Investigating and Processing Physical Evidence

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Chapter 2

• Investigating and Processing Physical Evidence

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Objectives

• Students should gain an understanding of
• Common objects found at a crime scene that
  qualify as physical evidence
• Different types of crime labs and their
  organization
• The functions performed by a forensic scientist
• Class and individual characteristics of physical
  evidence
• Reconstruction of a crime scene
• The admissibility of physical evidence and the
  role of an expert in court

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Introduction

• Physical evidence is merely one piece of the
  puzzle when investigators are trying to solve a
  case.
• It may be the most important factor in proving
  the link between the suspect and the victim.
• It may be essential to prove that the same
  suspect is linked to a series of incidents.

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Types of Evidence (1 of 2)

• Physical
• Tangible objects
• Documentary
• Any kind of writing, sound, video recording
• Demonstrative
• Real evidence used to illustrate, demonstrate, or
  recreate a prior event
• Testimony
• Witnesses speaking under oath in court

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Types of Evidence (2 of 2)

• It is extremely important to identify those items
  that might provide significant probative
  information related to the crime.
• Focus on those objects whose scientific analysis
  is likely to yield important clues.
• The judge is responsible for determining what
  evidence is and is not relevant to the crime
  under investigation.

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The Modern Crime Lab (1 of 4)

• Currently about 350 labs in United States
• Growth in number of labs driven by three factors
• Increase in crime rate
• Increase in drug-related crimes needing analysis
• Fewer confessions, leading to greater use of
  physical evidence for investigation

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The Modern Crime Lab (2 of 4)

• Major private labs
• Orchard Cellmark forensic DNA
• Battelle Corporation arson
• Sirchie Corporation fingerprinting, trace
  evidence
• Four federal labs
• FBI
• DEA
• ATF
• Postal Service

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The Modern Crime Lab (3 of 4)

• Typically labs have six divisions
• Biological/Serological
• Chemistry/Toxicology
• Trace Evidence
• Ballistics, Firearms, Tool Marks
• Latent Fingerprints
• Questioned Documents

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The Modern Crime Lab (4 of 4)

• Requirements for a crime lab
• Quality control manual
• Quality assurance manual
• Lab testing protocol
• Program for proficiency testing

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Functions of a Forensic Scientist(1 of 2)

• Processing evidence
• Recognize physical evidence
• Document crime scene and evidence
• Collect, preserve, inventory, package, and
  transport physical evidence
• Analyze the physical evidence
• Interpret the results
• Report the results
• Present expert testimony

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Functions of a Forensic Scientist(2 of 2)

• The primary goal in analyzing physical evidence
  is to make the facts of the case clear.
• Corpus delicti proves a crime took place
• Modus operandi characteristic of a particular
  criminal
• Physical evidence can direct the course of the
  investigation.
• Physical evidence can exonerate suspects.

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State of the Evidence

• Investigators must move quickly to identify and
  protect evidence.
• Biological evidence is most susceptible to
  change.
• Time and physical influences can affect other
  types of evidence as well.
• The forensic scientist seeks to show that the
  evidence (questioned sample) and the known sample
  (exemplar) have a common source.

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Why Examine Physical Evidence?

• Identification elucidating the physical or
  chemical identity of a substance
• Comparison subjecting evidence and reference
  material to the same tests to prove whether they
  share a common origin
• Definition of evidence determination of whether
  objects contribute information to the case

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Characteristics of Physical Evidence (1 of 2)

• Tests are valid if they meet three criteria
• They are reproducible.
• They are sensitive.
• They are specific.
• Scientists pick an appropriate test for each
  questioned sample.
• Test protocols are permanently recorded when they
  repeatedly yield reproducible, accurate results.

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Characteristics of Physical Evidence (2 of 2)

• Physical evidence can be used to associate a
  suspect to a crime or rule a person out as a
  suspect
• Class characteristics classified as belonging to
  a certain class of objects
• Individual characteristics associated with only
  a single source
• Examples tread, tool marks, impressions

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Crime Scene Reconstruction(1 of 5)

• Physical evidence
• May establish events that occurred before,
  during, and immediately after the crime
• May corroborate or refute witness reports
• Involves inductive and deductive reasoning,
  probability, statistics, and pattern analysis

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Crime Scene Reconstruction (2 of 5)

• Pattern evidence
• Reconstructs events that took place and the
  sequence in which they took place
• Generates investigative leads
• Explosion patterns
• Can establish direction in which blast traveled
• Indicate location of maximum damage
• Support analysis of the debris field
• Can identify the site of detonation
• Used to estimate the weight of the bomb

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Crime Scene Reconstruction(3 of 5)

• Firearms ballistics
• Determine cause of death
• Place shooter and victim at precise locations at
  the crime scene
• Entry and exit hole geometry
• Can indicate where the bullet entered the object
• Can be used to estimate the angle of entry

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Crime Scene Reconstruction(4 of 5)

• Bullet trajectory
• Insert rods into holes
• Use an attached string to estimate trajectory
• Bullet ricochet
• Can affect trajectory estimates
• Low-velocity, heavy bullets more ricochet
• High-velocity, lightweight bullets more likely
  to break on impact

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Crime Scene Reconstruction(5 of 5)

• Shell casings
• Produced by automatic and semiautomatic weapons
• Bloodstain patterns
• Active caused by blood that travels because of
  force or pumping of pressurized blood
• Passive caused by force of gravity
• Transfer caused by contact with an object that
  has wet blood on it

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Physical Evidence in Court (1 of 3)

• Evidence has value only when a forensic scientist
  can explain to a jury how the evidence was
  analyzed and what it means
• Federal rules of evidence apply in federal court

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Physical Evidence in Court (2 of 3)

• Frye standard
• Judge decided whether scientific techniques could
  be admitted as evidence
• Precedent was often taken into consideration
• Coppolino standard
• Court is allowed to admit a novel test or
  controversial theory if an adequate foundation of
  validity is laid

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Physical Evidence in Court (3 of 3)

• Daubert standard
• Pretrial hearing is conducted for scientific
  evidence
• Trial judge rules on admissibility
• Guidelines
• Has the technique been tested before?
• Has the technique been subject to peer review and
  publication?
• What is the potential rate of error?
• Do standards exist to verify the techniques
  results?
• Has it gained widespread acceptance within the
  scientific community?

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Expert Testimony (1 of 2)

• Must rest on reasonable scientific certainty
• Must present a truthful, persuasive opinion
• Cannot serve as an advocate for either side
• Must establish that he/she possesses a particular
  knowledge or skill or has expertise to explain
  the truth of issues presented
• Is qualified during voir dire

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Expert Testimony (2 of 2)

• Courts rely on the training and experience of
  experts to assess their knowledge.
• The opposing attorney will highlight weaknesses
  in the experts education and experience.
• The expert should explain complicated scientific
  data in clear, down-to-earth language.
• The opposing attorney will challenge the accuracy
  and interpretation of results, as well as the
  collection, transport, storage, and testing of
  evidence.