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Bureau of Criminal Apprehension

A Division of the Minnesota Department of Public Safety


The Trace Section uses a wide variety of instrumentation, microscopes, and chemical testing techniques to examine several different types of evidence.


The Chemical Testing Section performs fire debris analysis and chemical unknown analysis.  The identification or characterization of these unknown materials is accomplished by the examination of their physical properties, wet chemistry techniques, and/or using analytical instrumentation.


The Trace-Microscopy Section performs analysis on many types of materials.  Many of the examinations involve the comparison of two or more objects to see if they could have had a common source using chemical, physical, and/or optical characteristics of the material to draw conclusions.  The Locard Exchange Principal is one of the principal theories that this type of trace evidence is based on.  A French scientist, Edmund Locard (1877-1966) theorized that each time two objects come in contact with one another there is an exchange of material.  Trace evidence helps solve crimes by linking people, places, and things involved in a crime by the microscopic materials they transfer through contact.

Here are some examples:

  • A hair found on a hat left at a scene compared to a known hair sample from a suspect.
  • Glass found on a suspect’s clothes compared to glass from the scene.
  • Shoeprints left at a scene compared to shoes from the suspect. 
  • Tape from a bound victim compared to a roll of tape from the suspect’s car.