October 10, 2014 Physical Evidence

Transcription

1 Physical Evidence Common Types of Physical Evidence: - blood, semen, saliva - paint - documents - petroleum products - drugs - plastic bags - explosives - plastic, rubber, polymers - fibers - powder residue - fingerprints - serial numbers - firearms and ammunition - soil and minerals - glass - tool marks - hair - vehicle lights - impressions - wood/vegetation - organs/physiological fluids

2 Significance of Physical Evidence 1. Identification * its purpose is to determine the physical or chemical identity of a substance with as near absolute certainty as possible * requires the adoption of testing procedures that give characteristic results for specific standard materials * also requires that the number and type of tests needed to identify a substance be sufficient to exclude all other substances 2. Comparison - subjects a specimen and a standard/reference specimen to the same tests and examinations to determine whether or not they have common origin - two step procedure: combinations of properties are chosen from the specimens for comparison and then conclusions are drawn about the origins of the specimen (involves probability)

3 Individual vs Class Characteristics Individual Characteristics: - properties of evidence that can be attributed to a common source with a high degree of certainty EX: ridge characteristics of fingerprints; random striation markings on bullets/tools, irregular wear patterns in tire/ footwear impressions, handwriting characteristics Class Characteristics: - properties of evidence that can be associated with a group and never with a single source - weakness of forensic science is the inability of the examiner to assign exact probability values to the comparison of most class evidence EX: blood type, paint chips, brown hair, cotton fiber

4 The body of a woman was found with evidence of beating about the head and a stablike wound in the neck. Her husband was charged with the murder. The pathologist found a knife blade tip in the wound in the neck. The knife blade tip was compared with the broken blade of a penknife found in the trousers pocket of the accused. Note that in addition to the fit of the indentations on the edges, the scratch marks running across the bade tip correspond in detail to those on the broken blade.

5 The bound body of a young woman was recovered from a river. Her head was covered with a black polyethylene trash bag (shown on the right). Among the items recovered from one of several suspects was a black polyethylene trash bag (shown on the left). A side by side comparison of the two bags' extrusion marks and pigment bands showed them to be consecutively manufactured. This information allowed investigators to focus their attention on one suspect, who ultimately was convicted of the homicide.

6 Value of Class Physical Evidence - has the ability to corroborate events with data that is free of human bias and human error - defining significance of class physical evidence exact mathematical terms is difficult, if not impossible - dealing with more than one type of class evidence leads to an extremely high certainty that they originated from the same source - may also exclude or exonerate a person from suspicion

7 Tool Marks Tool Mark: any impression, cut, gouge or abrasion caused by a tool coming into contact with another object - most often are encountered at burglary scenes involving forcible entry - edges of tools display a series of microscopic irregularities that look like ridges and valleys - one of the major problems associated with tool mark comparisons is the difficulty in duplicating in the lab the tool mark left at the crime scene - when practical, entire object or part of object bearing a tool mark should be submitted to crime lab for examination - if it is impractical, need to take photos of mark and impressions

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11 Other Impressions - may include shoe, tire or fabric impression - primary consideration in collecting impressions at a crime scene is the preservation of the impression or its reproduction for later examination in the crime lab - should always be photographed to show observable details - if impressions are on surfaces that cannot be submitted to the lab, investigator needs to preserve the impression (uses a method similar to that of lifting finger prints) - if impressions are on soft earth, preservation is accomplished by photography and casting - test impressions compare the characteristics of the suspect item with the evidence impression - evidential value is determined by the number of class and individual characteristics found

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15 Human Bite Marks - human bite marks on skin and food are important items of evidence in homicide and rape cases - if a sufficient number of points of similarity between test and suspect bite marks are present, forensic odontologists may conclude that a bite mark was made by one individual

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