Transcript Forensic science

Forensic science
The application of science to those
criminal and civil laws that are
enforced by police agencies in a
criminal justice system.
Physical Evidence
Any and all objects that can establish that a
crime has been committed or can provide a
link between a crime and its victim or a crime
and its perpetrator
Classification of Physical
Evidence
Trace evidence
extremely small items
Direct evidence
eyewitness testimony
Prima facie evidence
aka Direct Evidence
Circumstantial evidence
Exculpatory evidence
implies a fact and is used to
incriminate a person
helps to prove that an
accused individual is not
guilty
Another commonly used
classification of physical
evidence
Corpus delicti
establishes that a crime
has been committed
Associative evidence
links suspect with the
crime.
Physical evidence utilization in
other areas of forensic
investigation
Provides investigative leads for a
case
Ties one crime to a similar crime or
connects one suspect with another
Corroborates statements from
witnesses to or victims of a crime
The elements of a crime help to
determine what will be useful as
evidence.
Besides knowing what types of
evidence to search for, it is necessary to
know where evidence is most likely to
be found.
Types of physical evidence
Blood
 Conventional serology:
 presence of blood in
stains
 species identification and
ABO grouping
SEM: erythrocytes & lymphocytes
 DNA analysis can associate
victim and/or suspect with each
other or with the crime scene
BLOODSTAIN PATTERNS
 How high and fast did the
blood fall
Types of physical evidence
BODY TISSUES

organ samples collected at
autopsy, including blood,
urine and stomach contents
 Toxicological analysis
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DRUGS & CONTROLLED
SUBSTANCES

plant materials, powders,
tablets, capsules

volatile compounds (ethanol,
methanol, isopropanol)
heavy metals (arsenic)
nonvolatile organic compounds
(drugs of abuse,
pharmaceuticals)
poisons (strychnine, cyanide)
 Trace drug presence,
identity, and quantity
Black tar heroine wrapped in cellophane
Types of physical
evidence
DOCUMENTS
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paper and ink analysis
handwriting comparison to
determine authenticity
 typed, handwritten and printed
materials for evidence of forgery
 indented writings, obliterated
or altered writings, used
carbon paper, burned or
charred paper
Obliterated writing examination
Types of physical evidence
HAIRS
 hairs analysis can
determine
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Human/animal
Race
Body area
Cosmetic treatments
Method of removal (crushed,
cut, burned, forcibly removed,
fallen out naturally)
Morphological features  Structural differences in
peoples’ hair
DNA analysis
 Positive identification
Toxicological
 Presence of drugs and poisons
examination
 FIBERS
 Type
 Color, composition construction
Types of physical evidence
Two hairs being examined
under a comparison
microscope
Flax fibers viewed with
polarized light
Types of physical evidence
FINGERPRINTS
 Ridge characteristics are one of
the strongest pieces of evidence to
identify a person
23 matching characteristics
FIRE DEBRIS &
EXPLOSIVES RESIDUE
EXAMINATIONS
 identification of accelerants
and explosive residues
Unburned accelerator liquid on a soot covered carpet
Types of physical evidence
FIREARMS & AMMUNITION
 individual microscopic marks
 identification, source,
operability of firearms.
 detection and characterization
of gunpowder residues
Photomicrograph: test
bullet - questioned bullet
 muzzle-to-garments distance
estimation
GLASS FRAGMENTS
 Cause of breakage

Direction of breakage force

Physical fitting

Glass fragment comparisons
Glass fracture produced
by a high-speed projectile
Types of physical evidence
PAINT & PAINT PRODUCTS
 analysis and comparison of
paint transferred from the
surface of an object to another
during the commission of a
crime:
 Suspect vehicle impacting a
victim vehicle; a pedestrian or a
stationary object
 Tool impacting stationary object
 Paint databases can help
identify the year, make and/or
color of a motor vehicle from a
chip of paint left at the scene.
Paint Layers on Wood Surface
Types of physical evidence
TOOLMARK IDENTIFICATION
 microscopic side-by-side
comparison
 attempts to link a particular tool
with a particular mark to the
exclusion of any other tool
ROPE & CORDAGE
 composition, construction,
color and diameter
Spacing between teeth in gripping -major
role in toolmark examinations
Manufacturer
Types of physical evidence
SOILS & MINERALS
 comparison between two or
more soils to determine if
they share a common origin
 color, texture, composition
comparison
Layers of soil exposed at a grave site. Each
layer must be sampled
WOOD
side or end matching,
fracture matching and
species identification.
Cross-section - Xylem
Types of physical evidence
OILS/GREASE &
COSMETIC PRODUCTS
SHOEPRINTS & TIRE
TREAD IMPRESSIONS
 have value for forensic
comparisons.
 can provide positive
identification of the
suspect’s shoes or tires from
the suspect’s vehicle.
 possess unique composition
for comparison
Shoeprint
collected
using a
gelatin lifter.
Characteristics of evidence
Class characteristics
Features, that place an
item into a specific
category
Individual characteristics features that distinguish
one item from another of
the same type
Class Characteristics of
Blood
Can apply the product rule to narrow the focus.
What percentage is O+?
36.6%
What percentage has all of these blood factors?
0.004%
Class Characteristics
Examples: new factory painted cars with
only two-layers of paint
Uses of Class Characteristics are usually
to corroborate the events with data that
are free of human bias
Individual Characteristics
Examples: DNA, fingerprint ridges,
grooves/striations on a bullet or toolmark,
irregular and random wear patterns on tire
or shoe impressions, handwriting
characteristics, fitting together pieces of
broken objects, matching the striations in
sequentially made plastic bags, etc.
Ex. the probability of two people having
the same fingerprints is 1 / 1060
Individual Characteristics
Individual Characteristics
Individual vs. Class
Characteristics
Individual vs. Class
Characteristics
The more objects that can place a suspect
at the scene, the higher the probability that
the person was there, even if only class
characteristics are present.
Individual and Class Characteristics can
often exonerate a person as well.
Processing physical
evidence
discovering, recognizing and examining it;
collecting, recording and identifying it;
packaging, conveying and storing it;
exhibiting it in court;
disposing of it when the case is closed.
RAGE!!