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.” (http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/july2009/review/2009_07_review01.htm). Next, the suspect weapon is test-fired into a tank of water or a cotton filled recovery box so that bullets, together with their cartridge casings (which serve as a control) may be analyzed. The test-fired bullets are then examined via comparison microscope. This will confirm or rule out a relationship between the control and the bullets retrieved from a crime scene.
According to the Firearms Tutorial, there are three possible results of comparison identification. Test-fired bullets and those retrieved from a crime scene may be related to the same weapon; may not be related to the same weapon; or not even be likened via this kind of test. It is not recommended that forensic examiners make assumptions founded primarily from test firing. Analyzing an image of the bullets and casings may be used to compare and even identify the weapon from which the bullet was fired. (http://library.med.utah.edu/WebPath/TUTORIAL/GUNS/GUNINTRO.html).
Additionally, standards have been established for consecutively matching striae, or CMS. Bullets contain striations as a result of being imprinted with markings on its journey through the barrel of a gun. Bullets are soft and somewhat malleable as they are typically made of lead. As a bullet twists upon firing, the striations will be determined by the direction in which the bullet spins. If the impressions on the bullet vary, then it is safe to say it could not have been fired from the same weapon (Saferstein, p. 209).
The inner workings of a firearm which are responsible for creating impressions on a bullet are as follows: the firing chamber, breech face, firing pin, ejector, extractor and the rifling of the barrel. Characteristics transferred to the cartridge case include: firing pin impression, center of firing pin impression, and ring of firing pin impression. In a study by Md Ghani et al in 2010, 96.7% of the time these features were able to connect the cartridge case to the firearm. (http://library.med.utah.edu/WebPath/TUTORIAL/GUNS/GUNLAB.html).
Tool marks, which are any “impression, cut, gouge, or abrasion caused by a tool coming into contact with another object” (Saferstein, p. 232) are frequently found at crime scenes involving burglary or other forced entry into a building or safe. Typically, tool marks can be made via the use of screwdrivers, knives, or crowbars. The result is an indentation left by the prying action of such implements. Because tools are manufactured with microscopic irregularities, they leave these markings on whatever soft surface they are pushed upon. Simple use of a comparison microscope can determine what appear to be ridges and valleys – thus resulting in a match, or a mismatch.
According to figure 8-26 of Forensic Science: From the Crime Scene to the Crime Lab, collecting a tool mark impression may be challenging. 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 of bloody foot impressions, there are various chemical enhancers that can be used that do not compromise DNA found in blood. With information available and an impression in hand, forensic examiners can access tire makes and models, and shoe brands and styles. In England, an automated shoe print identification system (SICAR, or shoeprint image capture and retrieval) searches databases for known and unknown footwear images. Images are displayed next to each other on a screen.
In conclusion, it is apparent that whatever evidence is collected is handled with extreme care in order to obtain accurate matches that will stand up to legal scrutiny. A good attorney for the defense will take every opportunity to try to disprove evidence that might convict his client.
Works Cited
Bangalor, S. et al. Firearms Tutorial. Retrieved from http://library.med.utah.edu/WebPath/TUTORIAL/GUNS/GUNINTRO.html
Bunch, S.G., Giroux, B.N., Murphy, D.P., Smith, E.D. (2009). Is a Match Really a Match? A Primer on the Procedures and Validity of Firearm and Toolmark Identification. Forensic Science Communications. July 2009 - Volume 11 - Number 3. Retrieved from http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/july2009/review/2009_07_review01.htm
Saferstein, R. (2009). Forensic Science: From the crime scene to the crime lab. Upper Saddle River, NJ. Pearson Prentice Hall.