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Forensic Science: Unit 1 Physical Evidence

Forensic Science: Unit 1 Physical Evidence. Forensic Science: Unit 1 Physical Evidence. EQs : What is the value of physical evidence? What physical evidence can or cannot prove? What are the classifications of physical evidence collected at a crime scene?

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Forensic Science: Unit 1 Physical Evidence

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  1. Forensic Science: Unit 1Physical Evidence

  2. Forensic Science: Unit 1Physical Evidence EQs: What is the value of physical evidence? What physical evidence can or cannot prove? What are the classifications of physical evidence collected at a crime scene? What is the significance of class evidence?

  3. Evidence

  4. Conducting a systematic search for evidence • How one conducts a crime scene • Depends on 3 factors: • Locale (Location of Criminal Activity & Physical Location of the Crime) • Size • Actions of victims & suspects • Recovering relevant physical evidence • Dependent upon the same 3 factors • Ex: Homicide • Weapon • Contact between victim & the assailant • Ex: Burglary • Tool marks at the point of entry • fingerprints • Ex: Hit-and-run • Outside and undercarriage of car

  5. Procedure for Collecting and Packaging Physical evidence • Consideration • Many items of evidence are obvious in their presence, but others may be detected only through examination in the crime laboratory. • Maintain the integrity of evidence • Best maintained when the item is kept in its original condition • Changes can arise from contamination, breakage, evaporation, scratching or bending

  6. Procedure for Collecting and Packaging Physical evidence • All items of evidence • Packaging evidence separately prevents damage through contact & prevents cross contamination • Handling trace evidence • Blood, hairs, fibers, glass, soil particles should not be removed from garments, weapons or other articles. Instead, the entire object should be sent to laboratory. • Exception: if evidence is stuck to an object in a precarious manner or stuck to a large object that can’t be moved.

  7. Tools for Evidence Collection • Forceps • Used to pick up small items • Containers for Trace evidence • Unbreakable pill bottles • Screw top glass bottles • Manila envelopes • Sealable plastic bags • What NOT to use • Do NOT use regular mailing envelopes – small particles can leak out of corners

  8. Special Considerations • Bloodstained materials • Wrapping paper, manila envelopes or paper bags • Moisture can encourage growth of mold, so need air to circulate • Charred debris • Needs air-tight container to prevent evaporation of petroleum products • Clothing • Must be air-dried to prevent growth of mold • Place individually in separate paper bags, so air can circulate

  9. Value of Physical Evidence Physical Evidence can: • prove a crime has been committed • back up (corroborate) witness testimony • link a suspect w/ a victim or crime scene • Determine the identity of people associated w/ a crime • Allow investigators to reconstruct a crime

  10. Manufactured Evidence: Man-made Fibers Explosives Firearms & ammunitions Glass Documents Paint Petroleum products Plastic bags Plastic, Rubber, & other polymer products Powder residues Vehicle (& parts) Serial numbers Chemical Evidence: Drugs Manmade poisons Common Types of Physical Evidence • Biological /Natural Evidence: • Blood • Semen • saliva • DNA • Organs, body parts • Other physiological fluids • Hair • Natural fibers • Soil & minerals • Wood & other vegetative matter • Impressions: • Fingerprints • tool marks • bite marks • tire marks • shoe prints • depressions in soft soils • all forms of tracks • gloves & other fabric impressions

  11. Examination of Physical Evidence 1. Identification of evidence: • The process of determining a substance’s physical or chemical identity (w/as near absolute certainty as existing analytical techniques will permit). 2 requirements: • 1. the adoption of test procedures that give characteristic results for specific standard materials; Standard Laboratory Procedure (SLP) • 2. the # of type of tests needed to ID a substance be sufficient to exclude other evidence.

  12. Examination of Physical Evidence 2. Comparison of evidence: • The process of ascertaining whether 2 or more objects have a common origin. • 2 steps: • 1. the use of a standard/reference sample • 2. draw a conclusion about the origin of the specimen.

  13. Evidence Why can’t one simple analytical scheme be applied to all types of evidence? • Not all evidence is the same-may have different categories • One test type may not be useful for all forms of evidence • Each type of evidence should be looked at individually for comparison

  14. Individual vs. Class Characteristics Individual characteristics: • Material that can be related to a single source; always involves a comparison. • Properties of evidence that can be attributed to a common source w/ an extremely high degree of certainty; narrowed to a single source or person. • Examples: • fingerprints • Random striations (markings) on bullets or tool marks • Comparison of irregular or random tire patterns or footwear impressions • Handwriting comparisons • Broken materials such as glass • ******Weakness: not always able to state w/ mathematical exactness (certainty) the probability that a specimens are of common origin.

  15. Individual vs. Class Characteristics Class characteristics: • Material or evidence that can be associated only w/ a group of items that share properties or characteristics; never w/ a single source. • Ex. Blood samples (blood type), tire tread marks, blood stains • Weakness: not individualized

  16. Probative Value • Definition: The ability of evidence to prove something that is material to a crime. • A way to increase the probative value of class evidence is to find as many different types of objects or evidences as possible to link the suspect to the crime or the victim.

  17. Product Rule • A formula for determining how frequently a certain combination of characteristics occur in a population. • It states that one must 1st determine the probability of each characteristic occurring separately, then multiply together the frequencies of all independently occurring characteristics. • The result is the overall frequency of the occurrence. • F1 x F2 x F3 x……..= Frequency of events • F= frequency as percentage (%)

  18. Product Rule:Multiply the frequencies of all independently occurring instances related to the event in order to determine the overall frequency of occurrence of an event or the likelihood of an occurrence. • 0.26 x 0.85 x 0.02= 0.0044 or .44% (1 in 200)

  19. Product RuleDetermine the frequency that these 3 blood factors will occur in a population-

  20. Product RuleDetermine the frequency that these 4 blood factors will occur in a population-

  21. Blood found at the crime scene has the following genetic markers, CSF1PO, TPOX, THO1 and vWA. A suspect was tested and has all four markers on his DNA as well. How frequent are these combinations found on DNA? What is the likelihood that someone else in his city has that same combination of genetic markers (pop. 85,000)?

  22. Forensic Databases • The ultimate contribution a criminalist can make is to link a suspect to a crime through comparative analysis. • Forensic Database: A collection of forensic evidence held by a governmental or private agency.

  23. Forensic Databases: Fingerprint databases • Integrated Automated Fingerprint Identification System (IAFIS) • A national fingerprint & criminal history system maintained by the FBI • Established in 1999. • Database of ~50 million subjects (how many fingerprints does this make)?

  24. Forensic Databases: DNA databases • Combined DNA Index System (CODIS)- 1988 by the FBI; enables federal, state & local crime labs to electronically exchange & compare DNA profiles, thereby linking crimes to each other & to convicted offenders.

  25. NIBIN • The National Integrated Ballistics Information Network (NIBIN): • Allows firearms analysts to acquire, digitize, & compare markings made by a firearm on bullets & cartridge casings recovered from a crime scene. • Maintained at the federal level by the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF)

  26. IBIS • Integrated Ballistic Identification System (IBIS) • A microscope & a computer captures an image of a bullet or casing. • The images are stored (on a regional server) and compared/ matched against other images in the regional database.

  27. PDQ • International Forensic Automotive Paint Data Query (PDQ): • Database that contains chemical & color information pertaining to original automotive paints. • Established & maintained by the Royal Canadian Mounted Police (MCMP) • Contains the make, model, year, & assembly plant of cars painted • Why /when might this database be used?

  28. SICAR • Shoeprint Image Capture And Retrieval (SICAR): • A nongovernmental computer retrieval system for comparing & identifying crime-scene shoe prints

  29. Review 1. Which database would be used to identify trace evidence of skin scrapings from under a fingernail? • A. IAFIS B. CODIS C. NIBIN D. PDG E. SICAR 2. Which database would be used to identify fingerprints found at a crime scene? • A . IAFIS B. CODIS C. NIBIN D. PDG E. SICAR 3. Which database would be used to match bullets casings found at a crime scene? • A . IAFIS B. CODIS C. NIBIN D. PDG E. SICAR 4. Which database would be used to match shoeprints found at a crime scene? • A . IAFIS B. CODIS C. NIBIN D. PDG E. SICAR 5. Which database would be used to match saliva found at a crime scene? • A . IAFIS B. CODIS C. NIBIN D. PDG E. SICAR

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