Almost everyone has heard of using fingerprints to identify people – it’s one of the oldest biometric identification technologies known to mankind. Today, fingerprint identification is used forensically to identify victims of crimes and place suspects at crime scenes. But how exactly does it work?
Before we can talk about fingerprint identification, we need to know a few things about fingerprints. A fingerprint is imprint of the pattern of flexible ridges on the pad of a finger.
The first characteristic that is critical to fingerprint identification is that fingerprints don’t change over time. That is, everyone’s fingerprints are the same from birth until death. The size of the finger changes, but the fingerprint pattern does not. The second critical characteristic is that no two fingerprints are ever exactly alike in every detail. Interestingly, even identical twins do not have the same fingerprints.
The uniqueness of fingerprints makes it possible to use prints from a crime scene to either connect a suspect to the scene or eliminate a suspect from consideration. Latent fingerprints (those left accidentally at the scene of a crime) can be collected from a wide variety of surfaces. Invisible fingerprints, such as those made from skin oils, can be processed to produce visible prints for identification. Even partial, smudged, or otherwise imperfect prints can help detectives make a case against a suspect.
In the context of crime scene investigation, the identification of latent fingerprints is the process of analyzing the latent prints against a database of fingerprints to try to find a match. The challenges involved in this task are numerous, particularly if the fingerprint is not clear and complete (and it usually isn’t!)
Before fingerprints can be identified, they must be detected.
When a crime is committed, crime scene investigators typically use adhesive powders to find fingerprints. This is often called ‘dusting for fingerprints’ because investigators use brushes to dust surfaces with powder. The powder sticks to the oils present in fresh fingerprints, making them visible. After locating a print, crime scene investigators photograph the print and ‘lift’ it using special lifting tape. They also write a short description of where the fingerprint was found and begin a meticulous record of the individuals who handle or transport the evidence as it makes its way to the forensic laboratory, where fingerprint identification occurs.
Forensic scientists who analyze fingerprints in the lab are typically called fingerprint examiners. The fingerprint examiner’s job begins when print arrives at the lab from the scene of the crime. A fingerprint examiner must first carefully mark the distinguishing features of the full or partial print.
The next step is to enter the print into a fingerprint identification system such as the Federal Bureau of Investigation’s Integrated Automated Fingerprint Identification System (IAFIS). Finally, the examiner must manually compare latent fingerprints with potential matches obtained by the system in order to make a positive identification.
The examiner must thoroughly document every step of the process and write a full report about the identification process and its conclusions. If the fingerprint evidence is used in a case that goes to trial, the examiner may be required to testify in court about the work. Fingerprint evidence can be critical in placing a suspect at the scene of the crime!
Fingerprint identification techniques continue to evolve with the increased availability and power of modern computers, and this field of forensic science promises to be exciting for years to come.
About the author: Emily Nelson earned an M.S. in Electrical Engineering from the Massachusetts Institute of Technology before beginning her career as a science writer.