1. Target 2-4 Monday October 30, 2017
Impression Evidence
Target 2-4
Monday
October 30, 2017

2. Target 2-4
I can match impressions by interpreting individual and class indicators.

3. Impressions
Markings left behind when 2 objects make contact with enough force
Can be used to identify objects that were at a crime scene

4. Types of Impressions
Prints show a pattern from the surface of an object
Fingerprints
Tire Tracks
Shoe Prints

5. Types of Impressions
Striations are scratches left on an object when another object moves past it
Tool Marks
Bullets

6. Collecting an Impression
Always take photographs first
Two-dimensional impressions (dust or other powders): electrostatic lift
Use oblique (angled) light to detect footprints
Use static electricity to stick dust to a thin plastic sheet
Photograph again, and be super careful! Electrostatic lifts are very delicate.

7. Electrostatic Lift

8. Electrostatic Lift

9. Collecting an Impression
Three-dimensional impressions (mud, dirt, etc.): cast
Use a precision putty to make a mold of the impression
Casts are more permanent and less fragile than electrostatic lifts
Can be used on any 3-D impression (including tool marks on walls, bullet holes, scratched floors, etc.)

10. Footprint Cast

11. Tool Mark Cast

12. Class Indicators
Class Indicators are information that identifies a class or type of object
Shoe size/brand
Tire make/model
Bullet caliber
Brand of pocket knife
Size of screwdriver
Some classes have sub-classes!

13. Class Indicators

14. Class Indicators – Sub-classes
Shoe Size
Number of molds
Size 6 89
Size 12 60
Size 6 1/2 69
Size 12 1/2 13
Size 7 78
Size 13 42
Size 7 1/2 49
Size 13 1/2 4
Size 8 64
Size 15 14
Size 8 1/2 75
Size 16
Size 9 87
Size 17 12
Size 9 1/2 90
Size 18
Size 10 94
Size 19 2
Size 10 1/2 83
Size 20 3
Size 11 77
Size 21
Size 11 1/2 40
Table 1: Nike Air Force I Molds (Men’s)
Nike Air Force 1 Molds (Men’s) source FBI.gov

15. Class Indicators
Class indicators can be determined based on the crime scene evidence
Once we have identified a class, we can obtain warrants to collect matching objects from suspects

16. Individual Indicators
Once we have a crime scene object AND a matching object from a suspect, we compare them for individual indicators.

17. Individual Indicators
Individual Indicators are specific details that identify a unique object
Wear marks on shoes
Defects, chips, or broken pieces
Bullet striations

18. Individual Indicators

19. Individual Indicators

20. Individual Indicators

21. Mini Investigation
Take off one of your shoes and examine the sole closely.
On one side of a clean piece of copy paper, list as many CLASS INDICATORS for your shoe as you can think of. For example: size, brand, patterns on sole
Put your shoe back on and step on the back side of the copy paper. Examine your footprint and circle (use color) all the INDIVIDUAL INDICATORS you can find.