Techniques for Collecting and Analyzing Fingerprints

Fingerprint evidence left behind by a suspect or victim may identify who was at a crime scene and what he or she touched. However, it is important for defense attorneys to know, and to inform the jury, that the techniques used to locate and identify fingerprints are far from a perfect science. An understanding of how fingerprints are located and lifted can help attorneys recognize if a flawed analysis was performed by investigators or lab technicians. Further, knowledge of the various fingerprint collection techniques is essential to successful cross-examination of crime scene technicians and fingerprint examiners. This post attempts to provide an overview of the techniques used to locate, lift, and identify a fingerprint.

Step 1: Locating the fingerprint

Locating a fingerprint often requires a vigilant and calculated search. However, in circumstances where the print is visible to the naked eye, finding a fingerprint is relatively easy. The more intricate searches take place when the print is present on a surface but not visible. The type of fingerprint left behind usually determines the amount of time and effort investigators must put into locating the print.

According to Forensic Science, there are three types of fingerprints. D.P. Lyle, Forensic Science (ABA Fundamentals), p. 255 (2012):

• Patent prints are easy to locate since they are visible to the naked eye. Patent prints occur when someone has a substance on their fingers such as grease, paint, blood, or ink that leaves a visible print on a surface.
• Plastic prints are also easy to locate but are less common than patent prints since they occur when someone touches an object such as wax, butter, or soap and leaves a three-dimensional impression of the finger on the object.
• Latent prints are the most common type of print and take the most effort to locate since they are invisible. Latent prints occur when someone touches any porous or nonporous surface. The natural oils and residue on fingers leave a deposit on surfaces which mirror the ridges and furrows that are present on the individual’s finger.

Investigators often follow a two-phase process when searching for fingerprints. The first phase involves looking for patent and plastic prints since they are visible. Often times, a flashlight is used during this phase. The second phase involves a blind search for latent prints, according to Scientific Evidence. Paul C. Giannelli, Edward J. Imwinkelried, Andrea Roth, and Jane Campbell Moriarty, Scientific Evidence, p. 949 § 16.03 (5th ed. 2012). To narrow the search, investigators usually focus on the entry and exits points that the suspect used and any items that appear to have been disturbed, such as overturned lamps or possible weapons. Id.

The type of surface being searched for fingerprints often determines the technique employed by investigators. Id. at 950.

Nonporous Surfaces:

A powder technique is usually used to identify latent prints on nonporous surfaces such as glass, marble, metal, plastic, and finished wood. Id. When powder is distributed on the surface, it adheres to the residue deposited from the finger’s touch, allowing investigators to find the print. Often times, to avoid smudging the print, a magnetic powder technique is used in which the powder is poured on the surface and then spread evenly over the surface using a magnetic force instead of spreading the powder with a brush. See Forensic Science by D.P Lyle p. 256. The color of the powder should contrast with the surface that is being searched to allow better visibility. For example, the investigator should use a white or grey powder if searching a black marble countertop for prints. See Scientific Evidence by Paul C. Giannelli p. 950.

Attorneys should find out whether the crime scene technician who collected prints using fingerprint powder used a disposable brush. If a brush is reused in different locations at a crime scene or reused at another crime scene, the brush can transfer trace amounts of DNA evidence.

Another popular technique for fingerprint location and identification used by both lab technicians and investigators at the crime scene is superglue fuming. Superglue fuming is a chemical process that exposes and fixes fingerprints on a nonporous surface. Id. at 959. In the lab, the process works by using an airtight tank, known as a fuming chamber, to heat up superglue (liquid cyanoacrylate) which releases gases that adhere to the oily residue of print, thereby creating an image of the fingerprint, according to this article. Superglue fuming can also be performed at the crime scene. Rather than using a fuming chamber, crime scene investigators may use a handheld wand that heats up superglue and a florescent dye, according to Forensic Science by D.P Lyle p. 256. Superglue fuming performed at the crime scene can be vital to preserve prints on items that are being sent to the lab via mail. One of the drawbacks is that if the evidence is fumed too long, it can distort the print, rendering it useless, according to this article. To read the procedure used by North Carolina State Crime Lab to conduct superglue fuming in a fuming chamber, click here. To read the procedure used by the North Carolina State Crime Lab to conduct superglue fuming using a portable wand, click here.

Porous Surfaces:

The powder technique is not as effective on porous surfaces such as fabric, unfinished wood, and paper. Instead, investigators often use chemical methods to locate the print such as iodine fuming, silver nitrate, or ninhydrin. When one of these chemicals comes into contact with the chemicals present in the fingerprint residue (natural oils, fats), the print become visual. See Scientific Evidence by Paul C. Giannelli p. 951-53.

Iodine fuming takes place in a fuming chamber. The process works by heating up solid crystal iodine which creates vapors that adhere to the oily residue of print, producing a brown colored print, according to Forensic Science by D.P Lyle p. 257. One of the drawbacks of using iodine fuming is that the print fades quickly after the fuming takes place and therefore must be photographed quickly. Alternatively, if the print is sprayed with a starch and water solution, it can be preserved for several weeks. Id.

Silver nitrate, when exposed to latent prints, reacts with the chloride of the salt molecules found in print residue, forming silver chloride. When exposed to ultraviolet light, silver chloride turns black or brown, making the print visible. Id. This method works particularly well on impressions left in cardboard and paper-like surfaces, according to Scientific Evidence by Paul C. Giannelli p. 952.

Ninhydrin is more commonly used than iodine fuming and silver nitrate techniques to locate a latent print. Id. The object on which the print is located can be dipped in or sprayed with a ninhydrin solution, which reacts with the oils in the print’s residue to create a bluish print. Forensic Science by D.P Lyle p. 257. One of the drawbacks of using ninhydrin is that the reaction is very slow, often taking several hours for the print to become visible. Id. To accelerate the reaction, the object containing the print can be heated to 80 to 100 degrees Fahrenheit. Id. To read the North Carolina State Crime Lab’s procedures for ninhydrin, click here and here.

A variety of other techniques are sometimes used. For example, laser illumination creates a contrast between the print and the surface which exposes the print. To learn more, see Scientific Evidence by Paul C. Giannelli p. 955-58.

Human Skin:

Locating and identifying fingerprints left on human skin is incredibly difficult. According to Scientific Evidence, the first major obstacle is finding the print since the oily residue left by fingers that creates the fingerprint itself is often present on human skin, making it difficult to create a contrast between the surface (skin) and the print. Further, after a print is left on human skin, the oily residue often disperses and is absorbed into the skin, blurring the print. Two hours is the maximum amount of time that a print on skin may be viable. See Feldman, Meloan, & Lambert, A New Method for Recovering Latent Fingerprints from Skin, 27 J. Forensic Sci. 806 (1982). For more information about current techniques used to identify fingerprints on human skin, see Scientific Evidence by Paul C. Giannelli p. 961-64.

Textured Surfaces:

Surfaces that are not flat or have a rough surface, such as a painting with brush strokes or a golf ball will make the process of identifying and collecting fingerprints more difficult, but not impossible. Click here to read about fingerprints collected from golf balls and other difficult surfaces.

Step 2: Photographing the fingerprint

After the print is located, it is vital that it is photographed before it is lifted. A photograph captures where the print was located in comparison to other objects and captures the orientation of the print. Further, a photograph can serve as a key piece of identification of a patent or plastic print and can be used to compare and possibly match the print to its source. Photographing the print’s location at the crime scene also guards against tampering of evidence. See Scientific Evidence by Paul C. Giannelli p. 964-65.

Step 3: Lifting the fingerprint

“Lifting a fingerprint” means to make a permanent impression of the fingerprint. Lifting a print can be accomplished on either flat surfaces or round surfaces. Lifting a print usually involves a rubber tape with an adhesive surface which is applied to the fingerprint, leaving an imprint on the tape. Often times, a flat object, such as a ruler, will be slowly swiped across the top of the tape to ensure that there are no bubbles or ripples in the tape that will affect the imprint. Next, the tape is carefully peeled off the surface and a plastic cover is placed on the adhesive side of the tape to prevent disruption of the print. Identification information and a description of the location of the print should be written on the back of the tape or card. Id. at 967-68.

After the print is lifted, it is converted into digital data that can be modified to create a clearer image.

Step 4: Comparing the fingerprint

The final step involves a close examination of the characteristics of the fingerprints. For more information about the system of friction ridge classification, click here to read chapter 5 in The Fingerprint Sourcebook by Scientific Working Group on Friction Ridge Analysis, Study and Technology (SWGFAST).

The fingerprint examination process utilizes the ACE-V method which stands for Analysis, Comparison, Evaluation and Verification to compare a print collected from a crime scene to a set of known prints. For a detailed description of the ACE-V method and how it is applied, click here to read chapter 9 of The Fingerprint Sourcebook. This post will not address critiques of the ACE-V method, but additional information that can be used to challenge this technique in court can be found here.

A system called the Automated Fingerprint Identification System (AFIS) was created to find a match to the print using a computer database. To learn more about the AFIS, click here to read chapter 6 of The Fingerprint Sourcebook.

For more information on the lab procedures employed by the North Carolina State Crime Lab in fingerprint collection and analysis, click here.