How Police Search For Fingerprints at a Crime Scene, and how they are matched

A very simple definition of a fingerprint impression is any impression of, or left by, the friction ridge skin of a finger or thumb. Friction ridges are the ridges and furrows that are on the pads of the fingers and thumbs. They are there to provide grip, hence the name friction ridges.

Almost everyone has fingerprints; they are formed while in the womb, and they are permanent, (short of scaring or mutilation to the skin), and they are unique to the individual.  Even identical twins, who have identical DNA, have different fingerprints.  

Fingerprint impressions can theoretically be found on most any surface that is smooth, clean, dry; preferably non-porous, and flat, and police have used fingerprints in criminal investigations as a means of identification for centuries.  Fingerprint identification is one of the most important criminal investigation tools due to the fact that fingerprint patterns don’t change over life, and no two individuals have ever found to have identical fingerprints, making them unique to an individual, regardless of their age.

Fingerprint impressions aren’t as easy to locate and collect, as TV would have us believe, and they are even harder to match to an individual. Fingerprint impression processing is art and science mix, as is the identification of fingerprints once located. Having said that, let’s look at how fingerprint impressions are left on a surface, and how we locate them, collect them, and then try to match them to someone.

Fingerprints fall into three basic categories, and they are categorized by what it takes to see them unaided. Fingerprints that are visible without powders or chemical process come in two basic forms, patent, and plastic. Prints you can’t see, and you have to process to observe are called latent prints. A patent print, and a plastic print, are clearly obvious during the inspection while the latent print is one that is not.

Patent prints are formed when blood, dirt, ink, paint, etc., are transferred from a finger, thumb, palm, onto a surface. Patent prints can be found on a wide variety of surfaces: smooth or rough, porous (such as paper, or wood) or nonporous (such as metal, glass or plastic). Patent prints are not physically collected, they are instead photographed beside a measuring devise, for scale.

A plastic print would be a print left in a soft malleable surface, like wet paint, or if your old enough to remember, silly putty. Silly Putty produced great plastic prints, and as a boy, I was fascinated with how I could leave a print on it that was clear as the matching one on my finger. Plastic prints are not physically collected, they are instead photographed beside a measuring devise, for scale.

Latent prints are generally formed when the body’s natural oils and constituents in sweat are transferred from a finger, or thumb onto a surface. Since the oils and amino acids deposited aren’t readily visible, we have to use fingerprint powders, or chemical reagents to make them visible. As mentioned above, smooth, clean, dry, and less porous, or nonporous surfaces are best mediums for successful recovery of latent fingerprints.

Magnetic Wand with black mag powder: Sirchie Fingerprint Labs

For general processing for latent prints, the go-to tool for the search is fingerprint power. The process of using the powder to process for latent prints is called ‘dusting for prints’ and it utilizes black graphite powder, aluminum flake powder, or magnetic powder. There are various colors to choose from to match the surface being fingerprinted, however black powder is still the workhorse for most technicians, and new technology has brought many variants in color into the mix.  My personal favorite non-magnetic powder is what is called bi-chromatic, and its used for almost any surface, it’s a combination of black and aluminum powders.

Let’s look at some ways and products to process a surface for latent prints:

White powder works especially well dark colored surfaces. White powder generally adheres well to the oils of the print, and provides a contrast on most surfaces. It works well on glass, metals, plastics, and dark colored surfaces.

Silver/Gray (aluminum) powder adheres to prints on non-traditional surfaces and provides good contrast to the background surface. It performs best on glass, plastic and rubber.

Bi-Chromatic powders are typically a combination of black and aluminum powders, and it will show black ridges on the lighter parts of a surface, and light colored ridges will appear on the dark part of the surface. The lifted print will be dark in color when placed on a white lift card. It works well on most all surfaces.

Black powder is the staple of fingerprint powders readily adheres to the oily residues from the fingers; it is the most versatile of all the powder colors, working well on porous and non-porous surfaces.  Black powder is a close runner up to the bi-chromatic powder in my mind.  

Magnetic-based powders, or mag powder, come in various colors, and if it could be used on ferrous metals, it’s all I would ever use. I love this powder because there are no bristles to touch the surface being printed, and there is no abrasive contact made with the latent print you’re trying to process. The powder is mixed with iron filings; you use a magnetic wand to attract the filings, creating a clump or ball of powder, you then ‘brush’ the ball of powder over the surface to be printed. Mag powder is great for delicate surfaces, but it can’t be used on ferrous metals, so processing any steel, like a vehicle exterior is out. Mag powder works on some porous surfaces and non-porous surfaces like plastic, Styrofoam, rubber. A word of caution here, you can’t mag powder very well in an upside down fashion; gravity fights the ball of powder.

Small Particle Reagent (SPR) is a suspension of fine molybdenum disulfide particles in a surface-active solution for light-colored surfaces and a white suspension for dark-colored surfaces. It adheres to the fatty constituents of latent fingerprints to form a gray or white image. It is sprayed onto the surface of the evidence, the micro-fine particles attach themselves to the latent print residues and the solution runs off. Lifts can be made directly without further processing.  I have used this with great success on wet items, and items that became wet after the latent was deposited.  It is messy, but works great in wet conditions.

In the case in forged checks, or letters. Amino acids and inorganic salts in the sweat react to a couple of chemical reagents, thus allowing us to develop and photograph latents prints from paper. Iodine fuming, Ninhydrin and DFO physical developer are widely used here, and can be very effective; you cannot lift these prints with tape, so they will be photographed with a scale. (Chemicals can cause ink to run, and alter your original, so we get a picture or make a photocopy of the paper before applying them)

Ninhydrin causes prints to turn purple, which makes them easily photographed. This is my preferred method for checking for latent prints on paper.

Iodine fuming produces a yellow or very faint orange color, and it fades somewhat quickly, these aren’t as easy to photograph.

DFO is a ninhydrin analog used for developing latent prints on porous surfaces. Its fluorochrome properties make it especially useful on multicolored surfaces where the background would interfere with subsequent photography. DFO reacts with the amino acids. DFO may produce lightly colored (pink) latent prints, but its real strength is in its fluorescent properties. DFO fluorescence may be produced using alternative light sources.

Physical developer is a silver-based aqueous reagent that reacts with the components sweat in latent fingerprints to form a silver-gray deposit. It can be used to develop latent prints on paper and other porous materials. Normally, physical developer is used on latent prints after they’ve been treated with ninhydrin. Just mix the solutions and immerse documents in the mixture.

Superglue processing, or fuming (Cyanoacrylate) often coupled with powders and certain dyes are also used to process non pourus items such as glass, plastic etc. (my least favorite method; super glue stinks, the vapors will adhere to anything in your fuming tank)

In addition to the methods identified above, there are special techniques for capturing prints from skin, clothing and other difficult surfaces. Amido Black, a non-specific protein stain that reacts with any protein present, is typically used for developing or enhancing bloody impressions.

Any recovered latent print should be photographed, then lifted from the surface with clear adhesive tape. The lifting tape is then placed sticky side down onto a lift card; this protects and preserve the print. Here, we get a small amount of powder on the brush, and very lightly move the soft brush over the surface to be checked. Latent prints show up immediately, and if you are careful, you don’t damage the ridges on the print. A brush can be used on any surface that isn’t greasy or wet, (grease and water will gun up the bristles)

 In criminal justice cases, computerized systems are used to search various local, state and national fingerprint databases for potential matches. The system is called Automated Fingerprint Identification System, or AFIS. Many of these systems provide a value indicating how close the match is, we used an NEC model that assigned a number from 001 to 9999, with the 9999’s being as close as you can get to a match. Regardless of what the program spits out though, a fingerprint examiner has to review the potential matches and make a final determination. I am a huge fan of AFIS, and have had great success with the Texas DPS system we used.

Fingerprint examination involves looking at the quality and quantity of information in order to find agreement or disagreement between the unknown print (from the crime scene or from a forged check, etc.) against known prints on file.

Fingerprint examiners use the ACE-V (analysis, comparison, evaluation and verification) method to reach a determination on each print, it’s a peer review system that verifies your work.

Analysis involves assessing a print to determine if it can be used for a comparison. If the print is suitable, the analysis continues, if it isn’t, then the analysis ends there.

Comparisons are then performed by the analyst between known and suspect prints in a side-by-side comparison. The analyst compares minutiae characteristics and locations to determine if they match.

Evaluation is where the examiner ultimately decides if the prints are from the same source (a match), from a different source (an exclusion) or they could inconclusive, not enough to say it’s a match or not.

Verification is when another fingerprint examiner independently analyzes, and compares the prints. They will either support the finding, or refute it.

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