- Crime Lab
- Crime Scene
- Death Penalty
- Digital Forensic Insider
- Digital Forensics
- Evidence Collection
- Expert Forensic Voices
- Forensic Anthropology
- Forensic Psychology
- Impression Evidence
- Medical Examiner
- Mobile Forensics
- Police Procedure
- Sexual Assault Investigations
- Witness Testimony
In previous issues, I’ve explored everything from death scene checklists to DNA collection to becoming a crime scene investigator. This month, instead of focusing on one topic, I’m going to touch on some of the basics at every crime scene.
I. Evidence: Dust Footprints
The first officer on the scene plays an essential role in the investigation of any crime. That officer can make or break your case, depending on the actions he takes—or fails to take—when he arrives at the scene. Say, for example, that that officer walks into the room at the point of entry. What if that room contains dust footprints? If the officer walks into the room without checking for prints, that evidence will be lost.
When patrol officers and/or first responders arrive at the scene, they should look for dust footprints by taking a flashlight and rolling it along the floor. If they simply shine it into the room from a standing position, they’re likely to miss the prints. When they find prints, they should mark them with plastic or disposable tent markers.
Next, remember to photograph the footprints. Even if you’re going to use an electrostatic dust lifter to retrieve the prints, photograph first. After all, collecting evidence is a one shot deal. If something goes wrong, that’s it; you don’t get a second chance. But if you document the evidence by photographing it, then you have something to fall back on.
When you photograph, set your camera on a tripod, use a side light to illuminate the print, and shoot at a ninety degree angle. Make sure you insert a scale so that you know how big the footprint is. You may need to experiment with the light position to find the best angle. After photographing, you can use an electrostatic dust lifter, if you have one, to retrieve the prints. Be sure to photograph the lift film. Then place the lift film in a shallow, covered box to keep dust off of the surface.
II. Preventing Contamination of Evidence
Clearly, finding and collecting as much evidence as possible is key. But in doing so, it’s all too easy to contaminate the results. How can we avoid contamination?
First, always wear gloves. By wearing gloves, you keep your fingerprints off the scene. Gloves also prevent cross-contamination if you change them every time you move from one type of evidence to another or from one activity to another. If you don’t, you can contaminate evidence for the current case and for future cases. For example, if you’re attending an autopsy and taking photos, you need to change your gloves each time you move from the camera to the body. Otherwise, you may transfer evidence from the body to your camera or vice versa.
Changing gloves can be difficult, but it’s necessary. I made the process easier by putting on one glove, then putting on another glove or two over it. Then when I needed to change gloves, I simply removed the top glove. I never took off the first glove until I was finished; if necessary, I would put a new glove on over the first. Taking these kinds of steps prevents the transfer of evidence from surface to surface.
Problems with contamination also happen with trace evidence. Trace evidence, such as hair and fiber, can help identify the suspect in your case. Since people lose, on average, 150 hairs per day, you may find such evidence at the scene. But remember that the average applies to officers as well as to suspects. And while a suspect may rip his clothing and leave fibers behind, officers working the scene may do the same. You don’t want to have to identify and eliminate hair and fibers that have come from officers. Instead, everyone working the scene should wear Tyvek suits.
One of the best ways you can prevent cross-contamination is by paying attention to your equipment. For example, let’s say that you’re working a scene and you’re dusting for fingerprints. As you move your brush over the area, you get a smudge, which you then swab for DNA. If you then use that same brush later at a second crime scene, you may transfer DNA from the earlier scene to the new scene. Now you have DNA at your current scene that you can’t account for. To avoid such a cross-contamination problem, always use a new brush or applicator.
Any equipment that comes into contact with evidence can pose a problem. For instance, tent markers should be disinfected every time you finish a scene or you can end up transferring blood from one scene to another or to you. If you’re not going to be diligent about disinfecting, get disposable markers instead. Go to my Website to learn how to make your own (www.csigizmos.com/products/evidencemarkers/dismarkers.html). You should also check your camera and your tripod. If there’s blood at the scene, for example, it can get on your tripod. Again, you need to disinfect. Another option is to make “booties” from disposable gloves and place them on the bottom of the tripod legs. Such measures will make your job much easier and protect your scene.
When it comes to processing a scene, you need to focus on the basics. Knowing what evidence to look for and how to prevent contamination is essential. When you pay attention to these areas, you’ll recover evidence that is often missed, but can be crucial in helping you tie your suspect to the crime and make 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