Fingerprints are impressions formed by the friction ridges on human fingers. These patterns serve as a distinctive biological signature, making them a valuable tool in personal identification. Their utility in forensic science stems from two principles: uniqueness and permanence.
No two individuals, not even identical twins, possess the exact same fingerprints. This individuality arises during fetal development, influenced by genetic factors and the unique environment within the womb. These patterns remain consistent throughout a person’s life. While minor superficial injuries do not alter the underlying ridge structure, only severe damage to the deeper dermal layer can permanently change a fingerprint. This combination of uniqueness and permanence makes fingerprints a reliable method for identifying individuals in forensic investigations.
Collecting Fingerprint Evidence
Collecting fingerprints from a crime scene is the initial step in forensic analysis. Fingerprints are categorized into three types based on their visibility and how they are left behind. Each type requires specific collection techniques to ensure preservation and analysis.
Latent Prints
Latent prints are the most common type encountered and are invisible to the unaided eye. They form from the transfer of natural oils and sweat from friction ridges onto a surface. Detecting latent prints involves specialized methods tailored to the surface type.
For non-porous surfaces like glass or metal, fine powders are dusted over the area, adhering to the residue and making the print visible. On porous surfaces such as paper or untreated wood, chemical treatments like ninhydrin are applied. These react with amino acids in sweat to produce a purple print. Cyanoacrylate fuming is another chemical technique used for non-porous items, creating a white, stable print. Additionally, alternate light sources (ALS) can illuminate components of latent print residue, causing them to fluoresce.
Patent Prints
Patent prints are readily visible, created when a finger coated in a foreign substance, such as blood, ink, or dirt, touches a surface. These prints are captured through high-resolution photography before any physical collection. If the print medium allows, they might be lifted using specialized tapes or gels.
Plastic Prints
Plastic prints are three-dimensional impressions left in soft materials, like wax, clay, fresh paint, or thick dust. Since these prints are visible and retain their shape, the primary collection method involves photography from various angles to document the impression. Forensic specialists may also create a cast of the impression to preserve the detail.
Analyzing and Comparing Fingerprints
Once fingerprint evidence is collected, the process shifts to analysis and comparison to determine if a match exists. This involves examining unique features within the friction ridge patterns. Primary characteristics used for comparison are minutiae points, which include ridge endings, bifurcations, and dots.
Automated Fingerprint Identification Systems (AFIS)
The initial comparison phase often involves Automated Fingerprint Identification Systems (AFIS). These computer databases store millions of fingerprint records from various sources, including criminal and civil applications. When an unknown fingerprint from a crime scene is entered, AFIS rapidly searches for potential matches by comparing minutiae points and ridge patterns, generating a list of candidate prints.
Human Examination (ACE-V)
Despite AFIS capabilities, a trained human forensic examiner always performs the final determination of a match. AFIS provides a filtering mechanism but does not make identifications. Examiners utilize a structured methodology known as ACE-V: Analysis, Comparison, Evaluation, and Verification.
Analysis: The examiner assesses the quality and quantity of discernible features in the unknown print.
Comparison: This involves a side-by-side examination of the unknown print with candidate prints from AFIS, focusing on agreement of ridge flow, pattern type, and minutiae location.
Evaluation: Based on observed similarities and differences, the examiner forms a conclusion regarding identification, exclusion, or inconclusive status.
Verification: An independent examination of the same prints by another qualified examiner confirms the initial conclusion, ensuring accuracy.
The Power of Fingerprints in Solving Crimes
Fingerprint evidence plays a significant role in criminal investigations and legal proceedings, providing direct links between individuals and crime scenes. When a fingerprint found at a crime scene is identified, it can lead to the identification of an unknown perpetrator at the scene.
Fingerprints can also connect seemingly unrelated criminal acts. If the same fingerprint is discovered at multiple crime scenes, investigators can establish a link between these cases, suggesting the involvement of the same individual and potentially identifying a serial offender. This capability allows law enforcement to understand criminal activity patterns and focus resources effectively.
Fingerprint evidence also serves to corroborate or refute other forms of evidence. For instance, a suspect’s alibi can be challenged if their fingerprint is found at the crime scene, or strengthened if their prints are absent where expected. Fingerprints can support or contradict witness testimonies. In a courtroom, expert witnesses present fingerprint analysis findings to judges and juries. This evidence contributes significantly to the legal process, aiding in determining guilt or innocence and supporting convictions or exonerations.