How can footwear evidence be used to solve crimes? Identification of a suspect (more on this later!) Number of perpetrators Location of prints may help reconstruct crime Confirm / rebut alibis or witness statements Sequencing events Finding other evidence Tracking a suspect Gait characteristics - how a person walks (stride, step length, step width) may indicate whether or not they were injured
How can footwear be individualized? Footwear size and brand can be easily determined. This is class evidence and has limited probative value. Remember: ○ Class evidence – evidence that narrows down items to a smaller group or category (e.g. red Ford Mustangs) ○ Probative value – ability to prove something
How can footwear be individualized? Footwear size and brand can be easily determined. This is class evidence and has limited probative value. How do you think we can individualize footprints? Wear patterns Cuts, scratches, gouges, embedded items
Vocab Check! Visible visible to the naked eye; also called patent. Example: print made with blood or mud Latent nearly invisible to the naked eye. Example: print made with fine layer of oil or dirt Plastic an impression in a moldable material. Example: print made in snow.
Obtaining Footprints Visible prints are typically just photographed. Photographs should be head-on (90 o angle) and should include a small ruler. Why?
Obtaining Footprints Latent prints must be visualized by: dusting with fingerprint powder or lifting with electrostatic lifting devices. Electrostatic lifting devices use electricity to attract the dust of a footprint onto special adhesive paper.
Obtaining Footprints Plastic prints (impressions) First the print is photographed (why?) Then, a cast is made with dental stone, a hard durable plaster. ○ Put a frame around the impression ○ Very soft impressions (e.g. impressions in mud or snow) are sprayed with shellac or lacquer to stabilize them ○ Pour the dental stone into the impression, and allow to set
Quick Review – Think, Pair, Share Scholar on the left answers … 1) Name three types of information that can be learned from footprints. 2) Describe characteristics of footprints be used to identify a suspect. Scholar on the right answers … 3) How is the processing of shoe prints similar to that of fingerprints? How is it different?
Categorizing Tool Marks Tools used during crime leave marks; some of the marks may be used to identify the tool. Indentation marks – Occur when a tool is pressed against a soft surface Example: a screwdriver tip wedged into a window seal It is often possible to tell the type & size of tool from indentation marks – which type of evidence? It is difficult to find individualized characteristics from indentation marks
Categorizing Tool Marks Sliding marks – Occur when a tool slides or scratches across a surface Example: Crowbar sliding against window seal Leaves a unique pattern of striations (lines) Class evidence (type of tool, size, etc.) usually cannot be determined
Categorizing Tool Marks Cutting marks – Left behind by tools that cut through material Combination of indentation and sliding Example: wire cutters Often leave behind unique striations
Preserving Tool Marks 1. Tool marks are photographed 2. Casts are made of tool marks using rubberized silicon because it retains the fine details of the impression 3. The suspect tools are used to scrape or cut a test surface. 4. The striations of the test surface and the evidence are compared using a microscope.
Quick Review: Thumbs up or Down 1. It is often possible to determine the size and type of tool used from sliding marks. False! Indentation marks can provide this information. 2. Striations from cutting marks can sometimes be used to determine exactly which tool was used in a crime. True! Both cutting and sliding marks may leave unique striations. 3. Dental stone is used to make casts of both shoe impressions and tool marks. False! Dental stone is used for shoe impressions, but rubberized silicone is used for tool marks.
Closure What were our objectives? Did we learn them? How do our objectives relate to our unit goal?
Exit Ticket Add to your “Do Now” page 1) List 3 ways shoe impressions can be used to solve crimes 2) Which types of tool marks are most likely to let investigators identify the exact tool used? Why?