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.
• Forensic crime labs maintain databases of the characteristics
and manufacturing details of materials found at crime scenes
for comparison.
•Wayne Williams was convicted of
murdering 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, however
most often the following is collected:
hair
fiber
soil
glass
paint
Trace Evidence
Fiber
The 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 Fibers
Natural 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 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:
(a) 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) nuclear 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
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.