Fingerprints at a Homicide Scene

If you're writing a murder mystery, you need to know how your investigator will go about solving the crime. I've already covered steps one through three. Today's post is on step four, the search for latent prints. I'm taking this information from the Whodunit book called Murder one: A Writer's Guide to Homicide by Mauro V. Corvasce and Joseph R. Paglino.

I brought this up in the last post, but not all cities have crime scene techs like on CSI. Many cities and towns use their own detectives to search for clues, then send stuff to the Crime Lab if it's necessary. All detectives and patrol officers and can hunt for physical evidence, but only qualified officers and detectives should dust for fingerprints. Unqualified personnel could seriously damage useful prints, making them useless, or worse, inadmissible in court. Make sure you do your research for the city your book is set in--some police departments use only specially trained officers or detectives called Evidence Specialists. Small towns sometimes send one of their officers into a few training programs and get them certified to dust for prints. Really small towns might just call the State Police and ask them to take on the investigation.

There are three main target areas for obtaining prints at a crime scene:

• weapons: guns (handle, trigger, shell casings, bullets still chambered), knives (handle, blade, inside the handle where the blade attaches), lead pipes, lamp bases, candlesticks, etc.
• points of entry or exit to the house/room, like doors and windows. Not just the door knob, but also the wood casing  and the areas around the knobs/latches where a criminal might touch to avoid the knob thinking he's so clever
• flat, hard surfaces like tabletops, drinking glasses, the mirror on the medicine cabinet (if it's likely the perp touched it)

There are several different types of fingerprint powders:

• black, for use on light colored objects
• white, for use on dark colored objects
• silver, for use on mirrors

Fingerprints don't adhere well to porous materials, despite the fact that CSI techs are capable of pulling prints off pretty much anything, so don't get too creative in this part of your story. For detailed instructions about how to lift a print, read page 7 of the book or watch CSI--they get it right with the dusting and lifting off hard surfaces. And while fingerprints are an essential part to any real-life investigation, try hard to not let your crime resolution rely solely on fingerprints. It's thoroughly believable, but it's a bit overdone anymore. Same thing with DNA. It's more interesting for the reader to rely on several clues, combining technical evidence with interviews and your protagonist's expertise. 

Anything I forgot? Comments?

-Sonja