How Criminalists “Touch” Evidence
This information might be better suited for a screenplay but I think you’ll find some value in it for novels as well. Forensics is about observation. Good forensics is understanding what you’ve observed. Try an experiment I’ve asked a number of interns to complete. Over the next few weeks watch how people handle certain objects. To simplify things you can watch for a certain activity (like opening doors) each day. How do people enter through a door? Do they grab the handle going in? What about coming out? If they hold the door open, where do they grab it? Don’t just look at buildings, watch people getting in and out of vehicles too.
The next day watch how people hold a glass or bottle while drinking from it. Do they grab it in approximately the same place each time? Do they clasp it with all of their fingers or just a few? The watch people reading a book, reading a piece of paper, or looking at a photograph. It may sound kind of goofy but as criminalists we want to understand how people handle things because that tells us where we expect to find fingerprints. More importantly, it tells us where not to touch. This only applies to smooth areas that can hold a fingerprint in the first place. If the area is textured (like the grips of some pistols) or if it is very small like the tab on a beer can we can safely assume no comparable fingerprint is there.
Notice how I handle the beer can by the tab while dusting for fingerprints
Think about it. If we handle objects in the same manner that the average person does then we run a greater risk of smudging the very evidence we’re searching for. That’s the last thing we want. For example, if I’m entering a crime scene (say a bank) through the door I’m not going to just grab the handle and yank it open. If I have to grab the handle I’ll grab the top corner or edge of the handle to get the door open a bit and then I may grab near the top of the frame to open it all the way. We never want to touch where the suspect likely touched.
After you’ve spent a week or so watching how people handle objects ask yourself how you’d do the same activity without touching the same areas they did. Many times we have to handle objects by the edges or by any area too small to hold a fingerprint. It makes our job a little more challenging sometimes but the reward can break a case wide open.
Posted on June 19, 2011, in The Crime Laboratory, The Crime Scene and tagged Crime Scene, csi, fiction, finger print, fingerprint, forensics, gun, thriller, tom adair. Bookmark the permalink. 4 Comments.
This all sounds rather common sense, but I never thought about actually observing where people put their hands for everyday activities, such as opening a building door or car door. Thanks for the tip.
I look forward to receiving the next post in my inbox. ( :
It takes a monumental geek to actually think about sitting and watching other people do simple tasks
I have one quick follow up. When you go in to a crime scene, what special clothing do you wear (or maybe that can be another post). I mean what goes over your shoes and are the gloves you use plastic?
I live in Japan and a lot of what I see are Japanese TV detectives. Here, they always pull out white cloth gloves when they are handling evidence. It makes me wonder what kind of differences exist between police procedure in one country vs. another. It certainly means as a writer that you need to research your department carefully.
I have noted (personally, not in posts) the differences between criminalists in various countries and it’s a fascinating topic. Certainly there are both cultural and legal influences for these decisions. To answer your question specifically, it really depends on the crime scene. I’ll say in Colorado it is rare to see criminalists wear disposable booties or full Tyvek (bunny suits) coveralls unless at a homicide scene. Even then, you may not wear them (if it is outdoors, or something like a drive-by shooting where there is likely to be little in the way of trace evidence from the suspect, or even when the suspect and victim co-habitate). When I have worn such gear it was for personal protection and not “evidentiary” in motive. Frankly, I’ve never bought into those arguments. All to often it is an “individual” decision unless there is a departmental policy requiring it in the situation.
In my experience, nearly all criminalists in developed countried wear latex gloves. Some wear vinyl if they have an allergy to latex. The gloves can be both powdered (inside) or powder-free. Mostly this just aids in putting them on. A lot of photographers will wear cotton gloves because “in the old days (i.e. 10 years ago) that’s what you wore in the photographic dark room while working with film negatives. Cotton gloves obviously absorb sweat and could transfer that residue to any surface you touched. In the DNA dominated world we operate in that could spell trouble. I’d be interested to see if any Japanese scientists have studied this. Have you ever contacted a local agency there? I’m wondering how “open” they are to civilian communications or public info sharing.