We’re continuing our series on CSI. In this post, we will look at different types of evidence you might write into a crime scene in your work. All of this information and more can be found in my book Fight Write, Round Two: Crafting Chaos, Combat and Crime.  Give it a look-see!

Types of Evidence

There are five types of evidence: real, documentary, demonstrative, testimonial and digital. Real evidence is physical evidence, something you can physically inspect. All of the examples we will look at today are physical evidence.

A Few Types of Physical Evidence

Footprints

Footprints fall under the category of both impression evidence and biological evidence. As biological evidence, they can tell a bit about the person’s stature, weight, gait, gender or possible deformity. In other words, if your character has a limp or drags a foot, that will be evident. Footprints can also suggest if something was carried. The tread of the shoe can also be vital evidence. Shoe prints may be left in blood, on a soft surface or discovered though the use of alternate light sourcing or chemicals. Foot/shoe prints are such vital evidence, there is actually such a thing as forensic podiatry.

Fingerprints

Most readers will understand the concept of fingerprint evidence. And because there’s no hard and fast rule for how long latent fingerprints last, (the kind you can’t see without the aid of alternate light sources etc) you have a lot of leeway in writing them in your work. The problem with fingerprint evidence, that can add an interesting wrinkle in your plot line, is that there is no proof that all fingerprints are unique. It can also be hard to match to a finger to a print or one print to another.  One study showed fingerprint analysis can be wrong .1% of the time. That may not seem like a lot until until you consider that, statistically, .1% would be 2,000 people currently in the US prison system because of an error of analysis. Also, just another little factoid, some people, because of a genetic defect, lack fingerprints.

CSI Effect

Blood

Blood is biological evidence that can be classified into three stain categories: passive, transfer and impact. Passive stains generally result from gravity’s effect on a body ie blood drops, pools and flows. Transfer stains result from objects coming into contact with the blood. Blood footprints and fingerprints are both transfer stains. Impact stains are the result of blood flying through the air as spattering. Impact stains can also be arterial spurts, gushes and splashes. While the DNA gathered from blood is crucial evidence, it doesn’t automatically tell you who committed a crime. DNA doesn’t identify a person on its own. It can only identify a person if that person’s DNA is catalogued somewhere such as CODIS – the FBIs Combined DNA Index System. But, even if that person’s DNA is in the system, there will be no address attached to the file.

         Other Biological Evidence

Other than blood, types of biological evidence are teeth, bone, sweat, saliva, urine, semen, as well as bodily tissue. In order to find bodily fluids that are not immediately visible, alternate light sources or chemicals might be used. Single wavelength light can highlight biological fluids. Luminol will only react to blood. BCIP (5-bromo-4-chloro-3-indolyl phosphate) reacts only with semen.

Trace Evidence

Trace evidence is small bits of evidence transferred during the crime.  The most common types of trace evidence are hairs, fibers, glass, plant material, paint chips, soil and fingerprints. The three most common are fibers, hair and glass.

          Hair

For hair to be used for DNA evidence, the more hair the better. For the most accurate testing, 6-10 hairs are needed with the root in tact. However, in 2019 there was a breakthrough in gleaning DNA from hair without the follicle in tact. How accessible that technology is  I don’t know. But, it’s out there. Hair is especially helpful as trace evidence is if a suspect is already in custody. Hair from the scene and from the suspect can be compared. Like any DNA evidence, hair shows that someone was at a crime scene. It is not proof that the person was at the crime scene when the crime happened. Unfortunately no DNA evidence is 100% reliable not even when the FBI tests it! The  FBI acknowledged that its testimony on microscopic hair analysis was flawed in at least 90% of cases. Ouch!

         Fibers

Fibers found at a crime scene can be compared to fibers found on the suspect or in a place the suspect frequents. It is less reliable than trace evidence such as hair. Also, it’s not uncommon for fibers to become airborne. If Albert is sitting next to Bob on a train, fibers from Albert’s sweater, or carpet fibers from Albert’s home stuck to his pants, can be transferred to Bob. If Bob commits a crime, Albert’s clothing and carpet fibers may be found at the scene. That’s not to say fibers can’t be valuable evidence. Carpet fibers were instrumental in convicting serial killer Randy Kraft.

CSI Myths

Firearms Evidence

If a firearm was used in a crime, a lot of valuable evidence can be left behind. The bullet itself can have markings peculiar to the gun from which it was discharged. However, the barrel of the gun can be tampered with to keep it from being matched with a bullet. The bullet casing can also be valuable evidence as its caliber will give information about the gun used. Ballistic evidence such as gun powder residue, angle of trajectory, distance from the target, bullet entrance and exit wounds, and damage — can be used to reconstruct the events of the crime and possibly negate a suspect’s version of events. And the gun itself, if it is in custody, can  corroborate or negate an alibi. If Bob says that his gun discharged accidentally, the trigger pressure can be analyzed to verify that. Yes, you can measure the pressure on the trigger. According to firearmshistory.blogspot.com, to measure the trigger pull force, the tester cocks the weapon (making sure it is unloaded first), simply fastens the hook end to the trigger and pulls the gauge backwards until the trigger releases the firing mechanism.

Drugs

Drugs at the crime scene can be valuable in helping piece together what took place as well as a possible motive for the crime. Drug evidence may be in the form of plant material, solids such as meth, cocaine, crack cocaine, and pharmaceuticals, liquids  or paraphernalia like smoking devices, straws, or spoons.

 

If you are writing a crime scene, step onto the page. Walk around. Look at the scene as an investigator would. Find the evidence. In the next post, we will look at the correct way to collect it. Until then, here’s how to cast an impression of a shoe print.