Crime Library: Criminal Minds and Methods

All about Fingerprints and Other Impressions

So How Does It Work?

A fingerprint (AP)
A fingerprint (AP)

In Hidden Evidence, David Owen points out that the undersides of our fingers (and feet) are covered with ridges and valleys. Some are continuous lines, some are interrupted, some fork, and some make formations like pockets and dots. It's the relationship of all of these formations—-called minutiae—-that gives the print its uniqueness. These patterns are divided into four basic groups, with subgroups, making eight overall pattern types:

  • Arch: a ridge that runs across the fingertip and curves up in the middle, and the tented arches have a spiked effect.
  • Whorl: an oval formation, often making a spiral pattern around a central point. There's a plain whorl and a central pocket loop whorl.
  • Loops: These have a stronger curve than arches, and they exit and enter the print on the same side. Radial loops slant toward the thumb and ulnar loops away from the thumb.
  • "Composites" mix two of the other patterns, while "accidentals" form an irregular pattern that's not classifiable.

As mentioned above, the way fingers leave readable impressions is from sweat mixed with amino acids from the body, by touching something like paint that clings to the skin, or pressing a finger into a moldable substance like soft wax that records it in three dimensions. Touching any surface transfers the perspiration present on the ridge and in the valleys, leaving an impression of the details. Sometimes it's only a partial impression, but that can be sufficient to provide a lead. According to Henry Lee, there are three basic forms:

  • visible prints (also called patent), as those made in ink or blood
  • latent prints are invisible, except under certain procedures
  • plastic prints are left in soft surfaces

The trick is to make invisible prints visible, which depends upon the type of material on which the prints were left. The more irregular or absorbent the surface, the more difficult it is to lift a good print, although advances are always being made. For example, it was once impossible to lift prints off plastic bags or human skin, but now there are ways to do it.

At first, prints were developed on nonporous surfaces using a soft brush with fine, gray-black dusting powder, and this is still practiced today. It works best with fresh prints, before the oils dry. The excess powder is blown off, leaving a clear impression from the powder that adheres. The print can then be photographed and lifted with a tape. It's placed onto a special print card for preservation.

Besides powder, other methods were developed for surfaces like paper and cardboard. Prints could also be developed with chemicals like iodine, ninhydrin, and silver nitrate. Then along came digital imaging, dye stains and fumes, and now more than forty methods are in use. Colored powders were developed to contrast with surface colors, and some powders or dyes even glow under alternative light sources. These days, surfaces are examined in darkness under a high-powered laser. The least destructive method is always tried first.

Super glue is also a technique in wide use, although it has some dangers. The super glue fuming method was discovered when a British police officer was repairing a broken tank. He discovered several fingerprints illuminated on its side. He then tried the glue in other areas, with success, and other scientists realized that super glue adheres to a print's amino acids.

Once the print is photographed and preserved, it can be used to try to identify its source. Generally that means either comparing it with a specific suspect or entering it into a computerized database like AFIS (Automated Fingerprint Identification System).

To make a match, an unknown print is examined against prints that have been identified as belonging to a certain person. The expert is looking for clear points of comparison. According to David Fisher in Hard Evidence, there may be as many as three hundred possible points of comparison on a single fingertip. While no minimum number has been established nationwide to say with certainty the origin of the unknown print, the more points of comparison the better. It takes only one dissimilar point to nix a match between the known and questioned samples.

Since print comparisons are a matter of interpretation, mistakes can certainly be made, so experts work with many cases to hone their abilities. Although fingerprint analysis has recently come under scrutiny in the courts, in all the years that it's been practiced no one has yet found two people with identical prints.

To compare them, the print technician must first make sure that prints are taken of everyone who was or who might have been at the scene. That includes taking them off any dead bodies. Henry Lee calls these "elimination prints." To take a print at a crime scene, an ink roller is run over the fingertips and the tips are then pressed against a card. At the police station, the person dips his fingers into printer's ink and then presses them onto a card, known as a ten-card. Then each hand is inked and its entire print preserved in the same way. The fingers are numbered one through ten, starting with the right thumb. The left thumb is number 6. Then they get coded, including any missing or scarred fingers.

Since 1972, fingerprints have been compared and retrieved via computer. State and local agencies built up automated fingerprint identification systems and the FBI opened the National Crime Information Center (NCIC), which expedited the exchange of information among law enforcement agencies. The FBI also introduced a standard system of fingerprint classification (FPC), so that information could be uniformly transmitted from one AFIS computer to another.

The computer scans and digitally encodes prints into a geometric pattern. In less than a second, the computer can compare a set of ten prints against a half million (although getting matches can take longer). At the end of the process, it comes up with a list of prints that closely match the original. Then the technicians make the final determination, which involves a point-by-point visual comparison.

Now even computerized images are coming under scrutiny and may yield to three-dimensionally scanned prints, such as those done by Livescan, but the databases have to be redone since the prints "read" differently.

Fingerprints alone have solved many cases, but one case that got nationwide attention was solved with only the print left by the palm. Let's take a look.


We're Following
Slender Man stabbing, Waukesha, Wisconsin
Gilberto Valle 'Cannibal Cop'