Presentation on theme: "Chapter 3: Physical Evidence"— 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 fluidsBlood, semen, salivaLiquid/dryAnimal/humanDocumentsHand-/typewritingPaper, ink, etc.DrugsAny illegal substanceSale, manufacture, distributionExplosivesDevices - explosive chargeExplosive objects/residuesFibersNatural/syntheticEstablishes connections between objects/peopleFingerprintsIndividual evidenceCan be visible/latent (hidden)Firearms/ammunitionGuns/casings/shells, etc.
3 Types of Physical Evidence (continued…) GlassGlass particles/ fragments in a crimeBullet holesHairAnimal/humanLink to a crime/sceneImpressionsTires/shoe prints/tracksBite marksOrgans/Physiological FluidsDetection of drugs/poisons or alcoholPaintLiquid/driedTransferred during a crimePetroleum ProductsSuspect/crime sceneGasoline, grease, etcResidues left at a crimePlastic bagsPolyethyleneHomicide/drugsPlastic/Rubber/PolymersRemains recoveredLink to crime/suspect
4 Types of Physical Evidence (continued…) Powder ResiduesFrom the discharge of a firearmSerial NumbersStolen propertyRestoration of erased ID numbersSoil and MineralsLink person/object to a particularlocationSoil embedded in shoes, etc.Tool MarksCauses an impressionTool in a crime leaves marksVehicle LightsHeadlights/taillightsOn/off at time of impactWood/Vegetative MatterFragments/shavings/twigs, etc. on clothing, shoes, etc.Link a person/object to a location
5 Key PointsBiological 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 IdentificationDetermine physical/chemical identityBody fluids can determine the species of originDetermine the chemical composition of a drug, residue, etc.
7 Identification Identification process Uniform testing procedures Scientific processSufficient number/type of tests conducted (multiple trials) depending upon type of evidenceComprehensive enough to exclude any uncertaintyEducation/experience of scientist also plays a role
8 Comparison Analysis Specimen in question compared to a known standard Usually a 2-step procedureProperties are selected to compare the evidence with the standard (reference)Conclusion(s) drawn about the origins of the sample/specimen
9 ProbabilityIf 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 sourceProbability plays a role in establishing evidence of valueFrequency of occurrence of an eventDefines 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 sourceUsed for elimination rather than identificationBlood, paint, hair and fibers are some examples
11 Evidence: Individual Characteristics Evidence from a single source with a high degree of probabilityCan be linked to a specific suspect and/or locationExamples are DNA and fingerprints
12 Key PointsTwo 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 evidenceVery few statistical data are available (mass-produced materials, etc.)Statistical databases must be created/updated to help evaluate the significance of physical evidencePersonal 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 possibleMethods/instruments are continually being revised/improvedObjects that exhibit significant diversity are appropriate for classification as physical evidenceAs 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 juryReliability and trustworthinessGiven great weight in jury deliberationsProper safeguards must be taken to protect the evidence to prevent bias against the accusedThorough collection and scientific evaluationFind as many characteristics as possible to compare one substance/piece of evidence to anotherDecided by the quality and composition of the evidence, case history, examiner’s experienceThere are practical limits to the properties/characteristics selected for comparison (sophisticated instruments help)Also used to exclude/exonerate someone
16 Key PointsThe 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 DNA Databases IAFIS (Integrated Automated Fingerprint ID System)Maintained by the FBI50 million suspects (500 million images!)Submitted by state, local and federal law enforcementMany other countries maintain a fingerprint database like IAFIS and these can be available through InterpolDNA DatabasesCODIS (Combined DNA Index System)Also maintained by the FBIEnables federal, state and local crime labs to electronically exchange and compare DNA profilesForensic Index and Offender Index to match3 million files with lots backlogged!Info from other countries available through Interpol
18 Other Databases NIBIN (National Integrated Ballistics Info Network) Maintained by the ATFAcquires, digitizes and compares markingsHeart of this is IBIN – Integrated Ballistics ID SystemMade up of a microscope and computer unit that can capture the image of a bullet/casing90,000 pieces of crime scene evidencePDQ (International Forensic Automotive Paint Data Query)SICAR (Shoeprint Image Capture and Retrieval)Privately run, comprehensive shoe sole database
19 Key PointsThe 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 ReconstructionPhysical evidence left behind is crucialSupports or contradicts the hypothesis and/or witness testimonyGenerate leads/assist the juryCollection/documentation of physical evidence is the foundation of reconstructionSupports likely sequence of events
21 Steps in Reconstruction Secure/protect the crime sceneIf not protected/secured, evidence could be rendered uselessProsecution is difficultProcessing the crime sceneCrime scene walk through/drawing/document all observationsContemplate events that might have occurredBag/tag physical evidencePhotograph the scene
22 Steps in Reconstruction (continued…) Personnel Involved in ReconstructionMedical examinerMake 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)CriminalistPlot the trajectory of a bullet/position of shooterAnalyze blood spatter, residues and other physical evidenceEstablish a relationship between suspect, victim and crime
23 Key PointsCrime-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.