Chapter 2

1. Chapter 2 Investigating and Processing Physical Evidence

2. 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

3. 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.

4. 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

5. 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.

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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.

12. 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.

13. 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

14. 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.

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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?

24. 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

25. 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.