Here we are at the last post in the series. Time flies, right?

Most of us are familiar with DNA, the individual blueprint for each of us. DNA testing has come so incredibly far in the last twenty years, almost to the point that we expect DNA to answer most if not all of our questions. That makes CAN an interesting place for writers to stymie the investigator (and the reader).

DNA is a powerful tool, yes, but not without exception in the field of criminal investigation. Consider the following scenario:

The Case of Sally Simms

Sally lives alone in her apartment. In the last three days, she’s had new appliances installed, new cable run for the internet, her boyfriend has visited several times, her sister and her sister’s boyfriend were over for dinner, and she had a coworker over for coffee.

Sally is found dead on the floor of her hallway. The science teams go through her apartment collecting everything they can find and Sally goes off to the autopsy table. The police begin with the list of possible suspects: Sally’s boyfriend, sister, sister’s boyfriend, the appliance installer, the cable installer, and the coworker.

DNA is everywhere. Each person in the apartment left their DNA behind. If Sally’s cause of death was a blow to the head, DNA is not helpful unless the object is found and DNA is recovered from parts of the object only the killer would have touched. Your police detective must turn to other investigative means.

During autopsy, should DNA from seminal fluid be found to match both the boyfriend and the sister’s boyfriend, Sally’s sister is going to want an explanation, but it doesn’t make either of them the killer, especially as Sally was dressed and in the hallway. Scrapings from under Sally’s fingernails might yield DNA, but if the suspect shows a scratch healed beyond the time frame of her murder, it might just mean Sally didn’t scrub her hands or under her nails.

Plausible DNA

Now, in most cases, DNA is eliminated because it has a plausible reason for being where it is. If other lines of investigation develop a suspect, DNA might support it, but not be conclusive. I don’t know about you, but I get a perverse kick out of writing crime stories in which DNA is not a major part of evidence, just because I can and I like the unexpected.

DNA is absolutely helpful in the case of serial crimes such as rape and murder, since unknown DNA from crime scenes is entered into and checked against CODIS, the Combined DNA Index. Once a suspect’s DNA is available for match, he or she could be connected to a series of crimes.

DNA can be collected from almost any surface and from almost any body fluid. Hair won’t give DNA but the hair root (if torn out) can. Naturally shed skin won’t give you much, but scrape your arm across an abrasive surface and it’s possible.  And when stranger DNA is present from a person who has no explanation for being at the location, good chances are you have your criminal.

Biologics

But there’s more. In recent years, we’ve made serious headway into biologics which, in addition to DNA and fingerprints, also includes palm prints, palm vein patterns, facial recognition, ear recognition (mostly in Europe still), Iris patterns, retina recognition, hand geometry, and odor or scent.

What was partially developed for security has become a database of biometric information. The Department of Homeland Security has IDENT, which includes biometric and associated biographic information. The Department of Defense has ABIS, the Automated Biometric Information System. CODIS, which also includes the National DNA Index, will, I suspect, branch into biometrics if it hasn’t already.

We already use biometrics to solve a crime such as Sally’s. Even if the killer wore gloves, should a lamp base or makeshift weapon turn up in good condition, it would be possible to match the size and shape of the hand to the person who wielded it. If her building has solid security camera coverage, facial recognition might identify or confirm a suspect entering and leaving the building near the time of the murder.

The Future is (Mostly) Now

The possibilities are as numerous as the writer’s imagined scenarios.  Consider shifting the story ten years into the future, and the writer could have a lot of fun with solving Sally’s murder.

I hope this series has been helpful. I’ve been working on a series titled Body of Evidence, but I’m not certain you would be interested in reading about the evidence a body can yield after death. Please comment below if you’d like me to post the Body of Evidence series. Though it’s not written in graphic language, some of the subject matter may prove uncomfortable for some readers.

Has anything in this series changed the way you look at criminal investigation from a storyteller’s point of view?

Other posts in this series:

Crime Scenes for Writers: Introduction

Crime Scenes for Writers: Reading the Scene

Crime Scenes for Writers: Fingerprints

Crime Scenes for Writers: Blood Evidence

Crime Scenes for Writers: Ballistics

Crime Scenes for Writers: Trace

Crime Scenes for Writers: DNA and  Biologics

Using trace evidence to solve the crime in your story is a great way to delay the payoff and finger the guilty party. I’ve given a list of the most common types of trace evidence and a brief description of each, though they could all be a post on their own.

Fiber

Clothing, leather, car interiors, carpet, and rope can all provide fibers for the investigators to discover and trace. The Mass Spectrometer is one tool used to identify the chemical makeup of a fiber. For example, carpet manufacturers’ and some fabric manufacturers’ product chemical signatures are in a database or researchable. This could narrow down the year, make, and model in the case of a car, or distribution information in the case of carpet and other fibers.  The most common use of fibers, however, is to match them from the crime scene to the suspect or the suspect’s environment.

Plant

Plants have DNA just like we do, so seeds or pods can often be linked to a specific plant. This might tell investigators where the suspect or the victim spent time as plants are often geographically specific.  Another use would be, for example, the discovery of seed pods in the suspect’s vehicle and matching them to the seed pods under the victim.

Insect

Larvae and living insects aren’t considered trace evidence, but webs, egg casings, insect parts, and evidence of bites or stings can yield evidence as to location, length of time the victim was at the dump site or crime scene, and even season if the crime occurred well before discover.

Hair

Humans and animals shed hair all the time, making hair one of the most common types of trace evidence. Once a suspect is identified, strands of hair can be matched. If the hair is pulled from the root, it may contain enough biological material to get a DNA sample. Pet hair found on or near the victim and also in the suspect’s environment (matched to a specific animal) can also boost the prosecutor’s case.

Fluids

Fluids include bodily secretions, of course, but other fluids include tree sap, cleaning products, chemicals, beverages, oil and gas, and water (pond water, ocean water, etc.). While these give information about the crime, I’ve always thought it would be interesting to use a fluid as the breakthrough piece of evidence. Something totally out of place would stump investigators and extend the tension of the story.

Particulates

I like this sort of evidence, and I’m intrigued about how to use particulates to good effect in a murder mystery. Here are four:

• Paint: In addition to binding chemicals, paint often includes a mineral or metal as a color ingredient. These can be matched to known paint products, vehicles, and other elements of locations. In the world of crime shows, I’ve seen it most often used to identify a vehicle, but wouldn’t it be fun to connect it to an artist’s studio or the statue in the town square?
• Soil: The great thing about soil is that it’s fairly localized depending on the particulates. Sandy, clay-based, loam, fertilized, mineral-rich . . . all give the investigators information they might not otherwise discover.
• Metals: Paint, vehicles, tools, clothing fasteners, soil, weapons, and much more, contain metals that can be traced and/or matched to the crime scene, dump site, or suspect’s environment. We see it used most often to identify, again, a vehicle, but flecks of gold might indicate a manufacturing process, and copper shavings might point to the plumbing or construction industry.

Trace evidence offers so many opportunities for the writer and suspense for the reader. It’s by far my favorite type of evidence to include in stories.

The last post in the series is DNA and Biologics. See you then!

How many ways can you think of to stump the reader and detective with trace evidence?

Other posts in this series:

Crime Scenes for Writers: Introduction

Crime Scenes for Writers: Reading the Scene

Crime Scenes for Writers: Fingerprints

Crime Scenes for Writers: Blood Evidence

Crime Scenes for Writers: Ballistics

Crime Scenes for Writers: Trace

Crime Scenes for Writers: DNA and  Biologics