Despite significant advancements in forensic sciences and crime scene investigation techniques, fingerprints remain one of the most reliable methods of identifying anybody who may have been at the scene of a crime.
This is an overview of latent prints, the type of fingerprints we typically associate with crime scene investigations and fingerprint identification.
What Are Latent Fingerprints?
It is important to that, in forensics, there are three different types of fingerprints:
- Plastic prints are fingerprints on soft surfaces, such as soap, wax, gum, etc. These are typically three-dimensional fingerprints which can be easily captured through simple photography.
- Visible prints (also known as patent prints) are fingerprints which an be visibly seen to the naked eye. Often, this occurs when blood, ink, dirt, and other substances are transferred from fingers onto the surface of an object. As with plastic fingerprints, these are commonly captured through simple photography.
- Latent prints are those which are invisible to the naked eye under normal circumstances. These are usually caused by the transfer of body oils onto a surface.
Unlike plastic prints and visible prints, latent fingerprints typically require advanced developing methods in order to capture them and submit for fingerprint identification.
Where Are Latent Fingerprints Found?
Latent fingerprints can be found on virtually all surfaces, regardless of their texture or type. However, the processing of latent fingerprint evidence may be different depending on the type of material it is found on.
In general, latent print evidence is typically categorized into two types, based on the material it is found on:
- Non-porous evidence is found on smooth surfaces such as doorknobs, car handles, electronics, etc. They are often much easier to develop and process because they lie directly on top of the surface in question.
- Porous evidence are latent prints that are found on porous surfaces, such as wood, paper, and cardboard. Because these surfaces are fairly permeable, they often absorb the body oils that make up latent fingerprints over time. As a result, processing porous evidence is considerably more complicated than non-porous evidence.
It is important to note that even though porous evidence may be processed, the quality and clarity of latent print evidence may suffer, and fingerprint identification is significantly more difficult. As a whole, non-porous evidence from smoother surfaces has a greater chance of being accurately processed and developed.
How Long Do Latent Fingerprints Last?
Under ideal conditions, latent fingerprints have the potential to remain on a surface for an incredibly long time. However, due to so many factors and circumstances that can impact the integrity of a latent print, predicting its viability cannot be accurately done.
Several factors play into the lifespan of a latent fingerprint:
- The matrix of the latent print. The make-up of the latent print can impact its preservation and longevity. Although most latent prints are comprised of body oils, some may be a mix of body oils and sweat, which can significant impact how long they last on a surface.
- The substrate the latent print is on. The substrate, or surface, may also play a factor. Although incredibly smooth surfaces like glass and metal may hold latent print incredibly well, others such as porous surfaces and some plastics may only hold latent prints for a couple days.
- Environmental factors. Temperature, humidity, moisture levels, and contact with any other natural contaminants can have a major impact on a latent print’s lifespan.
It is best to assume that all latent fingerprints are short-lived so that they are processed promptly.
How Are Latent Fingerprints Collected?
Latent fingerprints are most commonly collected in the following ways.
Arguably the oldest method of fingerprint collection, powders are still commonly used today to collect latent prints from smooth, non-porous surfaces. Traditionally, locations where latent prints may be found are dusted with a powder that bonds to the oils. Then, a piece of lifting tape is used to transfer the fingerprint to a card for preservation of the evidence.
Advances in crime scene investigations and forensic sciences have led to different types of powders that make it easier to develop and process latent prints. They have properties that react with heat, lighting, and other chemical reagents to improve the contrast and resolution of latent prints that are fading, older, or otherwise faint.
Alternate Light Sources
Before dusting with powders, many fingerprint examiners may use a special light to illuminate any locations where latent prints may be suspected. These alternate light sources emit a particular spectrum of light, such as ultraviolet (UV), to illuminate latent prints that may not otherwise be found.
Fuming is the process of exposing a piece of evidence to chemical vapors. These vapors react and adhere to any latent prints found on evidence, highlighting their presence on the object in question.
The most common form of fuming currently is cyanoacrylate processing. Cyanoacrylate is the industrial name for the family of super-adhesives known as Crazy Glue or Super Glue. When latent prints are exposed to cyanoacrylate vapor, the trace proteins and amino acids in the fingerprint react to form a raised, white material that is clearly visible to the human eye.
Ninhydrin dipping is form of chemical processing that is used primarily to develop porous evidence. The object in question is exposed to a chemical reagent called ninhydrin, and then processed with a hot iron. Under heat, latent prints exposed to ninhydrin turn a dark, clear purple on the porous surface.
Other variations of porous evidence processing include exposure to DFO, a compound which caused fluorescence in latent prints when exposed to blue-green light.
Technological advances in fingerprint processing have resulted in a number of other different chemicals and reagents that may be used to develop and process latent fingerprints. However, the safety of using some of them has been brought into question, as many can be quite hazardous to humans if they are repeatedly exposed to them.
Who Conducts the Fingerprint Analysis?
When latent prints are collected, they are typically handed over to fingerprint examiners. However, it is important to recognize that that is a catch-all term, much like crime scene investigators. Fingerprint examiners are usually forensic scientists or forensic science technicians. In rare occasions, law enforcement officers may also help with processing.
Proper training and experience is required in order to process, develop, and analyze latent fingerprints. Most agencies require fingerprint examiners to acquire a four-year college degree in one of the major sciences. Further, certification from a notable crime scene investigation organization or forensic science agency, such as the International Association for Identification (IAI), may be required.
How Are Fingerprints Analyzed?
The main objective of fingerprint analysis is fingerprint identification. Fingerprints are collected at the scene of the crime and cross-referenced, either with those of any arrested individuals or from a national database of fingerprints, to determine any potential suspects in the crime.
The fingerprint analysis process is as follows:
- Analysis of collected prints. Any collected, processed, and developed fingerprints are carefully analyzed to see if they are viable. The quality and clarity of the fingerprint are major factors during this process; those that are faded, unclear, or faint may not be usable in analysis. Additionally, latent fingerprints that may have been overprocessed may also not be usable because they may be too thick or too muddled.
- Comparison with other prints. Viable fingerprints are then compared with relevant fingerprints. These other fingerprints may be from any persons of interest at the scene of the crime, potential victims, or any others who may be potential suspects. In cases where no suspects may be found, the fingerprints are compared to those found in a database managed by notable law enforcement agencies.
- Evaluation. The evaluation of prints and their relationship to others is comprehensive, with fingerprint examiners extensively analyzing the smallest details in all prints. Ultimately, based on this information, the examiner gives a decision on whether or not there is a match.
- Verification. When an examiner declares a match, it is independently verified by another fingerprint examiner to ensure that the initial decision was a correct one.
Advances in software and technology have made evaluation and verification much easier for fingerprint examiners. Using programs with machine-learning algorithms, many examiners are able to take some of the guesswork out of comparing fingerprints.