Catching Criminals Using Fingerprinting

by Jack Lee (Retired Criminal Investigator)

Have you ever wondered how fingerprints are used to identify a criminal? Well, here’s the short version of the, who, what, when and where of fingerprinting. I think you will find it interesting.

In 1686, Malpighi, a professor of anatomy at the University of Bologna, noted in his treatise; ridges, spirals and loops in fingerprints. He made no mention of their value as a tool for individual identification. A layer of skin was named after him; “Malpighi” layer, which is approximately 1.8mm thick. In 1823, Purkinje, an anatomy professor at the University of Breslau, published his thesis discussing nine fingerprint patterns, but he too made no mention of the value of fingerprints for personal identification.

Using prints to detect a person’s identity didn’t begin until the last half of the 1800’s, Fauld wrote a scientific paper on fingerprinting for identification in 1880, but Herschel claimed he had been using print identification for nearly 20 years in India.

Even though these men discovered the potentials for identification it was Galton who introduced the minutiae features for single fingerprint classification in 1888. And the first real advance to a uniform or standardized system came in 1899 when Henry completed an extensive study in India that led to the “Henry System” that used 6 basic patterns and minutiae and that results in a sort of serial number unique to each person.

A nail to nail print could yield 11-12 or more unique points of identification making it highly unlikely that another person would have this exact series of identifying marks.

The 6 basic categories of prints are: (a) arch, (b) tented arch, (c) right loop, (d) left loop, (e) whorl, and (f) twin loop. Critical points in a fingerprint, called core and delta, are marked as squares and triangles. See the blow up of the finger print photo above.

Fingerprinting is the earliest form of biometric identifiers. Biometrics would be anything from eye color to foot size. Biometrics, or bio-identification, is merely the practice of measuring physical characteristics of a person to verify their identity.

There are currently two methods of taking a person’s fingerprint for classification. A rolled or inked print which is the oldest method and the “live” scan found most often in the military and border patrol. The latter works much like a computer scanner or copy machine. However, currently technology does not allow for scanning the outer edges of the finger as it approaches the nail simply because the glass surface of the scanner doesn’t make contact there. So the traditional method still works best, but when speed and economy demand it nothing beats the live scan method.

Latent print recovery: A fingerprint left by the oil residue of your skin can last decades in a cool area such as a basement, but it could evaporate in hours in direct sunlight in hot weather. It’s been my personal experience that an average print lasts about a week to 10 days and after that the quality drops off dramatically. Fingerprints left behind by the criminal are called latent prints because they are hidden and must be recovered.

A print that is left in dried blood, soft paint, tar, wax, etc., is referred to collectively as a plastic impression and there are various means of lifting those prints. One of the most common methods is to use dental casting material which is a rubber substance that sets quickly and can peeled away from the latent impression and then examined. This material lasts for decades. Photographing the latent print is another easy way to collect and preserve the latent print. Digital cameras make this the easiest of all methods and they produce instant results. If a picture didn’t come out to expectations you have the opportunity to continue photographing until it does. A good crime scene photographer can achieve amazing results in print recovery.

Most people have seen police dusting for latent prints at a crime scene. This typically involves using a camel or horse hair brush and a form of graphite similar to pencil lead.
The officer gently drags the print powder around the suspected area and hope it sticks to a hidden print. If it does come up as a visible print it’s either photographed or lifted using a print tape and placed on a 3X5 card or both. A good print can then be used in comparison with all prints of record.

How much of the actual fingerprint does it take to get a positive match? It all depends on the unique properties of the latent print. For example a “worl” with only 2 or 3 ridges visible could convince a jury when using a side by side microscopic comparison. The latent print falls far short of being classified, but when compared to a known person’s print it’s often quite valuable. To run a print for automated ID when the suspect is not known usually requires 6 to 8 points, but there really is no set minimum.

Will DNA replace the fingerprint? Not likely, although DNA is always present at the crime scene it is not always easy to find or recover. Currently the DNA data base pales in comparison to the fingerprint data base. So, it’s safe to say there will always be a place in identification for the fingerprint.