Physical Evidence Forensic Fuel Chapter 3

Lecture Highlights  Negative Controls  Comparison and Identification  Class vs. Individual Characteristics

Crime Scene  In most cases access to the crime scene lasts for about 4 hours  Can’t collect everything!  Want to collect evidence that is likely to –Determine  What happened  What was used  How it happened  Who did what  Rely on logic, common sense, experience, and knowledge of forensic capabilities –Every scene is different

Types of Physical Evidence  Body Fluids –Blood, semen, saliva  Documents  Drugs  Explosives  Fibers  Fingerprints  Guns & Ammo  Glass  Hair  Impressions Paint Gas, paint thinner Plastic bags Powder residues Serial nos. Soil Tool marks Vehicle lights Wood Clothing Etc.

Analysis of physical evidence serves 2 purposes  Identification –Determine the physical or chemical identity of a substance –Is it really heroin?  Comparison –Determining whether two objects have a similar origin –Is Hair from same person? –Is paint chip from that car?

Identification  How do you determine the identity of a substance? –Test it HeroineMethamphetamineSample Marquis Test

When can you be sure?  Identification requires that you are able to exclude all other possibilities –Could any other compound turn the Marquis reagent orange?  Yes  Therefore this test alone in not conclusive (definitive)  Can combine several tests to reach a definite conclusion regarding ID

Negative Control  When sample is attached to some material –Like clothing, sheets, carpet, auto seat, etc.  Need to collect a negative control –A sample of the same material without evidence on it  Bloody carpet –1 cut of carpet with blood –1 cut of same carpet without blood

Comparison  Determine whether two samples come from the same origin –Did the hair found at the crime scene come from the suspect? –Did the carpet fiber found on the victim come from the suspects apartment? –Did the blood found on the broken window come from the suspect?

Comparison of glass  Select properties of the evidence to compare –Puzzle fit, color, thickness, density, RI  Compare these properties  Assess the likelihood (probability) that these sample came from the same origin

Comparison of Hair Rabbit Hair from box in which victim was found Rabbit Hair from suspect’s coat Choose Properties Diameter Color Cuticle Structure Medula

STR DNA comparison Choose Properties Size of DNA fragments

Individual vs. Class Characteristics  Individual Characteristics –Evidence that can be associated with a common source with a high degree of probability  Class Characteristics –Associated only with a group –Not with a single source

Class or Individual?  Fingerprint characteristics –Individual  Blood Type (A, B, AB, O) –Class  Automobile paint chip (year, model, color) –Class  Wear patterns on footwear or tires –Individual  DNA –Individual  Fibers from carpet or clothing –Class  Matching tool marks, striations –Individual

Class Characteristics  Important –Even though circumstantial  Support the theory –Enough class evidence can be very compelling  Enough to exonerate innocent suspects –Victim’s blood is type O –Suspect’s blood is type A –Blood found at scene is type AB

Collection of Physical Evidence  Fibers  Hair  Glass  Flammable Liquids  Blood  Controlled Substances  Latent Fingerprints  Tool Marks  Paint

Evidence Collection

Collection of Fibers  Pick up with fingers of forceps  Place in a paper bindle – druggist fold  Place folded paper in coin envelope –Do not place loose fibers in envelope –Could get lost  Entire garment of cloth item  If fibers are attached to object – collect the object

Glass  Glass from different locations is kept in separate containers  Place small fragments in plastic containers  Large pieces placed in boxes separated with cotton or tissue to prevent breakage  Comparison pieces from window, headlight, bottle, etc.

Flammable Liquids  Clean glass vial with airtight seal –Avoid rubber seal or plastic container  Small samples of soil, wood, cloth, paper should be placed in small clean metal cans –Prevent loss of volatile components  Larger items (wood, upolstry, wallboard) can be heat sealed in KAPAK plastic

Blood  Liquid Pool –Absorb on clean, sterile gauze –Dry at room temp –Refrigerate or Freeze  Dried Blood Stain –If on clothing package entire article in brown paper bag –Small item –Large item  Scrape into druggists fold, seal in envelope  Moisten sterile swab, rub stain, dry, package

Maintain Chain of Custody  Log of what happened to evidence from the time it was collected to the time it appears in court