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Identification Techniques Used for Firearms, Tool Marks, and Other Impressions

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Since the early 20th century, the practice of firearm and toolmark identification or comparison has been a crucial factor in forensic science. Agencies such as the FBI have introduced a standard operating procedure which helps to identify weapons and toolmarks. There are certain criteria that must be met to be considered a match. These criteria include multi-level matching of the items brought from the scene of a crime with the test specimens. (http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/july2009/review/2009_07_review01.htm).
“The examination process typically begins when an examiner receives a suspect firearm, along with bullets (the projectiles) and spent cartridge cases recovered from a crime scene.”
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Examiners may photograph the mark. Liquid silicone material is best used to cast the mark. However, some fine details may still be lost in the cast. It is imperative that a crime-scene investigator never attempt to fit a suspect tool into a mark (Saferstien, p. 233). Should this happen, the mark may become altered and then the integrity of the evidence would be compromised. Best practice suggests packaging the tool and mark separately and to make sure that neither comes into contact with another hard surface so as to not damage characteristics.
Occasionally, investigators may find other types of impressions left at a crime scene. “This evidence may take the form of a shoe, tire, or fabric impression.” (Saferstien p. 235). It is crucial that the impression is preserved, and photographed with a scale. Several photographs should be taken that show obvious detail. A reproduction of the impression should be made and sent to the lab for investigation. If this is not possible, impressions may be preserved and lifted. Lifting material placed over an impression, or even a more advanced material that involves an electrostatic lifting device along with Mylar is available to help transfer it to a film.
Finally, impressions may be cast using a form of gypsum, used for tire or shoe marks, or Snow Impression Wax, for impressions in the snow (Saferstein, p. 238). In the case
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