Crime Scenes for Writers: Fingerprints

I’ve been aware that my finger prints are in a federal database for twenty-five years. My husband often jokes that I couldn’t get away with theft or murder because of it. If you were ever employed by an SEC controlled business such as I was, or have worked in a daycare in the last fifteen years, your prints are on file as well.

As far back as 1684, academics lectured on the subject of finger prints. In 1788, Johan Mayer declared our finger prints to be individually unique, but it wasn’t until 1892 that finger prints were used successfully in a criminal prosecution. The man who testified, Juan Vusetech, created a classification of loops, whorls, and arches still used today.

I should interject here that ears, a long-used identifier in Europe, are just as unique as finger prints. However, technology is advanced enough for the matching of foot, toe, and elbow prints as well. Scientists can also match whole or partial palm and heel prints as well, as long as there’s a print with which to compare.

There are two types of finger prints that can be recovered from a crime scene. The first, patent prints, are visible to the naked eye. The second type, called latent prints, must be developed to become visible.

The technique of dusting (using a fine brush of powder above the print, not actually on it) and then lifting a print with tape has been in use since the early 1900s. We’ve come a long way from the black powder and tape method, though. These days there are light powders for dark surfaces, fluorescent powders for textured or multi-colored surfaces (used in conjunction with a black light), and more recently, a laser technique that illuminates the salt and other compounds on the skin’s surface to reveal a clear, photographable print.

For surfaces suspected of holding prints that can’t be raised by more conventional methods, there are two more weapons in the lab’s arsenal. The first involves exposing the surface to vaporized superglue to reveal the print. The second, which is often used only when nothing else works, involves fine gold particles in a metal vacuum to “paint” the prints for visibility. Inroads and successes have also been achieved in lifting prints from skin and porous surfaces.

There are a few limitations to using fingerprints to identify a subject. Since most murders are committed by someone known to the victim, prints are useless unless it can be proven that the suspect has never been at the crime scene or come into contact with the items found there. If the suspect has, for example, often visited his now deceased neighbor, his prints are meaningless to prosecution as they could have been left at any time (unless of course they are left in the victim’s blood). This is a great opportunity for the writer to implicate or draw attention away from a suspect character. If, for example, the reader knows the suspect has been in the house, or has not, but evidence is either to the contrary OR excluded, you ratchet up your reader’s tension.

The other problem with prints is that, while they are unique, they can be quite similar. Print experts use a point system for comparison. Computers are able to match more, but the eight-point match is the baseline. Further points must be matched to eliminate similar prints. It is a time-consuming process and often completed by hand, not computer.

Society is quite aware that finger prints are easily left and easily found, leading more criminals to use gloves. While an actual print can’t be recovered from a gloved hand, size and grip are often evident as the gloves leave something behind as well. And, should the criminal foolishly leave his gloves behind, the prints can be recovered from inside the gloves or on their surface (as part of the process of putting the gloves on).

Prints have been recovered from places people don’t normally think about such as the inside of a belt buckle, a shell casing, the sticky side of duct tape (even after soaking in river water), larger jewelry pieces, paper, faucets, and the undersides of tables, chairs, and countertops. Basically, anywhere someone might rest a finger, a print might be recovered, and the technology will only improve with time.

Because prints are so commonplace and well known to most of us, I think they are often overlooked by both writers and readers, which is a shame. It’s an old method jazzed up by new technology, but still not as sexy as DNA or sensational trace evidence. Still, in the right circumstances, all a jury needs is one single print to convict.