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