PHYSICAL EVIDENCE

Learning Objectives Review the common types of physical evidence encountered at crime scenes Explain the difference between the identification and comparison of physical evidence Define and contrast individual and class characteristics of physical evidence Appreciate the value of class evidence as it relates to a criminal investigation List the national databases available to forensic scientists Understand the contributions the forensic pathologist, forensic anthropologist, and forensic entomologist can make to a death investigation

Why is Physical Evidence Important? The examination of physical evidence by a forensic scientist is usually done for identification or comparison

Types of Physical Evidence Blood, Semen, Saliva Documents Drugs Explosives Fibers Fingerprints Firearms and Ammunition Glass Tool Marks Vehicle Lights Plastic, Rubber and Other Polymers Hair Impressions Organs and Physiological Fluids Paint Petroleum Products Plastic Bags Powder Residues Serial Numbers Soil and Minerals Wood and Other Vegetative Matter

What is the Purpose of Identification of Physical Evidence? Identification has as its purpose the determination of the physical or chemical identity of a substance with as near absolute certainty as existing analytical techniques will permit. Chemical composition of drugs Identification of gasoline in residues recovered from the debris of a fire Identification of explosive residues (ex: TNT or dynamite) Identification of blood, semen, hair, wood would also include a determination of origin (did the bloodstain originate from a human, dog, cat…?)

Process of Identification Requires the adoption of testing procedures that give characteristic results for specific standard materials. Ex: if one wanted to know that a particular white powder was heroin, the test results on the powder must be identical to those tests performed on known samples of heroin Requires that the number and type of tests needed to identify a substance be sufficient to exclude all other substances. Ex: if the examiner concludes that the white powder is heroin, the test results must have been comprehensive enough to exclude anything else

Why is Comparison of Physical Evidence Important? A comparison analysis subjects a suspect specimen and a standard/reference specimen to the same tests and examinations for the ultimate purpose of determining whether they have to determine whether they have a common origin. Ex: a paint chip found on a hit-and-run victim’s garment may have to be compared with paint removed from a suspect vehicle Ex: hair found at a crime scene may be compared to the hairs removed from a suspect’s head

Comparison: A Two-Step Procedure Step 1: combinations of select properties are chosen from the suspect and the standard/reference specimen for comparison which and how many properties are selected will depend on the type of materials being examined Step 2: once the examination has been completed, the forensic scientist must draw a conclusion about the origins of the specimens (do they or do they not come from the same source?) if one or more of the properties selected for comparison do not agree, they are not from the same source if all of the properties do compare, they still may not be from the same source Role of probability (the frequency of occurrence of an event)

Class Versus Individual Characteristics CLASS CHARACTERISTICS Properties of evidence that can only be associated with a group and never with a single source Examples one layer paint chip on cars ABO Blood type INDIVIDUAL CHARACTERISTICS Properties of evidence that can be associated with a common source with an extremely high degree of certainty Examples fingerprints striation markings on bullets or tools irregular and random wear patterns in tire or footwear impressions multi-layer paint chip broken pieces of glass that fit like a puzzle handwriting characteristics

How to Tell the Difference Between Class and Individual Characteristics? A Class Characteristic can be associated with a GROUP of people Example: a red paint chip at a hit and run tells an investigator to focus on the group of people who drive red cars An Individual Characteristic leads to an INDIVIDUAL Example: a multi-layer paint chip at a hit and run is much more likely to belong to one car

Evidence with Individual Characteristics

Limitations to Identification of Evidence Most items of physical evidence CANNOT be linked definitively to a single person or object Most evidence retrieved has class characteristics But, as the amount of objects with class characteristics linking an individual to a crime increases, the probability of that individual’s involvement in the crime increases!

The Role of Probability Blood is a good example of evidence that can have class characteristics. Suppose two blood stains are compared and both are found to be of human origin, type A. The frequency of occurrence in the population of type A blood is 26% - hardly offering a basis for establishing the common origin of the stains. However, if other blood characteristics are also determined and are found to compare, the probability that the two samples originated from the same source increases. Product Rule: multiplying together the frequencies of independently occurring (genetic markers) to obtain an overall frequency of occurrence in a population

The Role of Probability Blood Factors Frequency A 25% EsD 85% PGM 2+ 2- 2%  

Forensic Databases Link police agencies across the country and around the world Provide a basis for comparing physical evidence from a crime scene Some Databases Include: Fingerprints (IAFIS) Integrated Automated Fingerprint Identification System DNA (CODIS) Combined DNA Index System Ballistics (NIBIN) National Integrated Ballistics Information Network Automotive Paint (PDQ) International Forensic Automotive Paint Data Query Shoe Prints (SICAR) Shoeprint Image Capture and Retrieval

The Murder Scene: Death Investigation The dead body of a victim is often the most important piece of physical evidence – the task of analyzing a victim’s corpse falls primarily to a forensic pathologist. This field involves the investigation of sudden, unnatural, unexplained, or violent deaths. Forensic pathologists, AKA medical examiners and/or coroners (a public officer whose primary function is to investigate any death thought to be of other than natural causes). Must answer these basic questions: Who is the victim? What injuries are present? When did the injuries occur? Why and how were the injuries produced? What is the cause of death?

Medical Examiner If the cause cannot be found through observation, an autopsy is normally performed. Autopsy: medical dissection and examination of a body to determine the cause of death The manner in which death occurred is classified into five categories: Natural Homicide Suicide Accident Undetermined http://www.youtube.com/watch?v=udi8IpP3Uy8

Three Stages of Decomposition after Death Rigor Mortis Livor Mortis Algor Mortis The medical examiner can often estimate the time of death by evaluating the stage of decomposition in which the victim was found.

Rigor Mortis A medical condition that occurs after death and results in the stiffening of muscle mass. Manifests itself within first 24 hours and disappears within 36 hours

Livor Mortis A medical condition that occurs after death and results in the settling of blood in areas of the body closest to the ground. Skin appears dark blue/purple in these areas Begins immediately and continues up to 12 hours after death Skin will not appear discolored in areas where body is restricted by clothing or by an object pressing against the body Useful in determining if the victim’s position was changed after death

Algor Mortis Process by which the body temperature continually cools after death until it reaches the ambient or room temperature. Rate of heat loss is affected by location and size of body, the victim’s clothing, and weather conditions Can only estimate time of death General Rule: beginning about an hour after death, body loses approximately 1 – 1.5°F per hour until the body reaches the environmental temperature.

Other Ways of Estimating Time of Death Potassium Levels in Ocular Fluid After death, cells within the inner surface of the eyeball release potassium into the ocular fluid. Food in Stomach Amount of food in the stomach can help estimate the person’s last meal

Forensic Anthropology Concerned primarily with the identification and examination of human skeletal remains Examination of bones may reveal: Sex Approximate Age Race Skeletal injury

Forensic Entomology The study of insects and their relation to a criminal investigation. Can be Used to Estimate Time of Death After decomposition begins, necrophilious insects are the first to infest the body, usually within 24 hours. Forensic entomologists can approximate how long a body has been left exposed by examining the stage of development of the fly larvae.

Summary Physical evidence: identification or comparison Comparative analysis determines whether a piece of evidence and a reference sample have the same origin Databases play a role in supplying references Several specialists evaluate the dead body as a piece of physical evidence