Case Analysis

Forensic Science 101: Identifying the Suspect - Part 1

Contributor Mimic continues her fascinating, and very informative series of articles on the subject of Forensic sciences. Here is the first part of the next subtopic.

Article by Mimic.
Page editor: XScribe.

If you think about it, you can't have a crime without a perpetrator. And if you have a perpetrator, that means there will be suspects who could potentially BE the perpetrator. But how does law enforcement narrow down the (almost always) large pool of suspects in order to arrive at the one "who dun it"? That's where forensic science works together with good, old-fashioned detecting.

"Fingering" the Culprit

Everyone's fingerprints are different. Even identical twins don't have the same prints. This isn't recent news, though. Fingerprints have been used to identify people since the ancient Babylonians recorded business transactions by pressing their fingertips into wet clay.1 When left undisturbed on a hard surface, fingerprints are nearly permanent; scientists have discovered usable latent prints in ancient tombs.2 Although fingerprints were used in India during the 1800s to identify illiterate workers, there wasn't any system in place for classifying and collecting prints as a means of formal identification Sir Francis Galton published a book in 1892 that was the first to split fingerprints into three pattern classifications of arches, loops, and whorls, but Sir Edward Henry is considered to be the father of modern fingerprinting. In 1896, he added tented arches to Galton's classifications and subdivided the loops even further.2 This system of classification allowed sets of prints to be compared to each other by matching specific points in the pattern. It is still in use today.

The advantage of utilizing fingerprints for identification was immediately embraced: They were first used to identify a criminal in 1892. While Bertillonage was still more popular, the murder of two children in Argentina provided an opportunity to prove finger printing's usefulness. Francesca Rojas insisted that a neighbor had killed her children in retaliation for her rejection of his advances, but when one alert official who was familiar with the new idea of fingerprinting matched a bloody thumbprint to Francesca herself, she confessed to murdering the children so she could marry her young lover. It would be nice to say that fingerprinting was universally embraced after that point, but it wasn't. Still, it was a good start.

Fingerprints come in three types: latent, visible, and plastic.2 Visible prints are just what they sound like--images left behind in blood, dirt, or another material where they're visible to the naked eye. Plastic prints are ones that have been molded into a surface where the impression is retained, such as mud, wet soap, or clay.1 The ones investigators find most often are latent prints, which are greasy imprints left behind on hard surfaces by oil in or on the fingertips or hands. Shiny surfaces are the best for a clear impression, but latent prints can be lifted or rendered visible from a number of different materials using several techniques. The standard powder we've all seen dusted about on cop shows is an organic compound, which adheres to the oil in the print. Once the impression is made visible, it can be "lifted" by pressing a piece of heavy tape onto the surface and then peeling it back, taking the print with it. For prints on porous surfaces, the imprinted item is placed inside a special chamber and subjected to fumes from iodine or ninhydrin (Super Glue) to raise the impression to the surface.2 Once detectives have a clear print to work with, it is passed along to a fingerprint specialist who will compare the pattern of impressions to possible suspects.

Anyone who is booked on suspicion of having committed a crime has their fingertips and palms "inked" and then rolled onto a standardized card, which the police department stores for later use. For a long time, checking latent prints against suspects meant manually flipping through one card of fingerprints after another with a magnifying glass to look for matching print patterns. If the actual criminal had never been arrested before, or was from a different state, there was no hope of ever finding a match. Things improved in the 1980's when the Japanese National Police Agency developed a computerized system of print-matching called the Automated Fingerprint Identification Systems (AFIS).1

It used Sir Edward Henry's matching parameters and was hundreds of times faster than manually checking cards, but there still was no way for police departments in different states, or even adjacent cities, to check each other's pool of available print records. In 1999, the Integrated AFIS was introduced, linking local, state and federal law enforcement to a computerized system which allows access to the FBI's main database of fingerprints, mug shots, and criminal histories from around the country.1 In as little as 30 minutes, a detective in Cleveland can match a fingerprint from a crime scene and find the name/address of the man he's looking for, or even that the suspect is already in custody for another crime. Quite a step up from spending weeks looking through hundreds of fingerprint cards!

For a long time, fingerprint analysis was the most recognized, and used, method of personal identification. It is still vital in criminal cases of all kinds when clear impressions are available, but there are other ways of identifying suspects that are every bit as useful as fingerprints.

Thicker Than Water

Deoxyribonucleic acid sounds like the stuff of science fiction. Better known as DNA, it is more like a detective's dream come true. Second only to fingerprints for its individuality, everyone (except identical twins) has a completely different, identifiable set of markers. DNA can be extracted from blood, saliva, semen, sweat, bone marrow, and most recently, tooth pulp. Until the discovery of DNA, identification using bodily fluids was limited to blood type, which was classified by Austrian-born scientist Karl Landsteiner in 1901, followed in 1925 by the discovery that blood type can be determined from other bodily fluids like saliva because about 80 percent of people "secrete" their blood type into other fluids. This meant investigators could include or eliminate possible suspects based on blood type, but they couldn't say with any certainty who was the culprit. DNA "fingerprinting" was a major revelation.

The existence of DNA has been known since at least 1911, when Phoebus Levene discovered two types of nucleic acid inside the nucleus of individual cells. Each nucleus also contains twenty-three pairs of chromosomes, with each pair being composed of one chromosome from the father's sperm and one from the mother's egg. In the 1950s, scientists discovered the four chemicals that are present in DNA--adenine, guanine, cytosine, and thymine, which are designated by the letter abbreviations A, G, C and T.2 While we all have sections of our DNA which are common to every human being, certain sections of an individual's chromosomal sequence are completely unique to that person (again, except for identical twins). The process for extracting DNA from a specimen is complicated and, while the time period has been reduced to a matter of days, it still takes much longer than CSI-type shows would have us believe, mainly due to the high volume of samples and the small number of laboratories authorized to carry out the testing. The way it all happens is this:

"Once DNA has been extracted from a specimen, it is mixed with a restriction enzyme that cuts the DNA chain at particular sequences. The fragments created by this process are placed in a gel, to which a high-voltage electrical current is applied. Shorter fragments move through the gel more quickly than longer fragments, and after a short time, the fragments will have lined up according to size. These are then lifted from the gel by a nylon membrane called a blot. After incubation, the membrane is treated with a radioactive genetic probe, which attaches to the polymorphic DNA fragments. Because the probe is radioactive, an X-ray photograph of the membrane will reveal the pieces of DNA that the probe has identified as dark bands, much like a supermarket bar code."2

While the existence of DNA had been known since the 1950s, the implications of its use weren't apparent until 1984, when British scientist Dr. Alec Jeffreys figured out how to make the chemical sequence visible on an X-ray slide.2 The chemical elements always line up in pairs, with A joined to T, and C joined to G. The pattern they create when aligned is what looks like a bar code, and no two patterns are completely alike, making it possible to identify a specific person by the pattern of their DNA. In 1986, the father of a mentally-challenged man in Leicester, England, thought back to an article he'd read about Jeffrey's process and wondered if it might help his son, who had recently confessed to the second of two brutal murders of young girls. The father believed his son was innocent, and that the police had the wrong man. There is some confusion about how Jeffreys became involved in the investigation, but eventually he was asked to do a DNA test on the suspect. The results showed that not only was the young man innocent of the first murder, he also hadn't killed the girl he'd confessed to murdering!2 Left without a suspect, the police realized that if DNA could tell them who hadn't committed the crimes, maybe it could also tell them who had. In one of the gutsiest moves in police history, the Leicester force made the decision to draw blood from every local male between the ages of sixteen and thirty-four. 2

Initially, they tested a thousand men. After a month of waiting, only a fourth of the samples had been checked because of the amount of time it took to complete the procedure. Undaunted, the police continued to collect samples and wait for results. Unknown to them, the man they were hunting for had pressured a co-worker into posing as him while giving blood, claiming that his previous convictions for indecent exposure would prejudice the police against him. When the co-worker told other people at his job about it, one of them went to the police, which made it possible for them to finally arrest Colin Pitchfork and get a real sample of his blood. By that point, they'd tested over 4500 men, but Pitchfork's DNA profile was the only match. In January of 1988, Colin Pitchfork pleaded guilty to both murders and was sentenced to life in prison. This case made forensic history twice, as the mentally-challenged man was the first person to ever be exonerated by DNA evidence, and because it also marked the first time DNA evidence was used to get a conviction anywhere in the world.

DNA evidence is not only used to help identify the suspect in a crime, it can also indentify the victim when their identity is unknown, or the source of blood stains at a crime scene, and in the past few years, it has been used to identity the father in paternity cases. Additionally, the Innocence Project works tirelessly to get criminal cases reexamined, especially if the conviction occurred before widespread DNA use. Since the first exoneration in 1989, 251 wrongly-convicted men have been cleared and released from prison thanks to DNA evidence.3

Today, inexpensive saliva samples are routinely taken from anyone sent to prison or arrested for particular classifications of crime, and entered into the FBI database where they can be checked against unsolved cases. This practice has raised some ethical issues dealing with when samples should be collected, how they should be stored, how they should be used, and whether or not we really want to branch out into generalized collection and storage of the entire population. These are good questions. Only time will tell how they are ultimately answered.

It would seem like fingerprints and DNA profiles would be enough to "get their man," but the investigator's bag of tools contains many additional methods of identifying suspects. We'll go into a few of those in part 2.

Editor's Notes
1 How Stuff Works: Forensic Science
2 Evans, Colin. The Casebook of Forensic Detection. John Wiley & Sons, NY: 1996. Print.
3 The Innocence Project: Statistics