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We all know that evidence from a crime scene can make or break a case. The amount of evidence identified, collected, and processed is dependent on the crime scene officer’s training or lack thereof. If you fail to fingerprint certain objects or surfaces, you can leave yourself open to questions from defense attorneys if your case goes to trial. You can prevent that ‘Pandora’s Box’ from opening if you are as thorough as possible. In this issue I’ll take a look at different types of prints and the decision-making that goes into processing them.
One of your first tasks is to make a preliminary survey of the crime scene to determine what needs to be processed. This initial survey will dictate what technique you’ll need to use to develop or lift prints. Depending on the technique needed, you may be able to process at the scene. For other techniques, you’ll need to take the evidence back to the office or send it to the lab. If you know what the experts can do, you’ll be sure to collect the proper evidence.
To begin with, let’s take a look at the types of prints you may find and how you should handle them. The three types of prints are latent, patent, and plastic. Of these, latent prints are the most common type found at a crime scene, so we’ll consider them first.
Of course, latent prints aren’t visible to the naked eye, so finding these prints may be easier said than done. Begin by looking for any items that appear to be out of place or the victim has advised you have been moved. Next consider any surfaces and materials the suspect may have come in contact with. Remember that latent prints can be lifted from nonporous materials like glass, metal, dressers, and other furniture, so make sure your search is thorough.
Once you identify the latent prints, you have to decide how to develop them. The type of evidence and your own resources and training will dictate how you proceed. Depending on the print, you may need to develop it mechanically or chemically. Prints left on nonporous surfaces are easily processed with brush and powder. Other prints require chemical processes like fluorescent dyes or superglue fuming. In some cases you can do the processing at the scene. For example, if you have a fuming wand, you can use it for small items at the scene, but you need to be sure that you have the proper ventilation. In some major cases you may need to superglue an entire room for latent prints; you may need to call in experts to assist you with that process. Usually, though, you’ll take the evidence back to the office or the lab—wherever you can process it with the proper equipment in a controlled environment.
When you’re working with latent prints, the age of the evidence can also be a factor in processing. For example, if you have fresh prints on documents, you can use standard fingerprinting powder or magnetic power. But because paper is porous, the prints will eventually seep into the surface and you won’t be able to lift them with powder. If you’re not sure of the age of the evidence, you can follow the PINS processing technique, which stands for powder, iodine, ninhydrin, silver nitrate. The chemical methods are best done back at your office or at the lab in a controlled environment. Begin with powder. If you don’t get a print, move on to iodine, then ninhydrin, and finally silver nitrate if necessary. Once you use one of the chemicals you can’t go backward, though.
The next category of prints is patent prints, which are readily visible because they contain a contaminant like blood or grease. Bloody or greasy prints left on walls or floors should be photographed, processed, and lifted at the scene if possible. Bloody prints can be processed with Amido Black (also known as Naphalene blue-black), which is a chemical process that reacts with the protein in blood, staining it blue-black. Processing a partial footprint or fingerprint with Amido Black may allow you to see much more detail than previously seen.
But in some cases the print can’t be lifted, so you have to photograph it and preserve it. For example, you can’t lift bloody prints left on glass, tile, and clothing and other fabrics, so you have to take them with you. I’ve also worked on cases where we’ve removed the entire windowsill or door to preserve blood prints.
Another type of patent print is a dust print. Dust prints may be visible on cars in rural areas where a lot of dust blows up or on interior surfaces that haven’t been cleaned well. These prints are fragile, so be sure to photograph them carefully. You can then use an electrostatic dust lifter to lift the print. Do not use the electrostatic dust lifter to lift dust prints off electronic equipment as it will ruin it.
Plastic prints, which are found in soft surfaces like putty, clay, and wet paint, are also visible to the naked eye. For these prints, you’ll need to make an impression. Again, don’t forget to photograph your evidence first. No matter how many times you’ve processed evidence and made impressions, you can still have something go wrong, so you need that photographic backup. Depending on the item you’re processing and your resources, you may be able to make your impression right at the scene. In other cases, you may need to take the item with you. Sometimes you have to get creative. I once discovered a beautiful fingerprint left in some butter. In order to use it, I had to put it in another container without disturbing it and then pack it in ice to get it back to the office. Then I had to freeze it so I could use Mikrosil to make an impression. Getting that kind of evidence may take some extra work on your part, but it can make all the difference in your cases.
When you’re working with print evidence, the decisions you make are crucial. For some evidence, you won’t have any choice—you’ll have to take it back to your office or lab. For other evidence, though, you will have a choice. Being prepared with the right equipment and knowledge will help you get the best evidence and build the strongest case.
Dick Warrington is in research and development and a crime scene consultant and training instructor for the Lynn Peavey Company. For the past several years, Dick has been teaching classes throughout the U.S. and Canada, trying to dispel some of those “you can’t do that” myths. Dick can be reached at firstname.lastname@example.org.