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Chapter 2. Investigating and Processing Physical Evidence. 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

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chapter 2

Chapter 2

Investigating and Processing Physical Evidence

  • 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
  • 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.
types of evidence 1 of 2
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
types of evidence 2 of 2
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.
the modern crime lab 1 of 4
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
the modern crime lab 2 of 4
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
the modern crime lab 3 of 4
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
the modern crime lab 4 of 4
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
functions of a forensic scientist 1 of 2
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
functions of a forensic scientist 2 of 2
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.
state of the evidence
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.
why examine physical evidence
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
characteristics of physical evidence 1 of 2
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.
characteristics of physical evidence 2 of 2
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
crime scene reconstruction 1 of 5
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
crime scene reconstruction 2 of 5
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
crime scene reconstruction 3 of 5
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
crime scene reconstruction 4 of 5
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
crime scene reconstruction 5 of 5
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
physical evidence in court 1 of 3
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
physical evidence in court 2 of 3
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
physical evidence in court 3 of 3
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 technique’s results?
    • Has it gained widespread acceptance within the scientific community?
expert testimony 1 of 2
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
expert testimony 2 of 2
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 expert’s 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.