Firearms And Fingerprints In The Crime

The roles of firearms and fingerprint examiners in the crime laboratory are numerous and varied. Firearms examiners are frequently called upon to do many things that do not fall under the traditional heading of "firearms identification." Examples include serial number restoration with chemical etchants, gunshot residue analysis for barium nitrate, and shooting incident reconstruction. Similarly, fingerprint examiners must be proficient in specialized methods of photography, the application of chemical enhancers (such as luminol and leuco crystal violet), and the use of forensic light sources, as well as being proficient in fingerprint comparison.

The presence or absence of fingerprints on items of evidence submitted to the crime lab by scene investigators is a primary concern and thus constitutes a large part of the daily workload of the crime lab. This requires that fingerprint examiners examine most evidence first, before anyone else handles it. In some instances, such as when guns and ammunition are involved, both fingerprint examiners and firearms examiners examine the same evidence, but it is the fingerprint examiner who works with it first.

Contrary to portrayals in the movies and on television, fingerprints are not always easy to find on certain items of evidence. In the author's experience identifiable fingerprints are found on guns, knives, clubs, and the like in less than 10 percent of the cases. This is due to a number of factors including the surface characteristics of the weapon and the way in which the weapon was handled. On the other hand it is not uncommon to find some evidence of the handling of weapons in the form of partial fingerprint impressions, even though these partial impressions usually fall short of being identifiable to a specific individual.

Some types of evidence simply do not tend to retain any evidence of fingerprints, even when touched. Textured surfaces such as vehicle steering wheels, plastic milk jugs, and suitcase handles seldom show any sign of even having been touched. Other problems can include intense heat, humidity, and/or precipitation, which effectively destroy the fingerprints. Thus the probability of finding identifiable fingerprints on fired cartridge cases left at the crime scene, for example, is remote; the combination of the curved surfaces and the heat produced upon discharge tends to vaporize the fingerprints.

Fingerprints are of two basic types: visible and invisible. Visible fingerprints are termed patent prints. These fingerprints do not require any special enhancement, such as chemical treatment, in order to be seen. Plastic prints are a special type of visible fingerprints and consist of impressions found in paint, putty, and other pliable materials. Fingerprints that are not ordinarily visible are termed latent fingerprints.

Visible Fingerprints
Patent fingerprint (Courtesy of the author)

Latent fingerprints are only visible following the application of specialized lighting, powders, or chemicals. The photographs show an example of a patent print, a plastic print, and a latent print.

Fingerprint examiners typically begin their day in the crime lab by reviewing the new cases that have come in since their last shift. An evidence submission form accompanies all evidence that has been submitted to the crime lab. This form lists each item and specifies which items are to be examined for fingerprints.

In cases where a suspect has been developed, that suspect's fingerprints will also be submitted in the form of inked impressions on special white fingerprint cards. Using these, the examiner can make a direct comparison of any fingerprints found on the evidence with the suspect's prints on the fingerprint cards.

In the event that no suspect has been developed prior to submission of the evidence, any fingerprints found by the examiner will be placed into an automated fingerprint identification system (AFIS), and a computer search will be conducted. If the fingerprints are determined to be similar to any prints in the database, the examiner will then make a "manual" comparison; in other words, the examiner, not the computer,

Physical Matching Forensic
Plastic fingerprint (Courtesy of the author)
Obliterate Fingerprints

visually compares each print identified by the computer against the suspect's known print to identify a match. If no identification can be made, the fingerprints go into an open case file in the database. They will remain there until either they are identified or the examiner removes them.

Fingerprint examiners use a wide variety of techniques, both physical and chemical, to develop and/or enhance fingerprints on a wide variety of surfaces. These will be discussed in chapter 3.

An additional crime lab role of fingerprint examiners is photographic documentation of the fingerprints that are located. Consequently the examiner must be well versed in the various aspects of photography. This particular duty is appropriate since the fingerprint examiner is really the only person who knows what areas of the prints are most important and hence must be visible in photographs that are made for documentation purposes.

Through the use of digital imaging, fingerprints found at crime scenes may be downloaded onto computer hard drives. By utilizing specialized software the examiner can then enhance the prints. Digital enhancement usually involves a combination of eliminating background

Latent Image Photography
Latent fingerprint photography (Courtesy of the author)

interference, such as patterns or grains, and increasing contrast between the fingerprint and the substrate (the surface that the print is on). In this way faint prints that would be otherwise unidentifiable become clear enough to identify.

When the examiner is able to identify fingerprints as belonging to a particular individual, the next step is independent verification. This means that another examiner is called upon to review the prints (known and questioned) and either confirm or deny the original identification. The requirements for making fingerprint identifications will be discussed in a later section.

The purpose of verification is twofold. First, confirmation of all identifications is part of a crime lab's quality assurance program and serves to minimize the potential for false identifications. The second reason is that if the original examiner is unavailable when the case goes to court, the verifying examiner can step in as needed and testify as to the identification.

Fingerprint examiners are also sometimes required to work with partially decomposed bodies in an effort to identify the deceased. This task can require actually removing the skin from the cadaver's fingers and using various techniques to make the skin suitable for producing inked fingerprints, which in turn are used in the identification process. An admittedly gruesome aspect of this can include slipping the finger skin over the gloved fingers of the examiner in order to make an inked impression of each. Clearly this is not a job for the squeamish!

Fingerprint examiners are also on occasion called to court for the purpose of identifying individuals who are about to be sentenced. This is done to verify that the correct individual is being sentenced. The examiner will usually take inked impressions from the suspect during a court recess, compare them to known prints, and then take the witness stand to testify that the individual before the court is in fact the accused.

Large police agencies often have fingerprint technicians, in addition to fingerprint examiners. Fingerprint technicians assist fingerprint examiners by taking the inked fingerprint impressions from suspects in crimes, but fingerprint examiners may be responsible for taking the inked impressions of arrested persons themselves. To record these impressions the technician or examiner applies ink directly to the subject's hands, or

Firearms Transfer Thumbprint Card
Portion of an inked fingerprint card (Courtesy of the author)

special inkless cards may be used. Individual fingers, as well as the palms and entire surface of the hands, are impressed onto the fingerprint cards, as shown in the photograph. These inked fingerprints are then entered into an AFIS database. This enables examiners to compare known fingerprints with latent fingerprints of unknown origin recovered at crime scenes. AFIS "hits" consist of lists of suspect prints exhibiting similarities to the prints entered into the database.

Firearms identification is similar to fingerprint examination only to the extent that both processes involve the comparison of known items with unknown items from the crime scene. Otherwise these are two entirely different fields of endeavor. For that reason two different individuals generally carry out firearms examinations and fingerprint examinations.

The term firearms identification is often used in conjunction with the term tool mark identification. In reality, much of firearms identification entails a specific area of tool mark identification. By definition a tool

Forensic Breech Face Marks

Ejector mark

Firing pin impression

Extractor mark

Extractor mark

Ejector mark

Firing pin impression

Forensic Breech Face Marks

Magazine marks

Chamber Breech marks face marks

Chamber Breech marks face marks

Magazine marks

Q Infobase Publishing

Top: Cutaway of a gun barrel (Courtesy of the author) Bottom: Cartridge case markings mark results from the contact of one surface with another, the harder of which is the "tool." Thus, in the case of a firearm and a bullet, the firearm (for instance the interior of the barrel) is the tool that produces tool marks on the surface of the bullet as it moves through the barrel upon discharge of the firearm. Likewise, the examination of firing pin impressions, magazine marks, extractor marks, ejector marks, breech face marks, and chamber marks on fired cartridge cases all constitute tool mark examinations.

Forensic Breech Face Marks
Bullets and cartridge cases (Courtesy of the author)

The fact that firearms identification also involves examinations other than tool marks accounts for the distinction between the two areas. The analysis of gunpowder patterns on clothing, the determination of cartridge case ejection patterns, and the measurement of trigger pull or establishing bullet trajectory are examples. Likewise, weapons function testing, shot pellet pattern testing, and serial number restoration are additional non-tool mark comparison aspects of firearms identification.

Firearms identification is often referred to as "forensic ballistics" or just "ballistics." This is actually a misnomer, as ballistics is limited to the study of projectile behavior. Ballistics, in the true sense, includes three different aspects: interior ballistics (bullet behavior within the confines of the barrel), exterior ballistics (bullet behavior upon exiting the barrel), and terminal ballistics (bullet behavior upon impacting a target). Wound ballistics is a specialized area of terminal ballistics relating to the behavior of bullets striking human or animal targets.

Tool mark examination, on the other hand, is conducted to determine whether a tool mark was made by, or could have been made by, a particular tool. Tools commonly examined and compared with questioned (evidence) tool marks include hand tools—such as pliers, saws, screwdrivers, pry bars, hammers, and chisels—and power tools— such as drills, air chisels, chain saws, and reciprocating saws.

Thus, firearms and tool mark examinations have similarities and differences. If firearms and/or ammunition components are being examined, then the term firearms examination is most appropriate. Likewise, when marks produced by something other than a firearm are involved tool mark examination is appropriate. Since the majority of the work in a crime lab involves firearms and ammunition components, tool mark examinations might be considered a secondary discipline. At the New York City Police Department (NYPD) Crime Laboratory, for example, the firearms examiners do not presently carry out any tool mark examinations due to the heavy load of firearms-related work.

Firearms examiners are frequently asked to predict the type of firearm used to fire various ammunition components (such as bullets, cartridge cases, or shot cups) found at crime scenes. Having some idea of the firearm that was used in a crime allows investigators to know what type of weapon to be on the look out for. For example, being able to determine that the weapon used to fire a bullet found at the scene has polygonal rifling helps narrow the number of possible weapons.

In order to be able to test-fire weapons with missing parts, to identify certain weapons based on their special characteristics, and to make other comparisons, firearms examiners must have access to weapons collections. Finding a needed part for test-firing an antique rifle with a rolling block action or a shotgun barrel with a particular choke, for example, is easy when such a weapon is available in a reference collection. Ammunition reference collections must also be created and maintained to allow firearms examiners to be able to make various comparisons. Collections should include center fire and rim fire cartridges (primer located in the annular rim), as well as shot shells (including bird shot). Weapons and ammunition reference collections require regular inventory for security purposes. The larger a reference collection is, the greater the need for security. The Los Angeles County Sheriff's Department, for example, has more than 100,000 specimens in its firearms reference collection, making maintenance and inventory a formidable task.

The use of databases in firearms examinations also requires some tedious work in order to enter the necessary data. The markings left on fired bullets and cartridge cases found at crime scenes and recovered at autopsies must be entered into a national database called the National Integrated Bullet Identification Network (NIBIN) so as to allow examiners around the country to search the database to see if the same weapon that fired the bullets and/or cartridge cases they are examining was used in other crimes. Street gangs in particular are notorious for retaining weapons and using them in crime after crime. This information in the database allows firearms examiners to relate different crimes on the basis of the markings found on bullets and cartridge cases left at the crime scenes.

Firearms examiners are also called upon to attempt the restoration of obliterated serial numbers on firearms as well as other items ranging from automobile components and motorcycles to electronic equipment. This requires various specialized techniques that depend on the particular material the serial number is on and the degree to which obliteration has occurred.

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Responses

  • zachary
    What types of cases do firearms examiners usually work in?
    6 years ago
  • Chanelle Macleod
    What work do tool mark examiners carry out?
    6 years ago
  • rhys
    What Are patent Fingerprints?
    5 years ago
  • Christin
    How to photograph latent fingerprints?
    5 years ago
  • mathias
    What units of crime laboratory would examine a revolver with fingerprints?
    4 years ago
  • amethyst
    Which forensic science laboratory examines fingerprints?
    4 years ago
  • douglas
    Where to mark fire arms found at the crime scene?
    4 years ago
  • Marjo
    Can cartridge cases found at a crime scene be worked for latent prints?
    3 years ago
  • ursula
    What is involved with forensic test firing of weapons?
    3 years ago
  • erminia
    Can you use fingerprint analysis on firearms?
    2 years ago
  • Herugar
    How to test a weapons found at a crime scene?
    4 months ago

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