Fingerprints are easy to find. They can even be located on a human body. The fingerprints are classified into three different categories due to the location they were found from and whether they are visible to the naked eye or not. Firstly, there are plastic prints that are three-dimensional. They are found on a soft surface such as fresh caulk, wet paint, wax, soap, and the like. Fingerprints are also found on a hard surface; they can either be latent or patent prints. The latter can be found on a variety of hard surfaces such as rough or smooth, and porous like wood, cloth and paper or non-porous like plastic, glass, and metal. They are formed when a liquid material such as paint, ink, dirt, and blood are transferred from one place to another, i.e., from finger to a solid surface.
The latent fingerprints, on the other hand, are formed by the natural oils and sweat on the skins of a human body. Due to the deposited materials, the prints are transferred from the skin to the surface. However, it is not possible to see these fingerprints with the naked eye as they are not visible. They can be found on a variety of surfaces. They are detected using a variety of materials such as chemical reagents, fingerprint powders, and alternate light sources. There are greater chances of finding and developing a latent fingerprint if the surface is less porous and smoother.
The most common method of finding and collecting a latent fingerprint is through dusting the non-porous or smooth surface with the fingerprint powder. There are several types of fingerprint powders that can be used in this regard such as black magnetic, aluminum flake, black granular, and others. The prints are made clear by dusting the powder; on clearance, the print must be photographed. Afterward, they are lifted from the surface using an adhesive tape. To preserve the fingerprint, the lifting tape is placed on a latent lift. These latent fingerprints are later examined as a part of the evidence in criminal cases and criminal profiling.
However, it must be kept in mind that dusting powder on to latent fingerprints can destroy or contaminate the source of the fingerprint. The opportunity to perform other techniques on the expression is also lost; these techniques could have provided the researchers and examiners with the additional information or a hidden print. Therefore, methods other than dusting fingerprint powder must be used to preserve the mark (Wilshire, 1996). The researchers have developed several other methods in this regard such as cyanoacrylate, i.e., superglue can be applied before using powders. Moreover, the alternate light source can be used for examining the area by the examiners.
The alternate light source has been becoming one of the researchers’ first choice in examining any surface such as railings, windows, doorknobs, door, etc. These are the LED or laser devices that emit a particular spectrum or wavelength of light. Some of these alternate light-emitting devices can also use different filters for providing a variety of spectra. These variety of spectra are important because they can be further processed with dye stains or powders and can also be photographed. For example, latent prints can be discovered from the computer equipment, chairs, desks or other objects at the crime scene with the help of blue light with an orange filter. It allows the fingerprint to appear clearly so it can be documented. It also enhances the appearance of the fingerprint.
Another option, other than dusting the latent fingerprint with either black magnetic, aluminum flake, black granular or others, is the use of cyanoacrylate, i.e., superglue that can be applied before using the powders. Similarly, fuming of the surface can also be performed before applying dye stains or powders. It involves exposing the surface to cyanoacrylate vapors. It is typically performed on the non-porous surfaces. The fumes (vapor) gets adhered to any prints present on the surface which can later be viewed with either a white or an oblique ambient light source. Similarly, chemical developers such as physical developer and ninhydrin are used on a porous surface such as paper to reveal latent fingerprints (Herod & Menzel, 1982). These chemicals react with the latent print residue including inorganic salts and amino acids. Ninhydrin turns the latent fingerprints blue on contact with them, and thus make them easier to be photographed. DFO, another chemical, turns the fingerprints into fluorescent images that are then illuminated using blue-green light. Other collection methods include staining the surface with Amido-Black – a non-specific protein. It particularly develops or enhances the bloody finger stains on a human body. In the future, methods such as vacuum metal deposition with the help of zinc and gold are giving hope to the examiners and investigators.
Once the fingerprints have been collected, in criminal justice cases, they are examined for potential matches on the local, state and national databases such as Automated Fingerprint Identification System using computerized systems. These systems provide the value indication about the closeness of the fingerprint with the match found from the database. The fingerprint and potential match are then examined by a fingerprint examiner to make a final determination. The examiners include police officers, technicians, and forensic scientists. However, having proper training and experience is a prerequisite for the examiners in this regard. The analysis is carried out by the crime laboratories and law enforcement agencies. If there is a need, the casework can be sent to private companies for high-profile handling cases, verifying results, and reducing backlogs.
The process of fingerprint examination includes looking at the quantity and quality of the information gathered from the source to find agreement or disagreement between the potential matches, i.e., known prints from the database and unknown prints from the crime scene. A small magnifier called a loupe is used for conducting the examination; it views minute details of the print. The experts count the friction ridges with a ridge which is a pointer. The ACE-V method is used for reaching a determination on the points of analysis, comparison, evaluation and verification by the fingerprint examiners.
A print is assessed to determine its utility for comparison with the help of analysis. If there is inadequate quality or quantity of the features, then the print is not suitable for comparison. On the other end, if the print is suitable, the features and their tolerances are indicated by the process of analysis which are later used in comparison. Tolerance is the amount of accepted variation. The analysis also uncovers physical features including creases and deltas that provide the examiners with the starting point for comparison where an analysts compares the suspect and known prints. Minute locations and characteristics are compared to determine if there is a match. Suspect prints are collected from the victims, persons of interest and other relevant people whereas known prints are taken out of the fingerprint database. The Automated Fingerprint Identification System is the largest database in the world and holds more than seventy-two million print records.
On comparing the two prints and performing detailed analysis on the potential matching characteristics between the two, the examiner evaluates the results to ultimately decide that if the prints are from the same source or different sources. It results in individualization or identification in the first scenario and exclusion in the second. Sometimes, the results are inconclusive as well. There are several reasons behind these results such as dissimilar features to be certain, insufficient number of corresponding, lack of comparable areas, and poor quality samples. The results of the first examiner are verified by another examiner by analyzing, comparing and evaluating the prints. The verification process either refutes or supports the conclusions of the original examiner.
There is a long history of admissibility of fingerprints as evidence in the legal cases in the United States. The 1910’s trial of Thomas Jennings for the murder of Clarence Hill was the first case in the country to use fingerprints as a part of the evidentiary support to the plaintiff’ side of arguments. The plaintiff had his house freshly painted when the incident occurred, and an impression of four fingerprints in the wet paint was found in the backyard railing of the house. The examination of the fingerprints held Thomas Jennings accused of the murder. In 1911, Daubert test was formed for the admissibility of the expert testimony in the context of fingerprint evidence in a legal case. Daubert test invites judges to examine that (I) if the evidence from the proffered expert was adequately tested, (II) if it had been subject to meaningful peer review, (III) if it had techniques and standards that controlled its operations, (IV) if it had a known error rate, and (V) if the results are accepted by the relevant community of experts. Before Dauber test, there was Frye test which questioned the admissibility of the fingerprinting technique by the relevant scientific community. However, Daubert test identified the importance of a logical and reasonable assessment of the fingerprint evidence in the legal cases by the judges and jurors.
In 2002, a surprising ruling was made by Judge Pollack on the admissibility of the fingerprints as evidence in a legal case. The former academic and distinguished judge issued a lengthy conclusion in United States v. Llera Plaza and found that fingerprinting was not a method of scientific evidence. It came as a shocking thought because fingerprints are scientific and conclusions of such examinations have previously been admissible in the courts (Melendez-Diaz v. Massachusetts, 2009) for nearly hundred years. They are often considered an indisputable and powerful form of evidence.
However, six weeks later, Judge Pollack made another surprising ruling when he denied his statement from a few weeks ago after a second evidentiary hearing in a blunt manner, “I disagree with myself.” He concluded in the United States v. Llera Plaza case that latent fingerprint examination has some defects but the opinions of the fingerprint identification experts are admissible evidence. By giving this statement, Judge Pollack rejected the challenges to the admissibility of the fingerprints and thus, preserved the status quo. Almost forty judges have considered that if fingerprint evidence meets the Daubert test which is the Supreme Court standard for accepting the expert evidence on fingerprint examination in legal cases. Every single judge of them, given Judge Pollack’s about-face, has concluded that fingerprinting passes the test.
However, some of the researchers and practitioner might argue the Judge Pollack’s first opinion was better due to the lack of research in testing and characterizing the fingerprinting identification techniques as scientific. He also found this technique hard to square with the Daubert test; the expert opinion used in the examination must be reliable and valid. Judge Pollack found that fingerprinting only meets one of the criteria of the Daubert test, i.e., general admissibility by the relevant community of experts. Judge Pollack judgment was correct despite being surprising though due to the lack of careful empirical examination of the key claims made by the examiners. However, it retains considerable cultural authority.
Although there is the successful and routine use of fingerprint evidence by the police that suggests that it is a powerful form of identification of the criminal, there are challenged at two points. Firstly, fingerprinting has not been challenged in the courts much frequently. Defense experts in fingerprint analysis were, almost, never used despite the presence of the adversarial testing in theory. Experts do not receive vigorous cross-examination either. Instead, the prosecutors and defendants, alike, take the accuracy of identification for granted. Secondly, Judge Pollack, in his first statement, indicated that Daubert test should not be the only criterion for the judges to use the evidence of fingerprinting without adversarial testing.
Therefore, the fingerprinting evidence as part of testimony in legal cases must carefully and empirically be tested and validated. Meaningful and difficult proficiency tests must be developed for the practitioners. The courts have not contested the idea the fingerprints can provide definite matches. In the early cases, some defendants argued that the method was not legitimate. However, they did not introduce experts from their side in the cases. Thus, there has been no spectacle of clashing experts on both ends of a legal case. The evidence concerning finding fingerprints of the defendant on the crime scene has rarely been challenged. Thus, the evidentiary value of fingerprinting grew to have cultural authority. Even the Frye test failed to question the long-lasting admissibility of the fingerprinting technique by the relevant scientific community, and now, there exists a hundred-years long acceptance of the method after the Daubert test. Therefore, the chances of the expert’s claims of infallibility were far surpassed.
References
Melendez-Diaz v. Massachusetts, 557 U.S. 305, 319 (2009)
People v. Jennings, 252 Ill. 534, 96 N.E. 1077 (1911)
Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993)
United States v. Llera Plaza 188 F.Supp.2d 549 (E.D.Pa. 2002)
Herod, D. W., & Menzel, E. R. (1982). Laser detection of latent fingerprints: ninhydrin followed by zinc chloride. Journal of Forensic Science, 27(3), 513-518.
Wilshire, B. (1996). Advances in fingerprint detection. Endeavour, 20(1), 12-15.
