1. PHYSICAL EVIDENCE

2. 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

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

4. 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

5. 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…?)

6. 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

7. 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

8. 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)

9. 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

10. 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

11. Evidence with Individual Characteristics

12. 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!

13. 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

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

15. 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

16. 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?

17. 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

18. 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.

19. 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

20. 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

21. 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.

22. 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

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

24. 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.

25. 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