1. Chapter 3: Physical Evidence
Any tangible objects that can be collected from a crime scene

2. Common Types of Physical Evidence
Body fluids
Blood, semen, saliva
Liquid/dry
Animal/human
Documents
Hand-/typewriting
Paper, ink, etc.
Drugs
Any illegal substance
Sale, manufacture, distribution
Explosives
Devices - explosive charge
Explosive objects/residues
Fibers
Natural/synthetic
Establishes connections between objects/people
Fingerprints
Individual evidence
Can be visible/latent (hidden)
Firearms/ammunition
Guns/casings/shells, etc.

3. Types of Physical Evidence (continued…)
Glass
Glass particles/ fragments in a crime
Bullet holes
Hair
Animal/human
Link to a crime/scene
Impressions
Tires/shoe prints/tracks
Bite marks
Organs/Physiological Fluids
Detection of drugs/poisons or alcohol
Paint
Liquid/dried
Transferred during a crime
Petroleum Products
Suspect/crime scene
Gasoline, grease, etc
Residues left at a crime
Plastic bags
Polyethylene
Homicide/drugs
Plastic/Rubber/Polymers
Remains recovered
Link to crime/suspect

4. Types of Physical Evidence (continued…)
Powder Residues
From the discharge of a firearm
Serial Numbers
Stolen property
Restoration of erased ID numbers
Soil and Minerals
Link person/object to a particular
location
Soil embedded in shoes, etc.
Tool Marks
Causes an impression
Tool in a crime leaves marks
Vehicle Lights
Headlights/taillights
On/off at time of impact
Wood/Vegetative Matter
Fragments/shavings/twigs, etc. on clothing, shoes, etc.
Link a person/object to a location

5. Key Points
Biological crime scene evidence includes blood, saliva, semen, DNA, hair, organs, and physiological fluids;
Impression crime-scene evidence includes tire markings, shoe prints, depressions in soft soils, all other forms of tracks, glove and other fabric impressions, tool marks and bite marks;
Manufactured items considered common items of crime-scene evidence include firearms, ammunition, fibers, paint, glass, petroleum products, plastic bags, rubber, polymers, and vehicle headlights/taillights.

6. Examination of Physical Evidence
Identification
Determine physical/chemical identity
Body fluids can determine the species of origin
Determine the chemical composition of a drug, residue, etc.

7. Identification Identification process Uniform testing procedures
Scientific process
Sufficient number/type of tests conducted (multiple trials) depending upon type of evidence
Comprehensive enough to exclude any uncertainty
Education/experience of scientist also plays a role

8. Comparison Analysis Specimen in question compared to a known standard
Usually a 2-step procedure
Properties are selected to compare the evidence with the standard (reference)
Conclusion(s) drawn about the origins of the sample/specimen

9. Probability
If one or more of the selected properties do not agree (between evidence and standard), the scientist will conclude that the specimens are not from the same source
Probability plays a role in establishing evidence of value
Frequency of occurrence of an event
Defines the odds of an event occurring and that the evidence is connected to that particular event

10. Evidence: Class Characteristics
Associated with a group and not a single source
Used for elimination rather than identification
Blood, paint, hair and fibers are some examples

11. Evidence: Individual Characteristics
Evidence from a single source with a high degree of probability
Can be linked to a specific suspect and/or location
Examples are DNA and fingerprints

12. Key Points
Two methods used by forensic scientists when examining physical evidence are identification and comparison.
Identification is the process of determining a substance’s chemical or physical identity to the exclusion of all other substances.
A comparison analysis determines whether a suspect specimen and a standard/reference specimen have a common origin.
Evidence that can be associated with a common source with an extremely high degree of probability is said to possess individual characteristics.
The overall frequency of occurrence of an event can be obtained by multiplying the frequencies of all independently occurring instances related to the event. This is known as the product rule.

13. Significance of Physical Evidence
Hard to assign exact or even approximate probability values to most class evidence
Very few statistical data are available (mass-produced materials, etc.)
Statistical databases must be created/updated to help evaluate the significance of physical evidence
Personal experience often plays a role in the evaluation – subjective – can detract from credibility rather than support

14. Assessing the Value of Evidence
Lies in its ability to corroborate events with data that is as free from human error as possible
Methods/instruments are continually being revised/improved
Objects that exhibit significant diversity are appropriate for classification as physical evidence
As the number/type of evidence increases to link a specific person to a crime scene, the probability of involvement increases dramatically

15. Cautions and Limitations
The weight/significance of evidence in a courtroom is usually decided by the jury
Reliability and trustworthiness
Given great weight in jury deliberations
Proper safeguards must be taken to protect the evidence to prevent bias against the accused
Thorough collection and scientific evaluation
Find as many characteristics as possible to compare one substance/piece of evidence to another
Decided by the quality and composition of the evidence, case history, examiner’s experience
There are practical limits to the properties/characteristics selected for comparison (sophisticated instruments help)
Also used to exclude/exonerate someone

16. Key Points
The value or class physical evidence lies in its ability to corroborate events with data in a manner that is, as nearly as possible, free of human error or bias.
As the number of different objects linking an individual to a crime scene increases, so does the likelihood of that individual’s involvement with a crime.
A person may be exonerated or excluded from suspicion if physical evidence collected at a crime scene is found to be different from standard/reference samples collected from that subject.

17. Forensic Databases Fingerprint Databases DNA Databases
IAFIS (Integrated Automated Fingerprint ID System)
Maintained by the FBI
50 million suspects (500 million images!)
Submitted by state, local and federal law enforcement
Many other countries maintain a fingerprint database like IAFIS and these can be available through Interpol
DNA Databases
CODIS (Combined DNA Index System)
Also maintained by the FBI
Enables federal, state and local crime labs to electronically exchange and compare DNA profiles
Forensic Index and Offender Index to match
3 million files with lots backlogged!
Info from other countries available through Interpol

18. Other Databases NIBIN (National Integrated Ballistics Info Network)
Maintained by the ATF
Acquires, digitizes and compares markings
Heart of this is IBIN – Integrated Ballistics ID System
Made up of a microscope and computer unit that can capture the image of a bullet/casing
90,000 pieces of crime scene evidence
PDQ (International Forensic Automotive Paint Data Query)
SICAR (Shoeprint Image Capture and Retrieval)
Privately run, comprehensive shoe sole database

19. Key Points
The creation of computerized databases for fingerprints, criminal histories, DNA profiles, markings on bullets and cartridges, automotive paints and shoe prints has dramatically enhanced the role of forensic science in criminal investigation.
AIFIS is a national fingerprint and criminal history database maintained by the FBI. AIFIS allows criminal investigators to compare fingerprints at a crime scene to an index of 500 million known prints. CODIS is the FBI’s DNA database. It enables federal, state and local crime labs to electronically exchange and compare DNA profiles, linking crimes to each other and to convicted offenders.

20. Crime Scene Reconstruction
Principles of Crime Scene Reconstruction
Physical evidence left behind is crucial
Supports or contradicts the hypothesis and/or witness testimony
Generate leads/assist the jury
Collection/documentation of physical evidence is the foundation of reconstruction
Supports likely sequence of events

21. Steps in Reconstruction
Secure/protect the crime scene
If not protected/secured, evidence could be rendered useless
Prosecution is difficult
Processing the crime scene
Crime scene walk through/drawing/document all observations
Contemplate events that might have occurred
Bag/tag physical evidence
Photograph the scene

22. Steps in Reconstruction (continued…)
Personnel Involved in Reconstruction
Medical examiner
Make observations about livor (settling of blood closest to the ground) as to whether the body was moved, etc.
Clothed/unclothed (livor will not develop if skin constricted by clothing)
Criminalist
Plot the trajectory of a bullet/position of shooter
Analyze blood spatter, residues and other physical evidence
Establish a relationship between suspect, victim and crime

23. Key Points
Crime-scene reconstruction relies on the combined efforts of medical examiners, criminalists, and law enforcement personnel to recover physical evidence and sort out the events surrounding the occurrence of a crime.
Examples of crime-scene reconstruction include determining whether a body was moved after death, determining whether a victim was clothed after death, analyzing bullet trajectory, analyzing blood spatter, determining the direction from which penetrated glass objects, estimating the distance of a shooter from a target, and locating primer residue on suspects.