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Chapter 3: Physical Evidence Any tangible objects that can be collected from a crime scene.

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Presentation on theme: "Chapter 3: Physical Evidence Any tangible objects that can be collected from a crime scene."— Presentation transcript:

1 Chapter 3: Physical Evidence Any tangible objects that can be collected from a crime scene

2 Common Types of Physical Evidence Body fluids –Blood, semen, saliva –Liquid/dry –Animal/human Documents –Hand-/typewriting –Paper, ink, etc. Drugs –Any illegal substance –Sale, manufacture, distribution Explosives –Devices - explosive charge –Explosive objects/residues Fibers –Natural/synthetic –Establishes connections between objects/people Fingerprints –Individual evidence –Can be visible/latent (hidden) Firearms/ammunition –Guns/casings/shells, etc.

3 Types of Physical Evidence (continued…) Glass –Glass particles/ fragments in a crime –Bullet holes Hair –Animal/human –Link to a crime/scene Impressions –Tires/shoe prints/tracks –Bite marks Organs/Physiological Fluids –Detection of drugs/poisons or alcohol Paint –Liquid/dried –Transferred during a crime Petroleum Products –Suspect/crime scene –Gasoline, grease, etc –Residues left at a crime Plastic bags –Polyethylene –Homicide/drugs Plastic/Rubber/Polymers –Remains recovered –Link to crime/suspect

4 Types of Physical Evidence (continued…) Powder Residues –From the discharge of a firearm Serial Numbers –Stolen property –Restoration of erased ID numbers Soil and Minerals –Link person/object to a particular location –Soil embedded in shoes, etc. Tool Marks –Causes an impression –Tool in a crime leaves marks Vehicle Lights –Headlights/taillights –On/off at time of impact Wood/Vegetative Matter –Fragments/shavings/twigs, etc. on clothing, shoes, etc. –Link a person/object to a location

5 Key Points Biological crime scene evidence includes blood, saliva, semen, DNA, hair, organs, and physiological fluids; Impression crime-scene evidence includes tire markings, shoe prints, depressions in soft soils, all other forms of tracks, glove and other fabric impressions, tool marks and bite marks; Manufactured items considered common items of crime-scene evidence include firearms, ammunition, fibers, paint, glass, petroleum products, plastic bags, rubber, polymers, and vehicle headlights/taillights.

6 Examination of Physical Evidence Identification –Determine physical/chemical identity –Body fluids can determine the species of origin –Determine the chemical composition of a drug, residue, etc.

7 Identification Identification process –Uniform testing procedures –Scientific process –Sufficient number/type of tests conducted (multiple trials) depending upon type of evidence –Comprehensive enough to exclude any uncertainty –Education/experience of scientist also plays a role

8 Comparison Analysis Specimen in question compared to a known standard Usually a 2-step procedure –Properties are selected to compare the evidence with the standard (reference) –Conclusion(s) drawn about the origins of the sample/specimen

9 Probability If one or more of the selected properties do not agree (between evidence and standard), the scientist will conclude that the specimens are not from the same source Probability plays a role in establishing evidence of value –Frequency of occurrence of an event –Defines the odds of an event occurring and that the evidence is connected to that particular event

10 Evidence: Class Characteristics Associated with a group and not a single source Used for elimination rather than identification Blood, paint, hair and fibers are some examples

11 Evidence: Individual Characteristics Evidence from a single source with a high degree of probability Can be linked to a specific suspect and/or location Examples are DNA and fingerprints

12 Key Points Two methods used by forensic scientists when examining physical evidence are identification and comparison. Identification is the process of determining a substance’s chemical or physical identity to the exclusion of all other substances. A comparison analysis determines whether a suspect specimen and a standard/reference specimen have a common origin. Evidence that can be associated with a common source with an extremely high degree of probability is said to possess individual characteristics. The overall frequency of occurrence of an event can be obtained by multiplying the frequencies of all independently occurring instances related to the event. This is known as the product rule.

13 Significance of Physical Evidence Hard to assign exact or even approximate probability values to most class evidence Very few statistical data are available (mass- produced materials, etc.) Statistical databases must be created/updated to help evaluate the significance of physical evidence Personal experience often plays a role in the evaluation – subjective – can detract from credibility rather than support

14 Assessing the Value of Evidence Lies in its ability to corroborate events with data that is as free from human error as possible Methods/instruments are continually being revised/improved Objects that exhibit significant diversity are appropriate for classification as physical evidence As the number/type of evidence increases to link a specific person to a crime scene, the probability of involvement increases dramatically

15 Cautions and Limitations The weight/significance of evidence in a courtroom is usually decided by the jury Reliability and trustworthiness Given great weight in jury deliberations Proper safeguards must be taken to protect the evidence to prevent bias against the accused –Thorough collection and scientific evaluation –Find as many characteristics as possible to compare one substance/piece of evidence to another –Decided by the quality and composition of the evidence, case history, examiner’s experience –There are practical limits to the properties/characteristics selected for comparison (sophisticated instruments help) Also used to exclude/exonerate someone

16 Key Points The value or class physical evidence lies in its ability to corroborate events with data in a manner that is, as nearly as possible, free of human error or bias. As the number of different objects linking an individual to a crime scene increases, so does the likelihood of that individual’s involvement with a crime. A person may be exonerated or excluded from suspicion if physical evidence collected at a crime scene is found to be different from standard/reference samples collected from that subject.

17 Forensic Databases Fingerprint Databases –IAFIS (Integrated Automated Fingerprint ID System) Maintained by the FBI 50 million suspects (500 million images!) Submitted by state, local and federal law enforcement Many other countries maintain a fingerprint database like IAFIS and these can be available through Interpol DNA Databases –CODIS (Combined DNA Index System) Also maintained by the FBI Enables federal, state and local crime labs to electronically exchange and compare DNA profiles Forensic Index and Offender Index to match 3 million files with lots backlogged! Info from other countries available through Interpol

18 Other Databases NIBIN (National Integrated Ballistics Info Network) –Maintained by the ATF –Acquires, digitizes and compares markings –Heart of this is IBIN – Integrated Ballistics ID System Made up of a microscope and computer unit that can capture the image of a bullet/casing 90,000 pieces of crime scene evidence PDQ (International Forensic Automotive Paint Data Query) SICAR (Shoeprint Image Capture and Retrieval) –Privately run, comprehensive shoe sole database

19 Key Points The creation of computerized databases for fingerprints, criminal histories, DNA profiles, markings on bullets and cartridges, automotive paints and shoe prints has dramatically enhanced the role of forensic science in criminal investigation. AIFIS is a national fingerprint and criminal history database maintained by the FBI. AIFIS allows criminal investigators to compare fingerprints at a crime scene to an index of 500 million known prints. CODIS is the FBI’s DNA database. It enables federal, state and local crime labs to electronically exchange and compare DNA profiles, linking crimes to each other and to convicted offenders.

20 Crime Scene Reconstruction Principles of Crime Scene Reconstruction –Physical evidence left behind is crucial –Supports or contradicts the hypothesis and/or witness testimony –Generate leads/assist the jury –Collection/documentation of physical evidence is the foundation of reconstruction –Supports likely sequence of events

21 Steps in Reconstruction Secure/protect the crime scene –If not protected/secured, evidence could be rendered useless –Prosecution is difficult Processing the crime scene –Crime scene walk through/drawing/document all observations –Contemplate events that might have occurred –Bag/tag physical evidence –Photograph the scene

22 Steps in Reconstruction (continued…) Personnel Involved in Reconstruction –Medical examiner Make observations about livor (settling of blood closest to the ground) as to whether the body was moved, etc. Clothed/unclothed (livor will not develop if skin constricted by clothing) –Criminalist Plot the trajectory of a bullet/position of shooter Analyze blood spatter, residues and other physical evidence –Establish a relationship between suspect, victim and crime

23 Key Points Crime-scene reconstruction relies on the combined efforts of medical examiners, criminalists, and law enforcement personnel to recover physical evidence and sort out the events surrounding the occurrence of a crime. Examples of crime-scene reconstruction include determining whether a body was moved after death, determining whether a victim was clothed after death, analyzing bullet trajectory, analyzing blood spatter, determining the direction from which penetrated glass objects, estimating the distance of a shooter from a target, and locating primer residue on suspects.


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