Chapter 3:
Physical Evidence
Introduction
It would be impossible to list all the objects that could
conceivably be of importance to a crime.
Almost anything can be Physical Evidence.
Although you cannot rely on a list of categories, it is useful to
discuss some of the most common types of physical evidence.
The purpose of recognizing physical evidence is so that it can
be ___________ and ____________.
The weight a given piece of evidence will have in a case will
be decided by a jury.
Types of Physical Evidence
Blood, semen, and saliva
Paint
Documents
Petroleum products
Drugs
Plastic bags
Explosives
Fibers
Plastic, rubber, and other
polymers
Fingerprints
Powder residues
Firearms and ammunition
Soil and minerals
Glass
Tool marks
Hair
Vehicle lights
Impressions
Wood and other vegetative
matter
Organs and physiological fluids
Classification of Physical Evidence
Biological Crime-Scene Evidence: includes blood, saliva,
semen, DNA, hair, organs, and other physiological fluids
Manufactured Items of Crime-Scene Evidence: firearms,
ammunition, fibers, paint, glass, petroleum, products,
plastic bags, rubber, polymers, and vehicle headlights.
Classification of Physical Evidence
Impression Crime-Scene Evidence: includes tire markings,
shoe prints, depressions in soft soils, all other forms of
tracks, glove and other fabric impressions, tool marks, and
bite marks
Purpose of Examining Physical
Evidence
Physical Evidence is usually examined by a forensic scientist
for _____________ or ______________
Identification : The process of determining a substance’s
physical or chemical identity
Examples: identifying the chemical composition of illicit drugs,
gasoline, the nature of an explosion, blood-human or animal?
The process of identification:
1. Testing procedures that give characteristic results for
specific standard materials.
2. The number and type of tests needed to identify a
substance be sufficient to exclude all other substances.
Purpose of Examining Physical
Evidence
Comparison: Deciding whether two or more objects have 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 or not they have a common origin.
Examples:?
Process of comparison analysis:
1. Combinations of select properties are chosen from the
suspect and the standard/reference specimen for
comparison.
2. Tests and examinations
3. The forensic scientist makes a conclusion with respect to
the origins.
Role of Probability
Probability: is the frequency of occurrence of an event.
To comprehend the evidential value of a comparison, one
must appreciate the role that probability has in ascertaining
the origins of two or more specimens.
Just because a comparison analysis determines the specimens to
be indistinguishable, it does not mean they 100% come from the
same source
So, it can only be concluded that a probability is so high as to
defy mathematical calculations or human comprehension
After classifying millions of fingerprints over the past
hundred years, no two have ever been exactly alike!
The probability of two individuals having the same fingerprints
is 1 out of 1 x 1060
Classifying Characteristics
Individual Characteristics: Properties of evidence that can be
associated to _____________ with an extremely high degree of
certainty (probability)
Examples: matching ridge characteristics of two fingerprints,
the comparison of handwriting characteristics, the fitting
together of irregular edges of broken objects (like puzzle pieces)
Class Characteristics- Properties of evidence associated only
with a group and never a single source.
Class Evidence
What is the probability that a nylon fiber originated from a
particular sweater, or that a paint chip came from a suspect car in a
hit and run?
There are very few statistical data available from which to derive this
information
In a mass-produced world, gathering this kind of data on class
evidence is increasingly elusive.
One of the primary endeavors of forensic scientists must be to
create and update statistical databases for evaluating the
significance of class physical evidence.
Most items of physical evidence retrieved at crime scenes
cannot be linked definitively to a single person or object.
The value of class physical evidence lies in its ability to provide
corroboration of events with data that are, as nearly as
possible, free of human error and bias.
Class Evidence
The chances are low of encountering two indistinguishable
items of physical evidence at a crime scene that actually
originated from different sources.
When one is dealing with more than one type of class evidence,
their collective presence may lead to an extremely high
certainty that they originated from the same source. ****
Product rule
Finally, the contribution of physical evidence is ultimately
determined in the courtroom.
Crossing Over
Crossing over the line from class to individual does not end the
discussions.
How many striations are necessary to individualize a mark to a
single tool and no other?
How many color layers individualize a paint chip to a
single car?
How many ridge characteristics individualize a fingerprint?
How many handwriting characteristics tie a person to a
signature?
These are all questions that defy simple answers and are the
basis of arguments.
Natural vs. Evidential Limits
There are practical limits to the properties and
characteristics the forensic scientist can select for
comparison.
Modern analytical techniques have become so sophisticated and
sensitive that natural variations in objects become almost infinite.
Carrying natural variations to the extreme, no two things in this
world are alike in every detail.
Evidential variations are not the same as natural variations.
Distinguishing variations of evidential use from natural
variations is not always an easy task.
Using Physical Evidence
As the number of objects linking an individual to a crime
scene increases, the probability of involvement increases
A person may be exonerated or excluded from suspicion if
physical evidence collected at a crime scene is found to be
different from standard/reference samples collected from
that subject.
Forensic Databases
• The Integrated Automated Fingerprint Identification System (IAFIS)- a
national fingerprint and criminal history system maintained by the
FBI.
• Used to compare fingerprints- a new one with one already in the system
• If a fingerprint is found, the latent-print examiner creates a digital image of
the print. Next, the examiner, with the aid of a coder, marks points on the
print to guide the computerized search. The print is then electronically
submitted to IAFIS and within minutes the search is completed against all
fingerprint images in IAFIS
• The Combined DNA Index System (CODIS)- enables federal, state,
and local crime laboratories to electronically exchange and compare
DNA profiles.
• Links crimes to each other and to convicted offenders
Forensic Databases
• The National Integrated Ballistics Information Network (NIBIN) allows firearm analysts to acquire, digitize, and compare markings
made by a firearm on bullets and cartridge casings.
• The International Forensic Automotive Paint Data Query (PDQ) database contains chemical and color information pertaining to original
automotive paints.
• SICAR (shoeprint image capture and retrieval) - a shoeprint database.
Reconstruction
Reconstruction: The method used to support a likely
sequence of events, at a crime scene, by observing and
evaluating physical evidence and statements made by
those involved with the incident.
Crime-scene reconstruction relies on the ______________
of medical examiners, criminalists, and law enforcement
personnel to recover physical evidence and to sort out the
events surrounding the occurrence of a crime.
Examples: whether a body was moved after death, determining whether a
victim was clothed after death, analyzing bullet trajectory, analyzing blood
splatter, determining the direction from which projectiles penetrated glass
objects, estimating the distance of a shooter from a target, etc.
The Role of Physical
Evidence
The physical evidence left behind at a crime scene
plays a crucial role in reconstructing the events that
took place surrounding the crime.
Although the evidence alone does not describe
everything that happened, it can support or
contradict accounts given by witnesses and/or
suspects.
Information obtained from physical evidence can
also generate leads and confirm the reconstruction of
a crime to a jury.
The collection and documentation of physical
evidence is the foundation of a reconstruction.
Summing It Up
Reconstruction is a team effort that involves
putting together many different pieces of a
puzzle.
The right connections have to be made among all
the parts involved so as to portray the
relationship among the victim, the suspect, and
the crime scene.
If successful, reconstruction can play a vital role
in aiding a jury to arrive at an appropriate
verdict.
The recognition, collection, and analysis of
physical evidence is the foundation to successful
reconstruction, but only part of the process.
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index=16&list=PL3w5A_ld90eAL11e5OAWKA8miX7
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