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Investigating and Processing Physical Evidence - PowerPoint PPT Presentation

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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 l.jpg

Chapter 2

Investigating and Processing Physical Evidence

Objectives l.jpg

  • 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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  • 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 technique’s 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 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.