Section 3.2: Forensic Science & The Crime Lab

Fundamentals of Criminal Investigation by Adam J. McKee

Understanding the capabilities of the crime laboratory is critical to the success of an investigation.  Knowing what can be analyzed and something about how it will be analyzed aids the investigator in sending quality samples to the lab.  No matter how good the lab, the quality of analysis can be no better than the quality of the samples submitted by the investigator.

While lab protocols vary from jurisdiction to jurisdiction, we can use the FBI’s submission protocols as a general template.

Requests for evidence examinations by the FBI’s laboratory must contain the following information:  The submitting contact person’s name, agency, address, and telephone number. Previous case-identification numbers, evidence submissions, and communications relating to the case should be provided.  Descriptions of the nature and the basic facts of the case as they pertain to evidence examinations should also be provided. The name(s) of and descriptive data about the individual(s) involved (subject, suspect, victim, or a combination of those categories) and the agency-assigned case-identification number should be included. The name of the prosecutor assigned should be provided, if available.  

A list of the evidence being submitted “herewith” (enclosed) or “under separate cover.” “Herewith” is limited to small items of evidence that are not endangered by transmitting in an envelope. Write on the envelope before placing evidence inside to avoid damaging or altering the evidence. The written communication should state: “Submitted herewith are the following items of evidence.”

“Separate cover” is used to ship numerous or bulky items of evidence. Include a copy of the communication requesting the examinations. The written communication should state: “Submitted under separate cover by [list the method of shipment] are the following items of evidence.”

In addition to the above information, what types of examinations are requested should be specified.  The lab should also be notified as to where the evidence should be returned and where the Laboratory report should be sent.

Forensic Science Principles

Trace evidence is any tiny fragment of physical evidence such as hairs or fibers from clothing or carpeting, small particles of glass, and so forth.  These minuscule items can help tell the story of what happened at the crime scene. Trace evidence can be transferred when two objects come in contact with each other or when some small particles are dispersed by action or movement.  For example, paint can be transferred from one vehicle to another when a collision happens. Similarly, hair can be left on a cloth during a physical attack, such as in rape cases.

Reconstruction of an event can be done with the aid of such evidence, or the evidence may indicate that a person or thing was present.  Therefore, trace evidence provides crucial information in solving cases. Collection of materials from a crime scene can yield information about where the various sample came from and help demonstrate how the evidence aids investigators in reconstructing the story.  Forensic scientists in the crime laboratory analyze the physical, optical, and chemical properties of trace evidence and use a variety of tools to find and compare samples and then look for the sources or the common origin of each item.

Most test methods require microscopy and/or chemical analysis because samples are often very small and because important properties are not readily apparent.  Using advanced laboratory methods, even the smallest piece of evidence is enough to provide information about what happened at a scene such as whether an item or body was moved, or whether someone was assaulted from the rear, side, or from the from the front.   Some laboratories consider fire accelerants as trace evidence, and others will include them in chemistry even though the same tests are conducted in both the laboratories. Suspects often interact with the victim while committing a crime as well as interacting with the environment of the crime scene. These interactions are very important to keep in mind.  

Trace evidence becomes very important during such interactions because the exchange of physical evidence can occur when anyone or anything comes into contact with something or someone else.   The significance of trace evidence in the investigation of criminal cases was first discovered by Edmond Locard, a French scientist. According to Locard’s exchange principle, wherever a criminal perpetrator steps, whatever he touches, and whatever he leaves behind will serve as a silent witness against him.  Not only his fingerprints and footprints can link him to the scene and the victim, but his hair, the fibers from his clothes, the glass he breaks, the tool marks he leaves, the paint he scratches, the blood or semen he deposits, can prove valuable to the investigator’s case.  

All of these and many more may be “a mute witness” against him.  Trace evidence does not forget, it does not get confused by the excitement or terror of the moment, and it is not subject to the limitations of human memory.   According to the exchange principle, there is always a transfer of evidence when two things come into contact.  If it is absent, it is because the investigator failed to find it, or lacked the tools and technology to detect it.  It is up to the investigator to find relevant trace evidence, and it is up to the laboratory technicians to correctly analyze it before it can ever be used as evidence in a criminal trial.  Between detection and analysis is the all-important task of preservation.

Trace evidence is important in accident investigations. Such investigations usually involve the movement of one part against another.  When there is such contact (as in an automobile collision) telltale marks are left behind. For example, fibers, glass, paint chips, fingerprints, tire impression, glove prints, hairs, cosmetics, plant fibers, soil, and other materials can be present as used to demonstrate linkages between people and things.  For example, in the case of a hit and run, investigators can use such evidence to link a suspect vehicle with the vehicle belonging to the victim.

Modern technology has worked wonders for investigators attempting to identify the characteristics of trace evidence. Comparison databases from scientists and manufacturers contain a growing number of samples of items such as paint, glass, and even soil.  These databases allow for a forensic sample (a sample from a crime scene) to be compared against known standards to provide solid and consistent classifications. For example, the National Automotive Paint File is an FBI database containing more than 45,000 samples of automotive paint from manufacturers dating back to the 1930s.

Law enforcement and forensic agencies are not the only ones that maintain databases of useful information.  Paint manufacturer Sherwin-Williams maintains a large database which can be very helpful in identifying a vehicle’s year, make, and model based on color.  Trace investigators must stay abreast of advances in manufacturing techniques, materials, coatings, and processes. Every item that can be touched or transported has the potential to become trace evidence.  Investigators and analysts must consider the potential that a product may have a new or updated version available.

Collection of Trace Evidence

The collection process begins with documentation of the crime scene and analysis of evidence locations.  Several materials are used for collecting evidence items. These materials include containers such as bags and envelopes.  Non-breakable, leakproof containers are used for the transportation of liquids. Evidence such as blood and plants (which are moist or wet) are usually collected in plastic containers and sent back to the area where evidence is stored.  

Once wet evidence reaches a secure location, it is removed and allowed to dry completely.  It is then repackaged in a new dry paper container. Note that the wet evidence should not be packed in plastic or paper containers for more than two hours because microorganisms start growing in it and destroy the evidence.  The investigator responsible for packaging and transport to the evidence room or laboratory should take precautions to prevent contamination of evidence packages. How investigators should “bag and tag” evidence items depend on the characteristics of the items and the conditions in which they are found.


Analysis of Trace Evidence

Trace material analysis usually starts with a visual examination of the evidence using macro photography followed by microscopic analysis.   there are different analytical methods based on the different types of material available for analysis. Common analytical equipment includes stereo microscopes,  scanning electron microscopes (SEM), and x-ray machines.   

Chemical analyses can be done using mass spectrometry, high-performance liquid chromatography (HPLC), and infrared spectroscopy (IR).  For example, such methods may be used to identify a small amount of explosives, volatile hydrocarbons, and other chemicals.  It is important to protect such samples from damage by using non-destructive testing. That is, non-destructive methods should always be used before making use of destructive methods.  Such analyses are usually conducted by technicians trained in forensic chemistry.  



Microscopy allows for the forensic examination of hair samples.  A key type of class evidence is the structure of the hair in question.  The forensic examination of hairs helps in the determination of origin, such as determining whether the hair is animal or human.  If a sample is of animal origin, the species and possibly breed of the animal can be determined.

When hairs are of human origin, racial characteristics, length, area of the body, and any treatment or damage can be determined.  Samples can be tested to determine the color, shape, and chemical composition of the hair. The presence of toxins, dyes, and hair treatments are also detectable.  This information can assist investigators in including or excluding particular individuals.

If the hair still has a follicle root attached, DNA testing may be used to identify a particular individual.   Otherwise, hair comparison is typically considered as class evidence and is most useful to exclude innocent suspects.    

Collected samples are sent to the laboratory along with control samples from a suspected individual.  Control samples should include hair from all parts of the head.  Pubic hair should also be collected. The pubic area should be combed for foreign hair prior to sample collection.   Hair samples are primarily collected using tweezers.

Hair samples are tested primarily by microscopic comparison and chemical analysis.  Microscopic comparison identifies the shape, color, texture, and other visual aspects of the sample.  Chemical analysis indicates the presence of toxins, drugs, dyes, and other chemicals.


Fibers are thread-like elements from fabric or other materials such as mattresses.  Fibers fall into three categories. Natural fibers come from animals (e.g., wool) or plants (e.g., cotton). synthetic fibers are completely man-made products including polyester and nylon.  Fibers that are manufactured by heavily processing natural materials to create fibers can also be classified as synthetic, such as rayon.  

Fiber examination helps in determining the origin of the fiber, and whether it is natural or synthetic.  Fibers are useful in the crime scene investigation because their origin can often be identified. For example, a carpet fiber on a person’s shoe can indicate the individual’s presence at a crime scene.  Fibers are very mobile and can become airborne, get brushed off, or fall from clothing. This mobility makes timely collection crucial to prevent loss of material or cross-contamination.

Fibers tend to cling to other fibers and hair, but may be easily brushed off.  When approaching a scene, investigators should try to pinpoint the most likely locations for deposited fibers.  For example, clothing from the victim or a suspected weapon are likely places to find fibers. Common collection methods include individual fiber collection using tweezers and vacuuming an area and sorting the materials at the laboratory.   

Trace evidence can also be gathered by tape lifting, but this is not an ideal method of collection due to the destructive nature of adhesives.  Samples that potentially hold fibers should be separately bagged to prevent cross-contamination.

trace evidence analysts often have only mere strands to work with.  From these strands, fiber testing can be done using a high-powered comparison microscope to compare texture in a side-by-side assessment.  Chemical analysis can determine the chemical composition of the fibers. In the case of synthetic fabric or carpet, this information can be used to trace the product to the manufacturer using standard databases, further enhancing the probative value of the evidence.



Source determination of glass requires the comparison of a known with the questioned sample to identify the type and source of the glass.  A variety of material is used in making of glass. These materials make it easy to differentiate one glass sample from another. The properties of glass vary with the temperatures to which it was exposed during its manufacturing.  Some basic properties of glass that can be observed without specialized equipment are color, thickness, and curvature. The optical properties of glass, which require more sophisticated analysis, depend on the methods used in the manufacturing process.  

Glass, then, can be evidence, and it can also be the location of evidence.  For example, investigators often find fingerprints and blood evidence on broken windows.  Broken glass fragments are very small and can be found in shoes, clothing, hair, and skin.  Collecting glass fragments from a crime scene can be valuable in connecting people and objects to places.  For example, windshields have a different color and composition than a drinking glass, so glass fragments on a suspect’s clothing could be compared to those collected at a hit-and-run scene to determine if that individual was present.  

Investigators (and examiners in the lab) often use magnification and light to find glass fragments on clothing.  

“Jigsaw” Matching

When things break or tear, they usually do so in a random way rather than in straight lines (or other predictable, uniform patterns).  When the pieces of a broken object (or a torn one) are large enough to observe, unique patterns may emerge such that the original shape can be reconstructed.  When the pieces of a broken object can be reconstructed like a jigsaw puzzle, the pieces can be regarded (beyond a reasonable doubt) as coming from the same source.  This type of pattern matching is possible with several types of evidence, but it is perhaps most commonly used with glass evidence and is often called fracture analysis.  Glass characteristics are usually considered as class evidence, but the presence of a demonstrable fracture pattern raises it to the level of individualized evidence.  Glass, then, can yield valuable information from fracture marks lines and patterns.


Modification History

File Created:  05/02/2019

Last Modified:  02/03/2024

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