Chapter 3 Physical Evidence

What is Physical Evidence?Any and all objects that can establish

that a crime has been committed

OR can provide a link between a crime and it’s victim or a crime and its perpetrator.

Common Types

1. Blood, semen, saliva

2. Documents

3. Drugs

4. Explosives

5. Fibers

6. Fingerprints

7. Firearms and ammunition

Common Types

8. Glass

9. Impressions

10. hair

11. Organs and Physiological fluids

12. Paint

13. Petroleum products

14. Plastic bags

15. Plastic, rubber, polymers

Common Types

16. Powder residues

17. Serial numbers

18. Soil and minerals

19. Tool marks

20. Vehicle lights

21. Wood and vegetative matter

IdentificationProcess of determining a substance’s

physical or chemical identity. Drug analysis Species determination: human or not Explosive residue analysis

IdentificationSteps Involved:

Design systematic analysis that will always test for that substanceTesting must eliminate all other

possibilitiesSome substances require 1-10 tests

FS must be prepared to render a conclusion with respect to the origin of the specimen

ComparisonAttempts to ascertain whether two or

more objects have a common origin.Use properties of suspect and control.Gives conclusions as probabilityTwo types of characteristics

Comparison:Two types of characteristics

Class characteristics: Substances can be associated with a group but

not individual source Blood types: use factors in blood A lot of these can ID suspects at a crime scene

Individual characteristics: Substances that are related at almost 100%

probability Fingerprints are 1x1060 that 2 peoples are the

same

Do Now 10/14:Get out Lab due today: hair analysisPut in team folders

The Product Rule (Probability)

Multiplying together the frequencies of independently occurring events

Rolling 2 dices and getting sixes1/6 x1/6= 1/36

The Product Rule (Probability)

O.J.’s bloodstain frequencies

Blood Factors Frequency

Type A blood 26%

EsD

estérase D

85%

PGM 2+2- 2%

Fiber Evidence and the Wayne Williams Trial Items from residence and station wagon.

Fiber Evidence and the Wayne Williams Trial

Tracking Carpet fiber between the two victims

Assuming: Carpet installed in one room12x5’ roomTotal sales divided between 10 states

SO 82 rooms with this carpet in GeorgiaProduct rule (probability) =1 : 7,792Very low chance

Crime scene reconstructionThe method used to support a likely

sequence of events by the observations and evaluation of physical evidence, as well as statements made by witnesses and those

involved with the incident.

Crime scene reconstruction

For CSRMedical examinerLaw Enforcement personnel,Criminalists

Finis’Finis’