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PHYSICAL EVIDENCE

Chapter 3

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Physical Evidence• It would be impossible to list all the objects

that could conceivably be of importance to a crime.

• Almost anything can be Physical Evidence.

• The purpose of recognizing physical evidence is so that it can be collected and analyzed.

• What is important? - It is difficult to ascertain the weight a given piece of evidence will have in a case as ultimately the weight will be decided by a jury.

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Types of Physical Evidence• Blood, semen, and

saliva• Documents• Drugs• Explosives• Fibers• Fingerprints• Firearms and

ammunition• Glass• Hair• Impressions

• Paint• Petroleum products• Plastic bags• Plastic, rubber, and

other polymers• Powder residues• Soil and minerals• Tool marks• Wood and other

vegetative matter

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Purpose of Examining Physical Evidence

• The examination of physical evidence by a forensic scientist is usually undertaken for identification or comparison purposes.

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Identification• The object of identification is to determine the

physical or chemical identity with as near absolute certainty as existing analytical techniques will permit. – The process of identification first requires

the adoption of testing procedures that give characteristic results for specific standard materials.

– Second, identification requires that the number and type of tests needed to identify a substance be sufficient to exclude all other substances.

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Common Types of Identification• The crime laboratory is frequently requested to

identify the chemical composition of an illicit drug.

• It may be asked to identify gasoline in residues recovered from the debris of a fire, or it may have to identify the nature of explosive residues—for example, dynamite or TNT.

• The identification of blood, semen, hair, or wood are also very common and, as a matter of routine, include a determination for species origin.

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Comparison

• A comparative analysis has the important role of determining whether or not a suspect specimen and a standard or reference specimen have a common origin.

• Both the standard/reference and the suspect specimen are subject to the same tests.

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Comparison

• The forensic comparison is actually a two-step procedure. – First, combinations of select properties are

chosen from the suspect and the standard/reference specimen for comparison.

– Second, once the examination has been completed, the forensic scientist must be prepared to render a conclusion with respect to the origins.

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Role of Probability• To comprehend the evidential value of a

comparison, one must appreciate the role that probability has in ascertaining the origins of two or more specimens.

• Simply defined, probability is the frequency of occurrence of an event.

• The lower probability evidentiary item has greater value in a court of law.

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Classifying Characteristics• Individual Characteristics

– Evidence with an extremely high degree of probability is said to possess individual characteristics.

• Class Characteristics

– Evidence associated only with a group is said to have class characteristics.

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Individual Characteristics• In all cases, it is not possible to state with

mathematical exactness the probability that the specimens are of common origin.

• It can only be concluded that this probability is so high that is cannot be a random event.

• You ultimately have to convince the Jury using this probability.

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Individual Characteristics• Examples: – the matching ridge characteristics of two

fingerprints– the comparison of random striation markings on

bullets or tool marks– the comparison of irregular and random wear

patterns in tire or footwear impressions– the comparison of handwriting characteristics– the fitting together of the irregular edges of broken

objects in the manner of a jigsaw puzzle – matching sequentially made plastic bags by striation

marks running across the bags

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Class Characteristics• Evidence is said to possess class characteristics

when it can be associated only with a group and never with a single source.

• Here again, probability is a determining factor.• Nevertheless, the high diversity of class

evidence in our environment makes their comparison very significant in the context of a criminal investigation.

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Class Evidence

• Most items of physical evidence retrieved at crime scenes cannot be linked definitively to a single person or object.

• The value of class physical evidence lies in its ability to provide corroboration of events with data that are, as nearly as possible, free of human error and bias.

• Finally, the contribution of physical evidence is ultimately determined in the courtroom.

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Crossing Over• When do you cross over the line from Individual

to Class evidence?• How many striations are necessary to

individualize a mark to a single tool and no other? – How many color layers individualize a paint chip to a

single car? – How many ridge characteristics individualize a

fingerprint?– How many handwriting characteristics tie a person to

a signature?

• These are all questions that defy simple answers and are the basis of arguments.

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Using Physical Evidence• As the number of different objects linking an

individual to a crime scene increases, so does the likelihood of that individual’s involvement with the crime.

• Just as important, 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.

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Reconstruction• Reconstruction - The method used to support a

likely sequence of events by the observation and evaluation of physical evidence, as well as statements made by those involved with the incident.

• Crime-scene reconstruction relies on the combined efforts of medical examiners, criminalists, and law enforcement personnel to recover physical evidence and to sort out the events surrounding the occurrence of a crime.

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The Role of Physical Evidence• The physical evidence left behind at a crime

scene plays a crucial role in reconstructing the events that took place surrounding the crime.

• Although the evidence alone does not describe everything that happened, it can support or contradict accounts given by witnesses and/or suspects.

• Information obtained from physical evidence can also generate leads and confirm the reconstruction of a crime to a jury.

• The collection and documentation of physical evidence is the foundation of a reconstruction.