Investigations | Section 3


Fundamentals of Criminal Investigations

Adam J. McKee, Ph.D.


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Section 3: Physical Evidence

Physical evidence has the potential to play a critical role in the investigation and resolution of a crime.  For this potential to be realized, action must be taken early on at the crime scene. It is critical that physical evidence be properly collected, documented, and preserved.  Increasingly advanced technologies have given investigators a much wider array of tools, but have also place added importance on proper collection and preservation techniques.  For evidence to ultimately have a significant role in court, it is critical that investigators follow an “objective, thorough, and thoughtful approach” (NIJ Technical Working Group on Crime Scene Investigation, 2000, p. 1).

To understand what evidence is used for and thus what is appropriate evidence to collect, the uses of evidence must be considered.  Physical evidence can be used to help reconstruct the crime scene, help determine if a crime has actually occurred, help link a suspect to a victim, help link a suspect to a crime scene, help link serial crimes, provide investigative leads, and provide facts to a jury.  Note the prevalence of the term link. It is safe to say that a major function of physical evidence is to provide linkages that, taken together, indicate guilt. It is rare that an investigator will encounter a crime where there is direct evidence of a suspect’s guilt.

Rather, the proof beyond a reasonable doubt necessary in court is established by a series of circumstantial evidence items demonstrate guilt only when taken together as a whole.  For example, a suspect’s fingerprint on a gun does not prove guilt directly. Standing alone, this evidence is not very compelling. But if we add to this the fact that the bullet removed from the victim’s body came from that gun and the facts that the suspect had motivation and opportunity to kill the victim, the overall picture becomes much more convincing.

Physical evidence can be categorized in several ways.  One particularly useful way to do this is to consider the physical nature of the evidence.  This often dictates which type of forensic science specialist will analyze the evidence when an investigator sends it to the crime lab.  Common categories include drug evidence, friction ridge evidence (fingerprints, palm prints, footprints, etc.), firearm evidence, biological evidence (blood, semen, etc.), trace evidence (microscopic transfer evidence), document evidence (paper, inks, handwriting, etc.), physical matching evidence, and toxicology evidence.

 

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File Created:  05/02/2019

Last Modified:  05/16/2019

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