Currently there are two tests that law enforcement uses to determine whether a person has fired a firearm. The first is where the hands are swabbed for gunshot residue. Unfortunately, this type of testing cannot determine when and what type of firearm was used. The second type is to look for residue on a persons clothing. This procedure can sometime prove valuable to determine the distance between the muzzle and the target. Often the residue is so minute that it will take a microscopic analyzation to include chemical testing.
There are limitations to residue testing. It will not tell you the type of firearm that was used. The good part is if bullets are found within the scene, they can be compared to other evidence and they can be traced to the type and caliber of the weapon. According to Brozek-Mucha (2017), the forensic expertise that may produce evidence of a person being involved in a shooting incident has always been the most demanding one and concerned the identification of particles as characteristic or consistent with gunshot residue.
2. Explain in detail how you would collect the following impression evidence located at a crime scene:
a. A shoe print in dry dirt
A shoe print left in dry dirt is called an impression. Upon examination there could be damage such as cracks or rocks stuck in the tread. The wear characteristics will show damage, extreme wear or tread loss and how the person walked in them (leaning harder on one side or another). In dry dirt the casting method will be used. After taking photographs at a 90-degree angle (this helps with the sizing comparison), a powdery stone material will be mixed with water and poured straight in the impression. After drying the impression will be removed and appear to look like a 3-D model of the print itself. Further enhancement can be done by using a photoshop app, powder or chemical stain.
b. Tool mark on a windowsill
Tool marks happen when a tool meets another object or surface and leaves an impression. Suspects typically use wire cutters, crowbars and screwdrivers to cut and pry their way into windows and doors. All these tools leave distinctive marks that can be identified. If possible, the entire windowsill should be cut out and taken into the lab. If you cannot do that take detailed photographs before you use liquid silicone casting to detail all the markings. You will need the details so you can check for striations in the tool itself if recovered. It is also important to keep the tool and the mark in separate containers.
c. Tire marks in soft earth
According to Byrd (2008), in order to recover larger, three-dimensional impressions such as tire marks in soft earth or muddy conditions, experts use casting. The process works in very much the same way as an orthodontist makes a model of a patient’s teeth: A substance is poured into the impression, hardened, and then removed, providing a cast of the print on the ground.
d. Shoe print on a loose piece of tile
This type of pattern evidence imprints can be collected by taking the entire surface area of the imprint. According to NFSTC (2013), examiners generally try to collect the entire object containing the imprint, such sheet of paper or cardboard with a shoe print.
e. Faint shoe print in dust on a colored linoleum floor
A shoe print that is barely visible can be collected using powders and chemicals, along with light sources to identify, and confirm the print. A lifting technique is used to transfer the imprint to another source, allowing for a clearer picture in the laboratory.
All evidence should be photographed at different angles before it is collected.
Brożek-Mucha, Z. (2017). Trends in analysis of gunshot residue for forensic purposes. Analytical and Bioanalytical Chemistry, 409(25), 5803–5811. https://doi.org/10.1007/s00216-017-0460-1
Byrd, Mike. “Crime Scene Investigations: Other Impression Evidence.” Crime Scene Investigator. March 2, 2000. (June 30, 2008) http://www.crime-scene-investigator.net/otherimpressionevidence.html
National Forensic Science Technology Center (2013). A Simplified Guide to Footwear and Tire Track Examination. Retrieved from http://www.forensicsciencesimplified.org/fwtt/FootwearTireTracks.pdf