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When you’re called to a crime scene, you never know what you’re going to find. But no matter the scene, it’s important to recover as much evidence as possible. Sometimes we get so caught up in looking for DNA and other obvious evidence that we overlook other evidence that can be just as important. Impression evidence is a good example. Collecting impression evidence is definitely worth the effort—once you do so, you have duplicate evidence that can help make your case.
When you arrive at the scene, carefully secure the perimeter and limit access. If impression evidence is present, you must do everything possible to preserve it. Next, work on locating the evidence. Impression evidence includes footprints, tire tracks, and tool marks. Often this evidence will be clearly visible. Tire tracks left in mud are a good example. But in some cases, this evidence may be hard to spot. Begin by thinking about the likely movements of a suspect at the scene: How did he enter? Where did he go? How did he exit? Examine all of these points for impression evidence. When you’re outside, look for tire tracks and footprints in the surrounding area. Pay special attention to entryways and windows. Are there signs of forced entry? These are the places where you might find tool marks. Don’t forget to examine hard surfaces such as window ledges, car doors, counter tops, chairs, carpets, tile, and wooden floors for footprints. These types of prints can be difficult to spot; you need to get down low to the surface with your flashlight. Hold the beam at an angle so that the light will cast shadows, allowing you to see the prints.
In many cases, CSOs simply photograph impression evidence once they find it. While it’s true that you absolutely must document all evidence you find with photographs, you shouldn’t stop there. You can collect, and thus duplicate evidence, by creating casts of three dimensional impressions (tool marks and footprints and tire tracks left in mud, sand, or snow) and electrostatic lifts of two dimensional impressions (dust footprints).
Figure 1: Tool mark lifted with AccuTrans
Always document your evidence with photographs before you try to collect it. Collecting impression evidence can be crucial to your case, but if you mess up during the process and you haven’t photographed the evidence, then you’re left with nothing. Take the time to do the job correctly. Take overall and medium range shots that show the location of the evidence in the scene, then move to close ups of the evidence itself. Remember that the quality of the close ups is crucial to the examiner. You need to capture as much detail as possible in your shots so that the examiner can make a comparison with the actual object.
When you’re ready to collect the evidence, you’ll need to use the proper equipment and materials based on the type of impression and where you find it. When you’re casting footprints and tire tracks, use dental stone. If the soil is soft or sandy, first use hairspray to firm it up so that you’ll be able to lift the cast. For moist or muddy soil, use a thin layer of cooking spray oil on the surface as a releasing agent. This will allow you to create the cast and then get it to come up without debris. If the print is in snow, apply Snow Print Wax before the dental stone. You can also use heated sulfur pellets to cast snow prints. This method works well, but it requires more preparation and additional equipment. Two additional materials used for casting include silicone (for footprints and tool marks) Accutrans and Mikrosil (for small items like tool marks, breech face marks on weapons and cartridge casings, and latent finger prints). For a detailed discussion of casting with these materials, please refer to my previous Forensic Magazine articles (April/May 2007 and June/July 2011).
Figure 2: A dust lift.
As noted above, use oblique lighting to identify dust footprints. Once you find the prints, collect them with an electrostatic dust lifter. You can use a commercial product like the PathFinder or use a stun gun set at a low voltage. Begin by placing the dark side of a piece of mylar film on top of the print. Apply a charge to the shiny side of the film. The charge will cause the dust print to adhere with a static charge to the film. Use a foam paint brush to lightly smooth out any air bubbles. Remove the lift film. You should have lifted your print. Be sure to photograph your lifting with side lighting. Because of the static charge, the lift will attract dust and other items to it. Protect it by placing it in a shallow box (such as a clean pizza box). Go to my Web site for more information and videos on electrostatic lifts (http://csigizmos.com/products/dustlifting.html).
Once the evidence is collected and documented, remember to search for the actual items that created the impressions. The examiner will use these for comparison with the photographs and the impressions. Use the details and patterns from the impressions to locate the corresponding items. If, for example, you have an impression from a work boot, you only need to collect the work boots you find, not every piece of foot wear.
Figure 3: A latent lift off a textured surface.
Impression evidence can certainly present many challenges for CSOs: it can be difficult to find, it is easily destroyed, and it takes extra effort to collect. But with the proper care and attention, you’ll be rewarded with evidence that may help seal your case.
Dick Warrington is in research and development and a crime scene consultant and training instructor for the Lynn Peavey Company. firstname.lastname@example.org