density of glass can be determined by flotation.

• A control glass particle is immersed and suspended ina liquid. The glass chips to be tested are added andcompared.

Density

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 glassfragments.

Trace Evidence

Glass Fracture Patterns

• Fractured glass can reveal information related to forceand direction of impact.

• At high velocities, a projectile leaves around, crater-shaped hole, wider on theexit side.

Trace Evidence

• At low velocities, a projectile leaves an irregular shapedhole, and the glass will most likely shatter.

Trace Evidence

Bullet Holes

• When a bullet enters glass it leaves tell tale signs ofRadial and Concentric Fractures.

• Radial fractures appear as straight linescoming from the center of the bullet hole.

• Concentric fractures appear as circlesaround 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 whenit encounters an existing fracture.

• So, which hole was made first?

• Answer: B

AB

Trace Evidence

Soil

• Because no two places on earth (literally) have precisely the same soil, soil from one area will be identifiably differentfrom soil collected in another location.

• Soil is class evidence unless an unusual combination ofrare particles is under consideration.

• Soil consists of naturally occurring rocks, minerals, organicmatter (rotting leaves, twigs, etc.), and includes asphalt,brick, and other man made materials.

Trace Evidence

Soil as Evidence

• Similar to glass, soil density is used to compare soilspecimens.

• Soil is added to a solution which varies in density itself,different soil particles will be suspended at different portionof the solution.

Trace Evidence

Paint

• Paint consists of a solid portion, pigment (color), anda liquid portion know as the solvent.

• For all intensive purposes paint exhibits class characteristics.

• In some cases car makes and models can be determinedwith paint left at crime scenes.

Trace Evidence

Paint

• Layering of paints or piecing together fragments can make evidence more individual.