I’m really interested in forensics and would love to write a short story on some kind of criminal investigator, but I realize I’ve never really gotten a grip on how the whole process works.
So, to ask morbidly: A witness finds a dead body with clear foul play, they call the police, what happens next?
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I’m a first year student in a forensic science course at my university and it is honestly a really interesting question you’re asking. The short answer is: it’s dependent on way too many factors and variables.
Let’s say you’re a crime scene investigator called to a murder scene and there’s a dead person. So, what do you do? Collect evidence and minimise contamination. There’s so many kinds of evidence you can collect that it’s a little ridiculous. There’s the obvious ones, like lifting fingerprints from the murder weapon or recovering a sample of blood to test whether it belongs to the victim or another individual.
Then you have other techniques and routes you can take, all dependent on the circumstances of the crime. Maybe you’ve noticed some shoe prints near the point of entry of the house. You can (and should) collect this evidence by lifting the shoe print with a special tape or by using a casting agent and making a mould of the impression of the shoe print if its an indentation foot print (a 3D foot print, like in snow or mud). With this print, you can study the sole pattern of the shoe and search for little indications of wear and tear on the bottom of the shoe that make that shoe unique to absolutely any other in the world. Something as small as a little cut in the sole of your shoe at a specific spot because you stepped on some broken glass one time will make your shoe unique to any other shoe. The police can then compare this footwear mark to others on a footwear database, or to a suspect’s shoes (if there is a suspect in this case). Forensic scientists can even learn about your gait (how you walk, such as putting more weight on the inside or outside of your soles) just by looking at the marks of your shoes.
If there’s a tool mark somewhere in the scene, like when a hammer is hit against the wall, the crime scene investigator can similarly cast a mould of the indentation and study the mould under a microscope. Every tool that is manufactured will have small striations on it that will make them identifiable, so you can possibly determine what tool (or at least what brand of tool) made that mark. Furthermore, you can study the head of this hammer to look for paint chips that are resting on it after it hit that wall. Obviously, if a suspect claimed to own this hammer but claimed to not have hit that wall, the presence of paint chips from that wall on the hammer would support the idea that the suspect was involved.
Let’s say you find little fibres from clothes at the murder scene. You can lift those fibres with a tape and study them under a microscope, looking at their colour and structure. Then, you can take a control sample of fibres from the suspect’s clothing and compare the fibres from the suspect’s clothing to the fibres at the scene. If they match up, the chances are that the suspect was at the scene.
Similar to fibres, the suspect’s clothing can be analysed for tiny shards of glass. These little glass fragments can be compared to the broken glass of a hypothetical broken window in a house using different scientific techniques like GRIM (glass refractive index measurement) to confirm that the the glass from the house and the glass from the clothing are of the same composition, and thus likely the same window.
There’s an abundance of ways evidence can be collected, but some of it becomes less available than others after time. Trace evidence like fibres can be blown away in the wind because they’re so small, so it’s important that a crime scene investigator collects the most fragile evidence first and avoids contamination of the scene by wearing PPE (personal protective equipment) and following a bunch of boring yet important guidelines and precautions. Once evidence is collected, it’s given over to forensic laboratories and the intelligence gained from that can then be circled back to the police if need be.
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