Physical Evidence

1
Physical Evidence

• Chapter 3

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Physical Evidence

• 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 collected and analyzed.
• It is difficult to ascertain the weight a given
  piece of evidence will have in a case as
  ultimately the weight will be decided by a jury.

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Common Types of Physical Evidence

• Blood, Semen, Saliva
• Documents
• Drugs
• Explosives
• Fibers
• Fingerprints
• Firearms ammunition
• Glass
• Serial Numbers
• Soil minerals
• Tool Marks

• Hair
• Impressions
• Organs physiological fluids
• Paint
• Petroleum products
• Plastic bags
• Plastic, rubber, other polymers
• Powder residues
• Vehicle lights
• Wood other vegetative matter

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The Examination of Physical Evidence

• Identification
• The process of determining a substances physical
  or chemical identity.
• Chemical composition
• Nature of explosives
• Bodily fluids
• Requires that the number type of tests needed
  to identify a substance be sufficient to exclude
  all other substances
• Each type of evidence requires different tests,
  and each test has a different degree of
  specificity

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The Examination of Physical Evidence

• Comparison
• A comparative analysis has the important role of
  determining whether or not a suspect specimen and
  a standard/reference specimen have a common
  origin.
• Both the standard/reference and the suspect
  specimen are subject to the same tests.
• The forensic comparison is actually a two-step
  procedure.
• First, combinations of select properties are
  chosen from the suspect and the
  standard/reference specimen for comparison.
• Second, once the examination has been completed,
  the forensic scientist must be prepared to render
  a conclusion with respect to the origins.

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Hair Comparison
A human hair can be associated with a particular
racial group based on established models for each
group. Forensic examiners differentiate between
hairs of Caucasoid (European ancestry), Mongoloid
(Asian ancestry), and Negroid (African ancestry)
origin, all of which exhibit microscopic
characteristics that distinguish one racial group
from another.
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Fiber Comparison
Cotton, Wool, and Man-made fibers
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Classifying Characteristics

• 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 a group is said to
  have class characteristics.

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Individual Characteristics

• 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.

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Individual Characteristics

• 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

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Class Characteristics

• Surprising to the inexperienced forensic
  scientist is the frequent inability of the
  laboratory to relate physical evidence to a
  common origin with a high degree of certainty.
• 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.
• Nevertheless, the high diversity of class
  evidence in our environment makes their
  comparison very significant in the context of a
  criminal investigation.

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Class Evidence

• One of the current weaknesses of forensic science
  is the inability of the examiner 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, 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, and in a
  mass-produced world, gathering this kind of data
  is increasingly elusive.

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Class Evidence

• 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.

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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.
• Finally, the contribution of physical evidence is
  ultimately determined in the courtroom.

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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.

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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.

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Using Physical Evidence

• As the number of different objects linking an
  individual to a crime scene increases, so does
  the likelihood of that individuals 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.

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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.

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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.

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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.
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A laser beam is used to determine the search area
for the position of a shooter who has fired a
bullet through a window and wounded a victim. The
bullet path is determined by lining up the
victims bullet wound with the bullet hole
present in the glass pane.
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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.