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PHYSICAL EVIDENCE
Learning Objectives
1) Review the common types of physical evidence
encountered at crime scenes
2) Explain the difference between the identification and
comparison of physical evidence
3) Define and contrast individual and class
characteristics of physical evidence
4) Appreciate the value of class evidence as it relates to a
criminal investigation
5) List the national databases available to forensic
scientists
6) Understand the contributions the forensic pathologist,
forensic anthropologist, and forensic entomologist can
make to a death investigation
Why is Physical Evidence Important?
The examination of physical evidence by
a forensic scientist is usually done for
identification or comparison
Types of Physical Evidence
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Blood, Semen, Saliva
Documents
Drugs
Explosives
Fibers
Fingerprints
Firearms and Ammunition
Glass
Tool Marks
Vehicle Lights
Plastic, Rubber and Other
Polymers
• Hair
• Impressions
• Organs and
Physiological Fluids
• Paint
• Petroleum Products
• Plastic Bags
• Powder Residues
• Serial Numbers
• Soil and Minerals
• Wood and Other
Vegetative Matter
What is the Purpose of
Identification of Physical Evidence?
Identification has as its purpose the
determination of the physical or chemical identity
of a substance with as near absolute certainty as
existing analytical techniques will permit.
• Chemical composition of drugs
• Identification of gasoline in residues
recovered from the debris of a fire
• Identification of explosive residues (ex: TNT
or dynamite)
Identification of blood, semen, hair, wood
would also include a determination of origin
(did the bloodstain originate from a human,
dog, cat…?)
Process of Identification
Requires the adoption of testing procedures
that give characteristic results for specific
standard materials.
– Ex: if one wanted to know that a particular white powder was
heroin, the test results on the powder must be identical to
those tests performed on known samples of heroin
Requires that the number and type of tests
needed to identify a substance be sufficient
to exclude all other substances.
– Ex: if the examiner concludes that the white powder
is heroin, the test results must have been
comprehensive enough to exclude anything else
Why is Comparison of
Physical Evidence Important?
A comparison analysis subjects a suspect
specimen and a standard/reference specimen to
the same tests and examinations for the ultimate
purpose of determining whether they have to
determine whether they have a common origin.
– Ex: a paint chip found on a hit-and-run victim’s
garment may have to be compared with paint
removed from a suspect vehicle
– Ex: hair found at a crime scene may be compared to
the hairs removed from a
suspect’s head
Comparison: A Two-Step Procedure
Step 1: combinations of select properties are chosen from
the suspect and the standard/reference specimen for
comparison
• which and how many properties are selected will
depend on the type of materials being examined
Step 2: once the examination has been completed, the
forensic scientist must draw a conclusion about the
origins of the specimens (do they or do they not come
from the same source?)
– if one or more of the properties selected for
comparison do not agree, they are not from the same
source
– if all of the properties do compare, they still may not
be from the same source
• Role of probability (the frequency of occurrence of
an event)
Class Versus Individual Characteristics
CLASS CHARACTERISTICS
Properties of evidence that
can only be associated
with a group and never
with a single source
Examples
• one layer paint chip
on cars
• ABO Blood type
INDIVIDUAL CHARACTERISTICS
Properties of evidence that
can be associated with a
common source with an
extremely high degree of
certainty
Examples
• fingerprints
• striation markings on
bullets or tools
• irregular and random
wear patterns in tire or
footwear impressions
• multi-layer paint chip
• broken pieces of glass
that fit like a puzzle
• handwriting
characteristics
How to Tell the Difference Between
Class and Individual Characteristics?
A Class Characteristic can be associated
with a GROUP of people
Example: a red paint chip at a hit and run
tells an investigator to focus on the group of
people who drive red cars
An Individual Characteristic leads to an
INDIVIDUAL
Example: a multi-layer paint chip at a hit and
run is much more likely to belong to one car
Evidence with Individual Characteristics
Limitations to Identification of Evidence
Most items of physical
evidence CANNOT be linked
definitively to a single person
or object
Most evidence retrieved has
class characteristics
But, as the amount of
objects with class
characteristics linking an
individual to a crime
increases, the probability
of that individual’s
involvement in the crime
increases!
The Role of Probability
Blood is a good example of evidence that can have class
characteristics. Suppose two blood stains are compared
and both are found to be of human origin, type A. The
frequency of occurrence in the population of type A blood
is 26% - hardly offering a basis for establishing the
common origin of the stains. However, if other blood
characteristics are also determined and are found to
compare, the probability that the two samples originated
from the same source increases.
Product Rule: multiplying together the frequencies of
independently occurring (genetic markers) to obtain an
overall frequency of occurrence in a population
The Role of Probability
Blood Factors
A
EsD
PGM 2+ 2-
Frequency
25%
85%
2%
Forensic Databases
• Link police agencies across the country and around the world
• Provide a basis for comparing physical evidence from a crime
scene
Some Databases Include:
– Fingerprints (IAFIS)
• Integrated Automated Fingerprint Identification System
– DNA (CODIS)
• Combined DNA Index System
– Ballistics (NIBIN)
• National Integrated Ballistics Information Network
– Automotive Paint (PDQ)
• International Forensic Automotive Paint Data Query
– Shoe Prints (SICAR)
• Shoeprint Image Capture and Retrieval
The Murder Scene:
Death Investigation
The dead body of a victim is often the most important piece of
physical evidence – the task of analyzing a victim’s corpse falls
primarily to a forensic pathologist.
•This field involves the investigation of sudden, unnatural,
unexplained, or violent deaths.
•Forensic pathologists, AKA medical examiners and/or coroners (a
public officer whose primary function is to investigate any death
thought to be of other than natural causes).
•Must answer these basic questions:
– Who is the victim?
– What injuries are present?
– When did the injuries occur?
– Why and how were the injuries produced?
– What is the cause of death?
Medical Examiner
• If the cause cannot be found
through observation, an
autopsy is normally performed.
• Autopsy: medical dissection
and examination of a body to
determine the cause of death
• The manner in which death
occurred is classified into five
categories:
Natural
Homicide
Suicide
Accident
Undetermined
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Three Stages of
Decomposition after Death
Rigor Mortis
Livor Mortis
Algor Mortis
The medical examiner can often
estimate the time of death by
evaluating the stage of
decomposition in which the victim
was found.
Rigor Mortis
A medical condition that
occurs after death and
results in the stiffening
of muscle mass.
•Manifests itself within
first 24 hours and
disappears within 36
hours
Livor Mortis
A medical condition that
occurs after death and
results in the settling of
blood in areas of the body
closest to the ground.
•Skin appears dark blue/purple in these areas
– Begins immediately and continues up to 12 hours
after death
– Skin will not appear discolored in areas where
body is restricted by clothing or by an object
pressing against the body
• Useful in determining if the victim’s position was
changed after death
Algor Mortis
Process by which the body temperature
continually cools after death until it reaches the
ambient or room temperature.
– Rate of heat loss is affected by location and
size of body, the victim’s clothing, and
weather conditions
• Can only estimate time of death
– General Rule: beginning about
an hour after death, body loses
approximately 1 – 1.5°F per hour
until the body reaches the
environmental temperature.
Other Ways of Estimating
Time of Death
• Potassium Levels in Ocular Fluid
– After death, cells within the inner surface of
the eyeball release potassium into the ocular
fluid.
• Food in Stomach
– Amount of food in the stomach can help
estimate the person’s last meal
Forensic Anthropology
Concerned primarily with the identification and
examination of human skeletal remains
• Examination of bones may reveal:
• Sex
• Approximate Age
• Race
• Skeletal injury
Forensic Entomology
The study of insects and their relation to a
criminal investigation.
Can be Used to Estimate Time of Death
• After decomposition begins, necrophilious insects are the first to
infest the body, usually within 24 hours.
• Forensic entomologists can approximate how long a body has been
left exposed by examining the stage of development of the fly larvae.
Summary
• Physical evidence: identification or
comparison
• Comparative analysis determines
whether a piece of evidence and a
reference sample have the same origin
• Databases play a role in supplying
references
• Several specialists evaluate the dead
body as a piece of physical evidence