Download Trace Evidence Slide Show from Class

Introduction to
Forensic Science
Physical Evidence
Value of Physical Evidence
• Vernon J. Gerberth, in Practical Homicide
Investigation, points out that physical
evidence is useful in:
– Helping to establish the scope of the crime
scene
– Placing a perpetrator at a scene
– Connecting a suspect with a weapon
– Supporting witness statements
– Connecting crime scene areas (abduction,
vehicle used, dump site)
Physical Evidence is Varied
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Blood, semen, and saliva
Documents
Drugs
Explosives
Fibers
Fingerprints
Firearms and ammunition
Glass
Hair
Impressions
Organs and physiological
fluids
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
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Paint
Petroleum products
Plastic bags
Plastic, rubber, and other
polymers
Powder residues
Soil and minerals
Tool marks
Vehicle lights
Wood and other vegetative
matter
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Trace Evidence
• No matter how much someone tries to clean up
a crime scene, something is generally left
behind.
• It may not always be detected, but it's difficult
to take any kind of violent action without
shedding something.
• Trace evidence, though often insufficient on its
own to make a case, may corroborate other
evidence or even prompt a confession.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Class and Individual Characteristic
Evidence
• Class: a group of objects or persons with
characteristic physical evidence common
to it
– Examples include soil and hair
• Individual characteristics can be identified
as having originated with a particular
person or source
– Establishes individuality
– Examples include fingerprints and footprints
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Categories of Evidence:
• Biological/Physiological
– Blood, semen, saliva and other bodily fluids.
• Chemical
– Narcotics, prescription drugs, powders associated
with firearms.
• Physical
– Fingerprints, footprints, impressions, toolmarks,
tiremarks, firearms evidence.
• Non-specific/miscellaneous
– Photography, dye marks, voice analysis.
Six Types of Evidence
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Trace evidence
Transfer evidence
Indented or impression evidence
Striated evidence
Geometric evidence
Chemical evidence
• All of these can be class or individual evidence
Trace Evidence
• Trace evidence is material found at a
crime scene or accident scene in small but
measurable amounts.
• This is important as it can definitively link
an individual or object to the scene.
Trace Evidence
• Because trace evidence can be any
number of things, from a fingerprint to
DNA to plant debris, there are numerous
different methods used for analysis.
• For some objects, there is a large
database available for comparisons
(fingerprints, DNA), while the science of
others has not advanced that far.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Locard’s Principle
• Every person who is physically involved in
a crime leaves some minute trace of his or
her presence, and often takes something
away.
• This is Dr. Edmond Locard's principle of
contact, proposed when he began his
forensic laboratory in 1910.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Transfer Evidence
• Transfer evidence is defined as any evidential
substance or particle such as blood, fluids, hairs,
fibers, paint, and skin that is exchanged
between an assailant and the victim or the
scene of the crime.
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From the criminal to the victim
From the victim to the criminal
Into or out of the crime scene
When forcible contact occurs between persons,
vehicles, or objects (e.g. paint in a car crash).
Fiber Analysis
• Most investigations center on fiber or hair, which
is easier to see than pollen or dirt.
• Cross transfers of fiber often occur in cases in
which there is person-to-person contact.
• Investigators hope that fiber traceable back to
the offender can be found at the crime scene, as
well as vice versa.
• Success in solving the crime often hinges on the
ability to narrow the sources for the type of fiber
found
Fiber Analysis
• The problem with fiber evidence is that
fibers are not usually unique.
• They cannot pinpoint an offender in any
definitive manner.
• There must be other factors involved,
such as evidence that the fibers can
corroborate or something unique to the
fibers that set them apart.
Fiber Analysis
• A better case can be made with class
evidence when the evidence has either a
greater number of identifying features or a
greater number of different types of
evidence.
• For example finding a mixture of white
dog and yellow dog and black cat hairs on
a thief from a household with those color
pets is stronger than just one type of hair.
Fiber Analysis
• Fibers are gathered at a crime scene with
tweezers, tape, or a vacuum.
• They generally come from clothing,
drapery, wigs, carpeting, furniture, and
blankets.
• For analysis, they are first determined to
be natural, manufactured, or a mix of
both.
Fiber Analysis
• Natural fibers come from plants (cotton)
or animals (wool).
• Manufactured fibers are synthetics like
rayon, acetate, and polyester, which are
made from long chains of molecules called
polymers.
• To determine the shape and color of fibers
from any of these fabrics, a microscopic
examination is made.
Fiber Analysis
• Generally, the analyst gets only a limited
number of fibers to work with—sometimes
only one.
• Whatever has been gathered from the
crime scene is then compared against
fibers from a suspect source, such as a car
or home, and the fibers are laid side by
side for visual inspection through a
microscope.
Fibers are Usually Class
Evidence
• Comparisons of fibers of the same type of
material include the presence of dyes and
a number of microscopic features.
• Comparisons can be made with fibers
from a fabric to individual fibers or from
fibers removed from another piece of
fabric.
But Can be Individual Evidence.
• The weave pattern of fabrics can also be
compared.
• Sometimes, when a piece has been torn
from a fabric, that piece can be matched
up with weave pattern and irregularities of
a mutual tear edge.
• Cordage (ropes, string, etc.) tears and
cuts can be matched if the ends are not
too frayed or distorted.
Soil Evidence
• Soil evidence is important when the
suspect drives/walks on unpaved areas
• It is picked up by:
– tire treads
– shoe bottoms
– pants cuffs
• It may also be located in:
– subject's vehicle
– articles in a suspect's trunk
Indented or Impression
Evidence
• Footprints or shoe
prints
• Writing surfaces, like
a pad of paper with
the top sheet
removed
• Tool marks
• Tire marks
• These can be class or
individual evidence
Fiber Impressions
• Fabric impressions may be found in a
number of situations.
• The impression of a victim's garment may
remain on the oil pan or on another flat
surface of a vehicle in an accident
investigation.
Collecting Fiber and Hair
Evidence
• Recovery of evidence should be the most
direct but least intrusive technique
practical. This could include picking,
scraping, or vacuuming.
• Wrap clothing or items to protect adhering
fibers if collection is to be done by
laboratory personnel.
Collecting Fiber and Hair
Evidence
• Since fiber evidence is generally small in
nature, care should be taken to prevent
loss or contamination.
• Several methods could be used in the
collecting of fiber evidence:
– visual searches,
– alternate light sources
– searches with additional magnification.
Striated Evidence
• Marks left on objects
that rub together,
when one object is
harder than the other.
• Striations on bullets
are a good example.
• Photomicrograph of
two bullets fired from
the same gun.
Geometric Evidence
• Pieces of Broken
Evidence
– Headlights with hit and
run accidents.
Chemical Evidence
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Narcotics
Flammables
Alcohol
Chemicals associated with clandestine
labs.
Factors to Consider for Trace
Evidence Examination
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The commonness of the material
The mode of transfer
Quantity found
Possible contamination from other sources
Whether someone attempted to remove the
material
• Certainty of identification of the material
• How the material was collected, preserved and
retained
• Background information of the case
Trace Evidence
• The quality of trace evidence examination depends on
the information obtained from the crime scene and the
many factors that affect the trace material.
• Using trace evidence an investigator can link a suspect
to a crime.
• There are many modes of collection of trace evidence.
Some of the more popular collection methods are
vacuuming, lifting with tape, and gathering by hand.
• Most trace evidence investigations involve the use of the
microscope and a spectrophotometer.
• As with hair identification and firearm identification, a
comparison microscope is a very helpful tool in
characterizing the evidence.
Trace Evidence can be Fragile and
Easily Lost
• E.g. footprints in the dust on a floor or in
the snow can be easily lost.
• Footwear prints and impressions should be
photographed:
– As part of the general scene
– Also photograph with a scale
• Dental Stone is used in casting
impressions
Individual vs. Class Evidence
• Individual Characteristics
– Evidence that can be associated to a common
source with an extremely high degree of
probability is said to possess individual
characteristics.
• Class Characteristics
– Evidence associated only with one group is
said to have class characteristics.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Probability
• To understand the accuracy of a comparison,
one must appreciate the role that probability has
in ascertaining the origins of two or more
specimens.
• Simply defined, probability is the frequency of
occurrence of an event.
• In flipping a coin, probability is easy to establish.
• The exact probability of some things are almost
impossible to define- of transferring dog hair
and blue jeans fiber for example.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Probability
• In all cases, it is not possible to state with
mathematical exactness the probability
that the specimens are of common origin.
• It can only be concluded that this
probability is so high as to defy
mathematical calculations or human
comprehension.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Individual Evidence
• Examples:
– The matching ridge characteristics of two fingerprints
– The comparison of random striation markings on
bullets or tool marks
– The comparison of irregular and random wear
patterns in tire or footwear impressions
– The comparison of handwriting characteristics
– The fitting together of the irregular edges of broken
objects in the manner of a jigsaw puzzle
– Matching sequentially made plastic bags by striation
marks running across the bags
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Class Evidence
• Evidence is said to possess class characteristics
when it can be associated only with a group and
never with a single source.
• Here again, probability is a determining factor.
• However, the high diversity of class evidence
often makes it very significant.
• However, a lab may be unable to classify
evidence to a common origin with a high degree
of certainty.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Class Evidence
• A current weaknesses of forensic science is the
inability to assign exact or even approximate
probability values to the comparison of most
class physical evidence.
• For example, what is the probability that a nylon
fiber originated from a particular sweater?
• There is little statistical data available from
which to derive this probability in a world with
so many mass-produced products.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Class Evidence
• One of the primary endeavors of forensic
scientists must be to create and update
statistical databases to evaluate the significance
of class physical evidence.
• Most physical evidence 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.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Class Evidence
• The chances are low of finding two
indistinguishable items of evidence at a crime
scene that originated from different sources.
• In 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.
• The final value of physical evidence to a case is
determined in the courtroom.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Crossing over the line from class to
individual evidence is difficult.
• 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 legal arguments.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Using Physical Evidence
• As the number of different objects linking an
individual to a crime scene increases, so does
the likelihood of that individual’s involvement
with the crime.
• Just as important, 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.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Forensic Databases
• The Integrated Automated Fingerprint Identification
System (IAFIS), a national fingerprint and criminal
history system maintained by the FBI.
• The Combined DNA Index System (CODIS) enables
federal, state, and local crime laboratories to
electronically exchange and compare DNA profiles.
• 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) is a
shoeprint database.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Reconstruction
• The method used to support a likely sequence of
events by the observation and evaluation of
physical evidence, as well as statements made
by those involved with the incident, is referred
to as reconstruction.
• Crime-scene reconstruction relies on the
combined efforts of medical examiners,
criminalists, and law enforcement personnel to
recover physical evidence and to sort out the
events surrounding the occurrence of a crime.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Reconstruction and Physical
Evidence
• The physical evidence plays a crucial role in
reconstructing the events that took place.
• Evidence should support the theory of the crime.
• Evidence may 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.
CRIMINALISTICS
An Introduction to Forensic Science, 9/E
By Richard Saferstein
PRENTICE HALL
©2007 Pearson Education, Inc.
Upper Saddle River, NJ 07458
Case Study: Sam Sheppard
• To determine if the weapon could have
been a flashlight or if the assailant was
right or left handed, experts did
reconstructions of the crime scene.