What Does a Crime Fiction Author Need to Know About Fingerprints?

Fingerprints are the cornerstone of forensic analysis. Fingerprints left at a crime scene can help determine if a person of interest was present at a crime scene, and give investigators valuable information about criminal history, parole records and violations, among others.

In order to write authentic crime fiction, it is essential that authors grasp the mechanics of fingerprint analysis, and as part of the Daily (w)rite guest post series I’m delighted to be featuring Lisa Black, bestselling author and crime scene investigator. She will speak tell us more about how to distinguish between fact and fiction when it comes to fingerprints in forensic mysteries.

Lisa Black is a full time CSI and NYT bestselling author of the Gardiner & Renner series and now the Locard Institute series. Myths and misinformation run rife in everyday media, making it difficult for an author to know where to look or what to believe. Lisa’s in-depth advice provides invaluable insights into details which are often overlooked. 

On to you, Lisa!

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Fingerprint Analysis: It’s Only Skin-Deep — Physical Characteristics

Let’s talk about fingerprints. Friction ridges are found only on the palmar surface of your hands and the soles of your feet. These patterns are permanent and do not change unless permanently scarred. Sand them off and they’ll grow back. They also develop at random and are not genetically dictated, which is why even identical twins have different fingerprints.

Fingerprint Analysis:  The Collection Process

90% of the time the collection of fingerprints is done just as you see on Dragnet: brush on powder, lift it off with tape (which is basically high-quality clear packaging-type tape) and put it on a white card to be scanned or photographed.

Your best surfaces are shiny porcelain, clean glass, or polished marble. Things go downhill from there. The rougher and more porous a surface gets, the less likely it will be to have fingerprints. When I go to a burglary, first, I look for things the burglar had to have touched—the window, maybe the windowsill, the jewelry box, the medicine cabinet door. Then, of all that, what is a good surface? I look for the jewelry cases that are shiny finished cardboard, not the ones covered in velveteen. The granite countertop, the toilet tank, the quartz sink rather than the cabinets or dresser drawers. Laminate and polished wood may look smooth, but instead it’s finely textured so that it won’t show fingerprints and require less housecleaning. I’ll still try and sometimes succeed, but the odds aren’t good.

Those things that are rougher and more porous—plastic cups, plastic bags, guns—can be processed with superglue in a fume chamber. These fingerprints are then visible, but if the white print is difficult to see it can be stained with fluorescent dye before photographing.

Fingerprint Analysis:  Identifying Individual Differences

When we look at these marks made by skin we might not see much (Figure 1) or a great deal—where the ridges end or divide (bifurcate), very short ridges, creases, scars are the pieces of information that we can use to compare one to the other (Figure 2). 

Fingerprint Analysis: Meeting Your Match — Comparing and Coordinating Patterns

We scan the latent fingerprints (the prints from evidence) into the system and search them. The computer comes up with the top 5 or 10 patterns which most closely coordinate with your evidence print. The computer’s choices might look nothing alike or they might look very similar; your true match might be second or third on the list. So I find one that looks possible. (Figure 3) I pick an ending ridge. I find another ending ridge above it with one ridge in between.

Then there’s a bifurcation to the left of that, crossing over one ridge in between, move up the same ridge to a ridge ending. Hop over one ridge to a bifurcation, then two ridges to a ridge ending, and so on. In the known fingerprint I find the ending ridge. There’s another ridge ending above it with one ridge in between. Then there’s a bifurcation to the left, but without a ridge in between as in the first fingerprint. These two prints were not made by the same finger.

In a different fingerprint (Figure 4) I look for the same starting point and find the next closest piece of information, another ridge ending right next to it. Then to the next, another ridge ending with no ridges in between, then a ridge ending crossing over 1 ridge in between, then up the same ridge to a ridge ending. Hop over one ridge to a bifurcation, then two ridges to a ridge ending, and so on. Then I look at the known fingerprint, the possible match. I find what seems to be the corresponding starting point, my ending ridge. An ending ridge right next to it, another ending ridge next to that, move over one ridge to another ending ridge, over one ridge to a bifurcation…eventually, moving through the whole print, I’m satisfied that all the information in the pattern corresponds between the two fingerprints.

Fingerprint Analysis:  The Side You Don’t See on TV 

If all this sounds monumentally tedious, that’s because it is and why on television they prefer the frenetic light show of a computer screen flashing through tons of fingerprints, which it does not do. The computer does its comparing off screen, hidden in its circuit boards. I’d guess 60% of the prints that the officers turn in are ‘not of value’—without sufficient information for comparison. No reflection on them, that’s just life. Perhaps 10% are the victim’s. A tiny fraction, embarrassingly, are the officers. When people swear they didn’t touch a thing, don’t believe them.

When I do find a matching fingerprint, the computer does not light up with a banner that says ‘match.’ The computer never decides when something matches. Only human beings decide if fingerprints, DNA or bullet casings match.

Yes, not so glamorous. But when I can put a specific person in a specific place, it’s all worth it.

Here’s an extract from Lisa’s second addition to the Locard Institute series — WHAT HARMS YOU:

“The walls were tiled from floor to ceiling, the subway tiles still glossy after all these years. Both were now covered in black fingerprint powder—not the entire walls, only from approximately three feet above the floor to seven or so feet above, the region where a human being’s hand would reasonably land. Ellie saw many roughly rectangular, random patches of relative clean where the fingerprint tape had been lifted, to be placed on a white card and examined because one of her ex-coworkers believed they saw a usable fingerprint there. She didn’t envy them that job, an exercise—nearly an exercise—in futility, since everyone connected with the Locard had likely used the hallway that week. Even those who carried their lunches might come by for the gym, the locker rooms, or to get something out of storage. Or the vault.

And now the cleaning staff would have to wash it off. That would be a sucky job—the fine powder could be washed off the glossy tile with soap and water, but the grout might need a scrub brush.”

Lisa Black’s second Locard book is on sale wherever books are sold. See www.lisa-black.com for more information or to contact her with your forensic questions!

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Do you write fiction? Do you write crime novels, thrillers or mysteries? Are fingerprints and forensics a part of your writing? As a reader, do you pay attention to investigative procedures in crime fiction? Do you have any questions for Lisa?

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My literary crime novel, The Blue Bar is on Kindle Unlimited now. Add it to Goodreads or snag a copy to make my day. The sequel, The Blue Monsoon is up for pre-orders! And if you’d like to read a book outside the series, you can check out You Beneath Your Skin.  Find all info about my books on my Amazon page or Linktree.

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