1. Physical Evidence

2. Physical Evidence: Learning Objectives
Review the common types of physical evidence encountered at crime scenes
Explain the differences 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 and explain the function of national databases available to forensic scientists

3. Physical Evidence: Vocabulary
Class characteristics
Comparison
Identification
Individual characteristics
Product rule
Rapid DNA

4. Amanda Knox: A Flawed Case of Murder
September 2007
Amanda Knox moves to Italy as a foreign exchange student with the University of Foreigners
Knox shares a cottage with 3 other women, including Meridith Krecher
Within weeks of her arrival, Amanda becomes romantically linked to Raffaele Sollecito

5. Amanda Knox: A Flawed case of Murder
November 1st, 2007—Meridith Krecher is brutally murdered
Her naked body was found in her bedroom covered by a bedspread soaked in blood with stab wounds to her throat

6. Amanda Knox: A Flawed Case of Murder
Police arrest and charge Rudy Guede, Amanda Knox, & Sollecito for the murder of Meridith Krecher
Guede’s DNA had been found in several places in Krecher’s room
Knox’s DNA had been found on the handle of a knife that had Krecher’s DNA on the blade
The knife was found in Sollecito’s apartment
Sollecito’s DNA was found on a bra clasp that belonged to Krecher

7. Amanda Knox: A Flawed Case of Murder
On December 5th, 2009—Amanda Knox is sentenced to 25 years for faking a break in, sexual violence, and murder
October 2011—Knox is acquitted
January 2014—retrial; Knox found guilty
March 2015—Knox is acquitted due to “glaring errors”, “investigative amnesia”, & “guilty omissions”

8. Common Types of Physical Evidence
Blood, semen, saliva
Documents
Drugs
Explosives
Fibers
Fingerprints
Firearms & Ammunition
Glass
Hair
Impressions
Organs & Fluids
Paint
Petroleum products
Plastic bags
Plastic, rubber, polymers
Powder residue
Serial numbers
Soil & minerals
Tool marks
Vehicle lights
Wood & vegetation

9. The Examination of Physical Evidence: Identification
Identification—process of determining a substance’s physical or chemical identity
Crime lab could be asked to:
Identify the chemical composition of drugs
Identify gasoline in residue recovered from arson
Identify the nature of explosive residues
Identify blood, semen, hair, or wood for species origin

10. The Examination of Evidence: Identification
Each type of evidence requires different tests—each test has its own degree of specificity
Identification procedures:
Determine tests needed for identification
Eliminate all other possibilities

11. Individual Characteristics
Evidence that can be associated with a common source possesses individual characteristics
Properties of evidence that can be attributed to a common source with an extremely high degree of certainty
Matching ridge characteristics of two fingerprints, striations (markings) on bullets or tool marks, handwriting characteristics, etc.

12. Class Characteristics
Class characteristics are the properties of evidence that can be associated only with a group and never a single source
EX. paint chips of a similar color can be traced back to a car model, but not a specific car

13. 0.44% of people have that specific combination
Product Rule
The product rule states that multiplying together the frequency of each factor in the blood sample will determine how common blood containing that combination of factors is in the general population
EX. Bloodstain found at the scene of the OJ Simpson case:
Factors Frequency
Type A %
EsD %
PGM %
0.26 x 0.85 x 0.02 = x 100 =
0.44% of people have that specific combination

14. The Significance of Physical Evidence
Forensic scientists must create and update databases for evaluating the significance of class physical evidence
Most of the items recovered at the scene cannot be linked to a specific person
The chances of finding class physical evidence is greater than physical evidence with individual characteristics
The more evidence linking an individual to a crime, the more likely the individual was involved

15. Cautions & Limitations on Dealing with Physical Evidence
Because of the image portrayed by TV and books, physical evidence is heavily weighted during jury deliberations
Physical evidence may exonerate (exclude) a person from suspicion
DEBATE: when does evidence cross the line between class and individual?

16. Forensic Databases
Crime labs are on the forefront of the investigation seeking to identify the perpetrator
Computerized databases link all 50 states & agencies around the world

17. Fingerprint Databases
Integrated Automated Fingerprint Identification System (IAFIS)
Launched in 1999
Fingerprints and corresponding criminal history information
About 500 million fingerprint images

18. DNA Databases Combined DNA Index System (CODIS)
Fully operational in 1998
Enables federal, state, and local crime labs to exchange and compare DNA profiles
Rapid DNA—a process for developing DNA profiles from a buccal swab in 90 minutes or less that are compatible with a CODIS search
TIME IS OF THE ESSENCE!

19. Other Databases
National Integrated Ballistics Information Network (NIBIN)
Compare markings made by a firearm on bullets & cartridge casings recovered from crime scenes
International Forensic Automotive Paint Data Query (PDQ)
Contains information about make, model, year, and assembly plant on more than 13,000 vehicles
National Missing & Unidentified Persons System (NamUs)
Missing Persons database, Unidentified Persons database, Unclaimed Persons database

20. The Examination of Evidence: Comparison
Comparison is the process of determining whether two or more objects have a common origin
A suspect may be placed at a location by noting similarities of a hair found at the crime scene to hairs from the suspect’s head
2 step process:
Combinations of properties from suspect and reference samples for comparison are chosen
Draw a conclusion about the origins of the specimens