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 –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

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.

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) 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) 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) 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) 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) 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) 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) 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 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? 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) 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) 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) 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) 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) 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) 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) 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) 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) 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) 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) 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) 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.