Anyone who has ever watched or listened to a crime drama or true crime series knows that fingerprints are a key piece of evidence often collected from crime scenes. They are a form of biometric information — a unique physical characteristic that can be used to identify an individual. Part of the unique characteristics are the distinct patterns visible with the naked eye; arches, loops, or whorls.

Because it is believed — but has not actually been proven — that no two people have identical fingerprints and people’s fingerprints don’t change over time (without interference from injury or surgery), their presence at a crime scene can provide valuable clues to help law enforcement and prosecutors place a suspect in a key location, often helping to secure a conviction.

There are three types of fingerprint evidence classifications that may be collected for further analysis and to try to match with records on file in fingerprint databases: latent, patent, and plastic. Let’s break them down.

Latent Fingerprints

Left by naturally occurring secretions such as sweat and oil on the skin’s surface, latent fingerprints are often difficult or impossible to see with the naked eye because they do not fluoresce on their own. They can be collected using powder dusting techniques or chemical processing with fluorescent compounds that make the fingerprints visible when viewed under different colors of light.

Patent Fingerprints

Patent fingerprints can be seen without processing, as they leave visible marks by transferring materials such as blood, dirt, ink, grease, etc., from the fingers to another surface. When someone gets arrested, police often use ink to capture that person’s fingerprints—another example of patent prints.

Plastic Fingerprints

Similar to patent fingerprints, plastic fingerprints can be seen with the naked eye. They are formed by fingers leaving three-dimensional impressions in materials such as wet paint, tar, soap, or wax. Anyone who has ever squished Play-Doh or Silly Putty in their hand has probably made their own plastic fingerprint without even realizing it!

Fingerprint Collection Methods

Depending on what type of surface the fingerprints are found on, different collection methods work best to pull the prints for use as evidence. 

For porous surfaces, chemical processing is required. Crime scene techs apply a compound called ninhydrin that enhances fingerprint visibility so photographs can be taken for further analysis.

There are also two types of non-porous surfaces: smooth and rough. On smooth surfaces, traditional powder processing is often used. Techs brush fingerprint powder over the prints to enhance their visibility, then lift the prints off the surface with special tape. On rough surfaces, the same powder-and-brush method is used, but instead of tape, techs use gel or silicone lifting materials that can get into the grooves of the rough surface and pull a more complete print for analysis.

Analysis and Comparison

Once fingerprints are successfully collected, a special examiner can analyze the prints of the unknown person for class and individual characteristics before comparing the characteristics to known fingerprints to try to make an identification.

Class characteristics refer to the three pattern types mentioned earlier: arches, loops, and whorls. One or multiple of these patterns can be found on every person’s fingerprints.

Individual characteristics include variations in fingerprint ridges such as ridge endings, dots, islands, and bifurcation that are unique to you.

If two fingerprints don’t fall into the same class, they cannot be considered a match, and an examiner would automatically exclude the known print as being a possible source for the unidentified print.

If two fingerprints do share a class, then an examiner would review individual characteristics to determine if there’s a match between the unidentified print and a known one. The two possible outcomes in this scenario would be a positive identification (where there is sufficient detail to compare and there are no unexplained discrepancies) and an inconclusive result (where there’s not enough detail to compare or there are unexplained discrepancies).

To be considered reliable, results typically need to be verified by two separate examiners conducting independent examinations. 

Does it sound like there’s a possibility for human error? Absolutely — which is why databases such as AFIS (Automated Fingerprint Identification System) have created ways to help assist trained examiners during their analysis. The databases speed up the process and can detect more individual characteristics with minimal effort, according to the FBI.

Not a ‘Perfect Science’

Despite its popularity as an identification tool, fingerprinting is not infallible. 

It is widely accepted that no two people share the same fingerprints, but there is no hard proof of that assertion. In addition, one of the biggest issues is that fingerprint collection is challenging and often prone to error and incomplete specimens. 

On top of that, there are no standardized criteria for analyzing fingerprints for possible matches. One examiner may declare a match if only 10 characteristics are identical, while another may require 15 or 20 points to match.

While DNA is rapidly overtaking fingerprints as the forensic evidence of choice in many criminal investigations, fingerprints are still widely used. Focusing on the collection quality and increasing the standardization of analysis methods are key to ensuring fingerprints remain a valuable evidentiary tool for law enforcement and the criminal justice system.



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