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Chapter 5

Fingerprints: Your Personal Signature

IN THIS CHAPTER

Defining fingerprints

Classifying and matching prints

Exposing and collecting prints

If you read, watch TV, or go to the movies, you know that fingerprints play a vital role in many mysteries, whether real-life or fictional. Fingerprints often are how police identify criminals and solve crimes, even crimes that are decades old.

Although fingerprint identification is now standard practice, acceptance of the individuality of fingerprints by police, scientists, and the courts didn’t happen overnight.

In this chapter, I explain the science behind fingerprinting, a little bit of its history, and some of the methods used for finding and lifting prints at the crime scene.

Getting a Grip on Fingerprints

Using a bright light and magnifying glass, take a close look at your finger pads (the fleshy surface of your fingers that you use for touching and gripping). You’ll see very fine lines that curve, circle, and arch. These lines are composed of narrow valleys called grooves and hills known as friction ridges. When you see an inked fingerprint, you’re looking at the pattern made by the friction ridges.

Friction ridges give your fingers traction, and this traction enables you to pick up a glass, pin, piece of paper, or speck of lint. Imagine trying to turn the pages of this book without being able to grip the paper. Without friction ridges, a glass would easily slip from your hands, and you’d have trouble gripping the broom you need to sweep up the mess. Without friction ridges, we’d all be butterfingers.

These ridges evolved so that human beings would be less clumsy and more efficient when working with their hands. In fact, the ability to grip sticks and throw stones was a matter of survival to your remote ancestors. After all, how can you hunt and kill an elk if your weapon continually slips from your fingertips or that stone you hurl lacks any predictable direction? The evolutionary development of friction ridges makes a tough and perilous existence just a little easier. Modern-day people use these ridges to twist doorknobs, hold pens, grasp a steering wheel, and throw baseballs.

Fingerprints have a utility beyond gripping a doorknob or throwing a baseball, however, and that utility became evident as the need for a reliable method of identification arose.

When the world was sparsely populated and people lived in small nomadic communities of 30 to 50, everyone knew everyone else. If you were an outsider and wanted to know who Joe was, anyone in the group you were visiting could easily point him out. But as populations grew, settled into ever-expanding cities, and developed systems of government, identifying others became much more difficult. No longer did everybody know Joe. A reliable method for identifying people became necessary, not only for fingering criminals, but for detecting fraud in contracts and pension payments and other situations where identity is important.

DEVELOPING THE SCIENCE: A TRIP THROUGH FINGERPRINT HISTORY

Fingerprints moved from being marks of authenticity in artwork to criminal signatures over a 3,000-year period that includes the following highlights:

Prehistory: Early potters identify (or sign) their works with an impressed fingerprint.
1000 BC: The Chinese sign legal documents (even criminal confessions) with hand and fingerprints. Whether the practice was ceremonial or a true method of identification is unclear.
1685: Marcello Malpighi, a professor of anatomy at the University of Bologne, first recognizes fingerprint patterns and describes them in terms of loops and whorls when writing about the “varying ridges and patterns” he saw on human fingertips.
1823: Johannes Purkinje of Breslau University establishes nine basic fingerprint patterns and rules for classifying them, thus forming the basis for modern classification systems.
1858: English civil servant Sir William Herschel requires natives in Bengal, India, to sign contracts with a hand imprint to prevent fraud in contracts and pension distributions. Perhaps the first European to note the individuality of handprints, he finds that his prints are unchanged after 50 years, an important fact in developing fingerprints as a forensic tool.
1880: Henry Faulds, physician and surgeon at Taukiji Hospital in Tokyo, Japan, writes that fingerprints can be used for personal identification and suggests they can be useful in identifying criminals. Faulds discovers that dusting latent (invisible) prints with powder exposes them. He uses that method to exonerate an accused thief whose prints don’t match one left on a window by the real thief, who later confessed based on the evidence.
1892: Sir Francis Galton publishes his classic textbook, Finger Prints, the first book on the topic, in which he describes patterns within the prints that he calls loops, arches, and whorls. More importantly, Galton offers convincing evidence that no two prints are identical.

Juan Vucetich, a police official in La Plata, Argentina, devises the fingerprint classification system that’s still in use in most of South America and ultimately publishes a book on the subject in 1894.

Argentina is the first country to report using fingerprints to solve a crime, the June 18, 1892, murders of the children of a woman named Rojas. She tries to pin the murders on a man named Velasquez, but an investigator discovers Rojas’s lover said he’d marry her if she had no children and finds a bloody fingerprint at the scene matching Rojas’s right thumb. She confesses.
1897: Herman Welcker finds that his own prints taken in 1897 are unchanged from the ones he’d taken 41 years earlier (1856), thus supporting the findings of Sir William Herschel.
1899: Sir Edward Henry devises a classification system based on five types of prints. His system is the basis for the ones used today in Britain and America.
1910: Thomas Jennings becomes the first person convicted in the United States on fingerprint evidence. Tried for murder in Chicago, Jennings is convicted, and the verdict is upheld on appeal, making his case a landmark in the use of fingerprint evidence in court.

Measuring bodies: A precursor to fingerprinting

One of the first attempts to identify and record differences among people was through anthropometry, the science of measuring humans.

Using anthropometry, French police officer Alphonse Bertillon developed the first truly organized system for identifying individuals in 1883. Believing that the human skeleton didn’t change in size from about age 20 until death and that each person’s measurements were unique, he created a system of body measurements that became known as bertillonage. According to Bertillon’s calculations, the odds of two people having the same bertillonage measurements were 286 million to one.

Bertillon thought everyone could be distinguished from one another by key measurements, such as the diameter of the head and the span of the outstretched arms. For many years, this system was accepted by many jurisdictions, but by the dawn of the 20th century, bertillonage was losing its luster. The inexact measurements varied according to who made them. And because the measurements of any two people of the same size, weight, and body type varied by fractions of a centimeter, the system’s flaws quickly appeared, and it soon was discontinued.

Anthropometry’s death knell sounded as a comical coincidence in the Will West Case. On May 1, 1903, Will West arrived as a new inmate in Leavenworth Penitentiary in Kansas. The records clerk apparently thought he looked familiar, but the new inmate denied having been in the prison before. Imagine the surprise among prison officials when Will’s anthropometry measurements exactly matched those of William West, another inmate at Leavenworth. The two men did indeed look eerily similar … as if they were twins. They were brought together in the same room, but they said they weren’t brothers. Fingerprints helped authorities distinguish between the two Wills. So much for measuring the diameter of a person’s head. Leavenworth immediately dumped anthropometry and switched to a fingerprint-based system for identifying prisoners. New York’s Sing Sing Prison followed a month later.

The similarity between Will and William West was not merely a bizarre coincidence. A report in The Journal of Police Science and Administration in 1980 revealed that the two actually were identical twins who had many fingerprint similarities and nearly identical ear configurations (unusual in any circumstance except with identical twins). Furthermore, each wrote letters to the same brother, same five sisters, and same Uncle George.

Toward the end of the nineteenth century, Bertillon reluctantly agreed to add fingerprints to his bertillonage profile. However, he added only those of the right hand. Big mistake.

On August 21, 1911, Leonardo da Vinci’s famous painting Mona Lisa was stolen from the Louvre Museum in Paris. The thief left a clear thumbprint on the glass that had protected da Vinci’s masterpiece. Alphonse Bertillon added his profiles to the investigation, but unfortunately, he had no classification system to aid investigators searching through his thousands of data cards. He and his assistants spent several months digging through his files to no avail. Two years later, police apprehended the thief, Vicenzo Perugia. His prints matched the one found at the crime scene. It turns out that Perugia’s prints were among those in Bertillon’s possession all the time. No match had turned up because the print found at the scene was from Perugia’s left thumb, and Bertillon’s files contained only that of Perugia’s right thumb.

Using ridge patterns

The usefulness of fingerprints for identification depends upon three principles:

A fingerprint is individual and is not shared by any two people. Every person has a unique set of fingerprints. Even though identical twins have the same DNA, they have different fingerprints.
A fingerprint remains unchanged throughout life. The fingerprints you’re born with are the same ones you die with. If someone burns or shaves off the pads of his fingers, the prints disappear for a while, but as the skin repairs itself and wounds heal, the print reappears.

However, more severe damage that involves deeper layers of the skin may leave permanent scars and prevent prints from reemerging. Nonetheless, completely obliterating a print is difficult, and any scars left behind by attempts to do so create new individual characteristics that an examiner can use for making a match.
Fingerprints exhibit general patterns that provide a basis for classification. General patterns exist within every person’s prints, and all people share these patterns to varying degrees. Prints therefore can be systematically classified, reducing the number of records that must be searched when looking for a match.

Making Matching Easier: Classifying Prints

The purpose of any classification system is to add order to chaos by finding common traits among items. For instance, libraries organize books by subject matter, which keeps readers from having to wander aimlessly through the shelves for hours or (more likely) days. Lumberyards categorize wood by type, width, and length, and grocery stores stock pasta sauce with the linguine and spaghetti. You get the idea.

The same is true of fingerprint files. They’re useful only if they can be stored in great numbers and quickly searched for a match. The Federal Bureau of Investigation (FBI) has more than 200 million fingerprint files. Imagine slogging through those! Organizing the prints into groups makes searching for a match to an unknown print much easier.

Grouping by arches, loops, and whorls

In 1685, Marcello Malphigi recognized patterns in fingerprints and named them loops and whorls. The arch pattern followed more than 200 years later, when Sir Francis Galton identified it in 1892.

Whorls, loops, and arches (see Figure 5-1) are still the basis for fingerprint matching and identification, because although everyone has them, how they have them is unique. Each person has a different number of these types of patterns, and the patterns vary from fingertip to fingertip on each person.

Screen captures depicting plain arch, single loop, target whorl, tented arch, double loop, and spiral whorl patterns. — Illustration by Nan Owen.

FIGURE 5-1: Patterns commonly found in fingerprints are the basis for classification.

Arches

Arches are ridgelines that rise in the center to create a wavelike pattern. Arches are subgrouped into plain and tented varieties. Tented arches have a sharper central rise than do plain arches. Only 5 percent of all pattern types are arches.

Loops

Loops are comprised of one or more ridges that double back on themselves. About 60 percent of patterns in human fingerprints are loops. They’re subdivided into two types depending upon the direction the ridges flow in relation to the two bones of the forearm, the radius and the ulna:

Radial loops flow toward the radius or the thumb side.
Ulnar loops flow toward the ulna or little finger side.

Whorls

Whorls look like little whirlpools of ridgelines. They make up 35 percent of patterns seen in human fingerprints and are subgrouped into four categories:

Plain whorls are either concentric circles like a bull’s-eye or spirals like a wound spring.
Central pocket loop whorls resemble a loop with a whorl at its end.
Double loop whorls include two loops that collide to produce an S-shaped pattern.
Accidental loop whorls are slightly different from other whorls and are irregular.

Developing the Henry System

Sir Edward Henry, an inspector general of the British police in India’s Bengal province, worked on a fingerprint classification system for many years. The Henry System, which he completed in 1899, still is used (with a few modifications) in the United States and Great Britain.

Using the Henry System, individual prints are assigned scores based on where whorls show up within a ten-finger set of prints. Whorls on certain fingers get higher scores than whorls on other fingers. The total score is used to narrow down matches into all the sets with the same score.

This system separates fingerprint files into 1,024 groups, thus narrowing the focus of fingerprint searches. The actual matching still is done by hand, meaning the system doesn’t make the match but simply reduces the number of files the fingerprint examiner must wade through.

Now, in the computer age, prints are digitized and stored as computer data. This allows for much faster sorting and matching. Still, if a match is found, the final determination is made by a human visually comparing the prints in question.

Speeding up identification: AFIS

Because criminals almost never leave behind a full set of prints, systems that rely on prints from all ten fingers are less than perfect. Enter the Automated Fingerprint Identification System (AFIS).

AFIS came about during the 1960s as a result of collaboration between the FBI and the then National Bureau of Standards (now the National Institute of Standards and Technologies, or NIST). With growing numbers of print sets being housed in the FBI’s database, a better method for storing, retrieving, and matching fingerprints became a necessity. Imagine hundreds of agents sitting at hundreds of desks looking through thousands of print cards, searching for a match to only a partial print left behind by an accused serial killer. The wheels of justice would indeed move slowly.

The AFIS computer scans and digitally encodes fingerprints, storing that information in massive databases. It can search thousands of these files every second while attempting to match them to an unknown ten-print set or even a single or partial print. Current AFIS computers search through a batch of 500,000 prints in less than a second.

After the computer establishes a match, an agent trained in fingerprint evaluation then hand-checks the file or files. Even in the computer age, the final match is made by the trained eye of a fingerprint expert.

PUTTING AFIS TO WORK: THE NIGHT STALKER CASE

Between June 1984 and August 1985, a series of brutal rapes and murders occurred throughout Southern California, putting the entire region in a state of fear and shock. The killer became known as the Night Stalker.

The Night Stalker entered victims’ homes at night through unlocked doors, often after cutting the phone line. After shooting and killing any adult males present, he then raped the female victims. The assaults often took place in the bed where the woman’s spouse lay dead. The Night Stalker sometimes, but not always, killed the female victims. His final victims were a couple from Mission Viejo, California. The Night Stalker raped the woman while she lay in bed next to her husband, who had been shot in the head. Both she and her injured husband survived the attack. As fate would have it, a teenage neighbor saw a man in black drive away from the murder scene and wrote down the license plate number. The vehicle, which had been stolen earlier, was found abandoned two days later, and a partial fingerprint was lifted from the vehicle’s interior.

At that time, the Los Angeles Police Department had just installed a new AFIS that was capable of comparing more than 60,000 prints per second. The partial print from the car registered a match within minutes. Performing a similar search by hand (without AFIS) and finding a match against the 1.7 million print cards in Los Angeles would have taken a fingerprint expert 67 years.

The Night Stalker was Richard Ramirez, a 25-year-old drifter from El Paso, Texas. After his picture went out over the media, residents of East Los Angeles recognized and overpowered him as he attempted to steal another car. Police arrived in time to save him from an angry mob. On November 7, 1989, a jury sentenced him to death.

Not only can the computer match prints with dizzying speed, it can improve the quality of the print through minor digital manipulations. Brightness and contrast can be enhanced, and fuzzy images can be made sharper, all of which results in a clearer print that is more easily used in matching.

Despite this system’s incredible power, all is not rosy in AFIS Land. Early in the system’s development, several different AFIS manufacturers designed and supplied the computers, and not all of these computers are compatible. Added to this, not all jurisdictions subscribe to the AFIS system. This means that a set of prints in New York may belong to a criminal whose prints are stored in a Chicago database, but because the two systems can’t search each other’s databases, no match can be found. The FBI, which has a national database within the United States, is working with NIST to standardize the different systems so that searches can be conducted of fingerprint databases in all jurisdictions. Still not perfect, the system continues to improve year by year.

Locating Those Sneaky Prints

Fingerprints come in three general types that depend on how and where they were left. For example, a print left in grease on a wall is easier to find than one left on a garbage bag without any visible substance present to enhance its visibility.

Here’s the breakdown of what types of prints investigators are likely to find and where:

Patent prints occur when a substance such as blood, ink, paint, dirt, or grease on the fingers of the perpetrator leaves behind a readily visible print.
Plastic prints have a three-dimensional quality and occur when the perpetrator impresses a print into a soft substance such as wax, putty, caulk, soap, cold butter, or even dust.
Latent prints are invisible and can’t be seen without special lighting or processing.

Patent and plastic prints can be photographed, and the photo can be used for matching. Often the print is lighted at an angle to increase contrast, but little else is needed to make these prints recordable. If a criminal doesn’t leave behind a visible print, however, identification still is possible, but it’s certainly a lot trickier. Tools for tracking down prints can be as simple as a flashlight or black powder, as sophisticated as chemical reactions and lasers, or as goofy as Super Glue.

Which method investigators use depends upon the surface beneath the print. For harder surfaces, powders typically are used, and chemicals often are needed on more porous surfaces. Some prints show up under ultraviolet light or even a plain old flashlight.

Seeking latent prints

Just because you don’t see it right away doesn’t mean it isn’t there. Fingers constantly are coated with sweat and oils that are left behind on everything you touch.

The inner surfaces of your fingers, palms, and even the soles of your feet contain friction ridges lined with pores that serve as outlet openings of your sweat glands. Although these particular glands secrete sweat that has fewer oils, your fingers nevertheless pick up oils, salts, and grime when you touch your hair, face, or any area of your body served by more oil-rich sweat glands. You deposit these residues whenever you touch another surface.

The best surfaces on which to find latent prints are hard, smooth surfaces, such as a murder weapon, tools, or objects left behind or potentially touched by a criminal, opened drawers, out-of-place furniture, and entry and exit points. In short, any place the perpetrator may have touched and therefore left a print.

The simple approach is best when trying to reveal a latent print. An angled light from a flashlight, with or without the aid of a magnifying glass, may bring it into focus. Because the ridge pattern of a latent print actually fluoresces (glows) when exposed to laser and ultraviolet lights, these light sources may pop it into view. Think back to the ’70s, if you’re old enough: This effect is just like looking at black-light posters. A print exposed in this manner can be photographed and then matched with a known print on a database.

Powdering the print

Fingerprint powders adhere to moisture and oils of the residue in a latent print and thereby expose the pattern of the friction ridges. The powders come in a variety of colors and types. Criminalists use the color that gives the greatest degree of contrast with the background surface. Black powder, which is made from carbon black or charcoal, and gray powder, made from aluminum or titanium powder, are used most often.

Magnetically sensitive powders also are used, and they too come in varying colors. These powders are used with a special magnetic brush that is not actually a brush at all. It has no bristles, and thus doesn’t come into contact with the print, making it less likely to damage or smear the print. It is more like a magic wand that loosely holds the powder by a weak magnetic force. This so-called brush deposits the powder when it’s passed over the print, and the print snaps into view.

Other specialized powders are fluorescent. After they are applied, the print fluoresces (glows) under a laser light.

After the powdering process is complete, the print is either photographed or lifted. Lifting is done by gently laying the sticky surface of a strip of transparent tape over the print. As the tape is peeled off, the print pattern sticks to the tape, which is then placed on a card for later examination and matching. Because smears can render a print unusable, lifting a print takes a very steady hand. If the print is on an irregular surface, where tape lifting can be a problem, technicians may employ a gel-lifter or the silicon-based Mikrosil Casting Kit. This is also used for collecting tool-mark impressions.

Using chemistry to expose prints

Latent prints found on more porous surfaces are treated with chemicals that reveal print patterns by reacting with some component of the print residue. The reaction creates another compound that is more clearly visible. Common chemicals used for exposing prints include: cyanoacrylate vapor, iodine fuming, ninhydrin, and silver nitrate.

Cyanoacrylate vapor

What is cyanoacrylate? You probably know it by its trade name, Super Glue. Yes, that Super Glue … the kind you can buy in virtually any hardware or hobby store. It turns out Super Glue is 98 percent cyanoacrylate, which has become an extremely useful forensic tool. When heated or mixed with sodium hydroxide, cyanoacrylate releases vapors that bind to amino acids that are present in print residues, thus forming a white latent print.

After a print has been exposed with cyanoacrylate, it can be photographed as is or treated with a fluorescent dye that binds to the print. The print then glows under a laser or ultraviolet light.

The item that’s to be checked for prints in this manner often is exposed to the vapor in something called a fuming chamber. The resulting fumed print is quite hard and stable, as you’d probably expect from Super Glue. Instead of setting up a fuming box at the crime scene, police now frequently use a handheld, wand-shaped gadget that heats a small cartridge of cyanoacrylate mixed with a fluorescent dye. The wand releases fumes that are directed at the latent print, enabling the criminalist to fix and dye the print at the same time.

Iodine fuming

When heated, solid crystal iodine releases iodine vapors into a fuming chamber, where the iodine fumes combine with oils in the latent print to produce a brownish print. This kind of print fades quickly, so it must be photographed right away or fixed by spraying it with a solution of starch in water, which preserves the print for several weeks or months.

Ninhydrin

Ninhydrin (triketohydrindene hydrate) is a staple of law enforcement investigators and has been used for years to reveal latent prints. The object with the supposed latent print is dipped in or sprayed with a ninhydrin solution. Because the reaction between the ninhydrin and the oils of the print is extremely slow, the latent print may take several hours to appear as a purple-blue print. Heating the object to a temperature of 80 to 100 degrees Fahrenheit speeds up this process. Ninhydrin is very useful in exposing prints left on paper.

Silver nitrate

Silver nitrate is a component of black-and-white photographic film. When investigators expose a latent print to silver nitrate, the chloride in salt (sodium chloride) molecules present in the print residue reacts with the silver nitrate and forms silver chloride. This colorless compound develops, or becomes visible, when it’s exposed to ultraviolet light, revealing a black or reddish-brown print.

Cleaning up the print: Digital techniques

More often than not, a print or partial print is unclear. Its minute details may be fuzzy or difficult to see. Digital technology has stepped up and helped remedy this problem. Prints are scanned into a computer and then subjected to one of many programs that can enhance, improve, and clean up the computer-generated image of the print. Changing the light, contrast, clarity, and background patterns can make a previously obscured print jump into clear view, speeding up the matching process and making it more accurate, to boot.

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