What would increase the probative value of natural fiber evidence

1Forensic Science and Criminalistics

Associated Press

Learning Objectives After reading this chapter, you should be able to do the following:

▪ Define forensic science and how it contributes to a case, as well as explain the CSI Effect and the scientific method.

▪ Summarize the history of forensic science and contributors to the field. ▪ List and describe some forensic science specialties. ▪ Identify the elements of a forensic investigation, how physical evidence can be produced, and forensic analysis.

▪ Describe the work and work product of a forensic scientist. ▪ Describe the U.S. court system, and the key rulings on physical evidence admissibility through expert testimony.

▪ List and discuss major issues in forensic science today.

149

6Trace and Materials Evidence

Alice S./BSIP/SuperStock

Learning Outcomes After reading this chapter, you should be able to

▪ Briefly explain the probability of chance duplication and the chain of custody with trace evidence. ▪ Explain the structure of hair and how it is collected and analyzed. ▪ Describe natural and human-made fibers and how they are collected and analyzed. ▪ Discuss the structure of glass and its collection and analysis. ▪ Summarize paint structure and how it is collected and analyzed. ▪ Explain the importance of soil and its collection and analysis. ▪ Describe the limitations a trace analyst should observe when presenting the results of trace evidence analysis.

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Section 6.1Probability and the Chain of Custody

Introduction While there can be many types of trace evidence, this chapter will cover some of the most common types, including hairs, fibers, glass, and soil. Trace evidence by definition relates to something small. It encompasses evidence such as particles of glass from a broken win- dow embedded in the sole of a suspect’s shoes, or particles of glass in a victim’s wounds that must be compared to a broken windshield on an automobile suspected of being involved in a hit-and-run accident. It also includes a few hairs from a victim found on a suspect’s jacket, or a few fibers from the suspect’s jacket found on the victim’s clothing. It is worth noting that while the term trace does refer to a small quantity, the term means that the forensic lab is capable of analyzing small quantities of a substance. It doesn’t necessarily mean that you always find the evidence in small quantities in casework.

While this type of evidence can be important to the solution of a crime, care must be taken in its use. Trace evidence is for the most part class evidence, also known as circumstantial evi- dence. It can help point officers in the right direction in their investigations—for instance, by exonerating an innocent person—but it is not on its own usually enough to close a case. Also, remember that because the materials being dealt with are small, extreme care must be taken to avoid loss or contamination of the evidence.

Although forensic scientists strive to devise methods applicable to small quantities of this kind of evidence, evidence could be present in larger amounts. For this reason, we also refer to this category as materials evidence. Many types of transferred evidence fall into this broad category. One of the features that justifies placing it into the same category is that the methods used to analyze trace and materials evidence are similar. One principal technique is micro- scopy—the use of microscopes. There are several types of microscopes used for different purposes in examining trace evidence. You are probably familiar with the “biological” micro- scope, a compound microscope often used to look at biological specimens. These microscopes usually have one eyepiece. There are also stereoscopic binocular microscopes, which have two eyepieces for “stereo” vision and provide views of specimens at various magnifications. A variant of the light microscope called a polarized light microscope (or PLM) allows the same visualization as a regular light microscope but also the measurement of more sophisticated optical and physical properties of specimens. Another type of microscope is the comparison microscope. This instrument uses two connected optical bridges and allows a person to view two different specimens, side by side in the viewer, simultaneously. It is ideal for doing micro- scopic comparisons and can be used for hairs and fibers as well. Finally, there are electron microscopes. They employ a beam of electrons rather than a beam of light and permit very high magnifications of specimens. There are some limited uses for “scanning” electron micro- scopes in forensic work. For example, they may be used to confirm the identity of gunshot residue particles. We will discuss comparison and electron microscopes in Chapter 9. Here, we’ll cover the concept of probability of duplication and the importance of maintaining the chain of custody for these types of evidence.

6.1 Probability and the Chain of Custody Trace evidence was briefly described in Chapter 1, defining the scope of criminalistics. It is often used in forensic analysis because of the Locard exchange principle (see Chapter 1.4),

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Section 6.1Probability and the Chain of Custody

and it can be used to tie a suspect to a victim or crime scene, a victim to a crime scene, or—in lucky cases—tie the suspect, victim, and crime scene together simultaneously. In the same way, trace evidence can indicate disassociations. While this sounds simple in principle, most often this type of evidence is circumstantial, defined only in terms of class rather than of indi- vidual characteristics. In other words, the evidence will relate to a group of similar objects rather than an individual object at a scene or a specific suspect. This evidence will only indi- cate proof of a point at issue in a court case, rather than provide conclusive proof. Oftentimes, interpretation of trace evidence comparisons comes down to the probability of chance dupli- cation. What is the likelihood that two people share hair characteristics? How many jackets were made of a particular cloth in a particular color? This information must be determined by forensic or other scientists, in some manner, for presentation to a judge or jury. The trace analyst is generally unable to walk into court and say, “This is the jacket, to the exclusion of all others.” However, the analyst can provide the circumstantial basis for the evidence that points to a suspect, while warning the trier of fact that there could be others. A guilty verdict can be achieved if the buildup of this circumstantial evidence can lead the judge or jury to conclude beyond a reasonable doubt that the evidence points to the suspect instead of another indi- vidual. Trace evidence can also be exclusionary. A trace analyst can use a comparison sample taken from a known source to tell if the known was not the source of the evidence. This finding is important because it is definite. If a textile object is excluded as the source of a fiber, this conclusion is absolute.

It is often difficult to determine how common or rare an item really is. Sometimes, the research has just not been done. In other cases, it cannot be done. For example, if carpet fiber from a car trunk is taken as evidence, an analyst can find out how many carpets of that kind and color were installed in a certain make, model, and year of car. What likely cannot be determined is the number of vehicles still in use and where they are located. If the crime occurred in Los Angeles, the national number matters less than the greater Los Angeles number. Investigators can go to the department of motor vehicles and find out how many of these vehicles are regis- tered in the county, but their distribution among the population remains unknown, and it is difficult to know if the vehicle from which the fiber came is registered in Los Angeles. So, while some frequencies may be given to the court, there is uncertainty in them. Often, several kinds of trace are present in a case and can provide good circumstantial evidence. Therefore, trace evidence of several types can be used to assist in the association of a suspect with various aspects of the crime scene, rather than depending on one item alone to convince a judge or jury that a suspect was at the scene. The logic is that it is unlikely for several different varieties of trace evidence to match exemplars by chance, all at the same time and in the same case.

As noted with other types of evidence, the chain of custody is crucial to the successful pros- ecution of a case. Additionally, the chain of custody allows the scientist and everyone else

Think About It

Since trace evidence does not generally allow a scientist to reach an individualization con- clusion, should we dismiss it in favor of evidence that will lead to an undisputable conclu- sion about the inclusion or exclusion of a suspect? Why or why not?

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Section 6.1Probability and the Chain of Custody

who handled the evidence to demonstrate that there was most likely no contamination of the evidence or loss of important evidence and that the evidence was not changed in some detrimental manner. How might this be done with trace evidence? This can be accomplished by maintaining the chain of custody, which starts with proper collection, documentation, and preservation.

Collection The location and collection of the evidence is important in any case. During the examination of large items, trace materials may be located and can be used as evidence. Fibers or hairs on clothing or soil, glass or paint on the ground or elsewhere in a yard—these can help link the suspect to the victim or crime scene. Some of this evidence can easily be picked up using tweezers, while some, such as paint on a wall, must be scraped, taking care to remove the sample all the way to the wall surface. This type of evidence can also tie remote crime scenes or transport mechanisms to the victim or suspect. Conversely, if there is contamination of the evidence in some manner, the evidence could lead to an erroneous conclusion by the analyst.

When locating and collecting trace evidence, the crime scene technician must be aware of the possibility of loss or contamination and take a few precautions. First, he or she must wear proper garb when on the scene. This clothing—sort of like “space suit” garb—should be worn to prevent hairs or fibers from being left on the evidence by the investigator. Also, the investigator should wear nonobstructive headgear so as not to shed hairs onto the evidence. Second, when processing multiple items, victims, or suspects, the items of collected evidence should never share the same space and should not be handled by the investigator at the same time in the same protective clothing. Any postincident contact that transfers hairs and fibers between the victim and suspect will render the evidence useless in court. Lastly, if it appears there is trace evidence on some larger object, such as a bedspread, the entire object should be packaged as evidence. If large items are collected, they should be handled gently and pack- aged in plastic, or if biological fluids are present, paper containers of appropriate size should be used. Individual hairs and fibers that are collected can be stored in druggist folds or small coin envelopes, which will help prevent loss of this evidence. The druggist fold containing the evidence is often packaged in a sealable secondary container, such as a ziplock bag.

Generally speaking, there are two approaches to the collection of trace evidence items. The first involves collecting in its entirety whatever object has the trace on it. For instance, a crime scene technician would collect a bedspread that has hairs or fibers on it, a shoe that has soil on it, or a bicycle that has a paint smear on it from being hit by a car. The second approach involves collecting the trace itself, and not the object on which it is deposited. It is always bet- ter to collect the intact item, but sometimes it isn’t possible. What if a paint smear were on the side of a building? Proper documentation—notes, sketches, photography—precedes collec- tion and packaging, as discussed in Chapter 2.

Preservation Packaging evidence was also discussed in Chapter 2; it helps preserve the evidence. It is always important to package evidence correctly so that the item is not contaminated or

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Section 6.1Probability and the Chain of Custody

compromised in any way and so that it cannot work its way out of the package. For example, a single hair placed in a paper envelope could fall through the paper envelope seams at the end if it is not sealed with evidence tape. Investigators and crime scene technicians must also remember that trace evidence alone does not have much value. The lab needs a com- parison specimen—a specimen of the supposed source of the evidence. If the trace item is a hair, the lab will need known hairs, such as from the victim or suspect. If the item is glass, the lab needs a specimen of the item from which the glass is thought to have come, if possible.

Once an item of evidence arrives at the forensic science laboratory, it will be handled in a manner that will also prevent loss and contamination. Some laboratories use clean rooms, which are rooms set aside to handle trace evidence that are kept free of possible contaminants, such as fibers and other materials, for processing. They may use small vacuum cleaners fitted with filters to collect the evidence for packaging. Or they may use tape to collect the evidence. In this method, called tape-lifting, a piece of wide adhesive tape is placed on the surface of the item of evidence, and any hairs, fibers, or other debris will stick to the tape. The tape will be repeatedly placed on the item in a pattern to collect all possible trace evidence from the object. The tape can then be folded over upon itself and effectively sealed against contamination. The tape can be observed under a microscope, and any items that appear to be of probative value (meaningful in the case at hand) can be collected from the tape lift. This is accomplished by using a scalpel to slit the tape next to the item, removing that item using forceps, mounting the item on a microscope slide, and pressing the tape closed to reseal it with no contamination or loss of evidence.

Generally, hairs, fibers, glass fragments, soil, paint particles, and other nonbiological trace items can be packaged in ziplock containers. These can be labeled and sealed with evidence tape. They are also transparent, so you can see what is in them without opening them. Often, the ziplock bag will be a secondary container. That is, the trace itself will be first folded up in a paper container, which is then placed into the ziplock bag. The whole concept is to contain the trace items or material, keep it together, not allow it to move around or transfer to other surfaces or get out of its container, and make it reasonably easy for the forensic analyst to open the container, sample the material, and examine it.

Remember, trace evidence is by nature small and easily transferred. There are examples in many court jurisdictions of defendants convicted of crimes only to find out later that trace evidence had been mishandled and the verdict was wrong because of contamination. Simi- larly, there have been cases where the evidence was lost during collection or handling and could not be used during the case. Hairs and fibers have been seen in a number of cases

imageBROKER/Superstock Tape-lifting is one way to prevent losing very small pieces of evidence. What do you think might be some benefits and drawbacks to this method compared to others?

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Section 6.2Hair

in which contamination or loss was an issue. Proper handling, packaging, and documenta- tion will help establish that nothing has occurred that will compromise the evidence prior to analysis and prevent the evidence from being used in the courtroom because of a technicality.

Probability of chance duplication and the chain of custody can impact the significance of trace evidence in a case and in court. Each of the major types of trace evidence will be discussed with these concepts in mind. This will provide you with the proper methods to collect and preserve each type of trace evidence, as well as analysis methods to help determine the probability of chance duplication of evidence. This can help either exclude the suspect, crime scene, and/ or victim from having contributed to the evidence or include them as possible contributors.

As with the other disciplines in forensic science today, the various areas of trace analysis are represented in the OSACs under the NIST, which have been previously discussed. The Materi- als (Trace) subcommittee is under the Chemistry/Instrumental Analysis section of the OSACs (https://www.nist.gov/topics/forensic-science/osac-organizational-structure).

6.2 Hair One of the most common types of trace evidence is hair. Hair is found on mammals and is an outgrowth of the epidermis (skin) composed of keratin and other proteins. It appears as a long thin filament and can serve as a covering for the animal on which it is found. Most hairs can be easily differentiated from one another; the microscopic structure of a human hair can be used to distinguish it from any other species’ hair. Since humans continually shed hairs through the day, it is a type of evidence that a suspect might not know was left at a crime scene. Of course, the case could be made that since everyone sheds hair, anyone with access to a crime scene could have left it. The examiner’s analysis is necessary to determine whether a hair could have come from a suspect or the suspect can be excluded as a hair depositor, based on the analysis.

Structure and Growth Phases of Hair Examination of a hair will reveal that there are three layers. The outermost layer is called the cuticle. It is composed of keratin-filled scales. These are very tough and give hair the ability to retain its structure and be resistant to chemical action. Because of these two characteristics, hair may be found at crime scenes for a long period of time. The scales overlap toward the tip of the hair, so if no root is present, the scientist can determine which end is the root and which end is the tip. This outer layer also has characteristics that allow an analyst to determine if a hair has originated from a human or an animal. As seen in Figure 6.1, the scales that make up the cuticle in animals are regular and, in many cases, appear to take up most of the circumfer- ence of the hair. The cuticle of humans is irregular in shape. The scales are rough and are not seen as a pattern. This is a helpful characteristic to use in the preliminary determination of whether the hair is human.

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Section 6.2Hair

Figure 6.1: Hair samples

As can be seen, the outer layer of hair is made up of scales. The scales in human hair are irregularly shaped, while those in other animal hair, such as cat hair, are more uniform in shape. What other similarities and differences can you see between these two hair samples?

a) Science Picture Co/Superstock; b) POWER AND SYRED/SCIENCE PHOTO LIBRARY

Hairs can be presumptively characterized according to species using microscopy. Ideally, a comparison microscope is used to compare hairs found at the crime scene with hairs from the victim or suspect or to compare unknown hairs with known hairs to see if a particular type of animal might have left them behind at the scene. Knowing the possible species of origin for a hair can be useful if there is evidentiary value to knowing whether that hair came from a cat or dog instead of a human. In many cases, the trace evidence can tie a suspect to a victim or crime scene because something he or she leaves behind is not part of the victim’s lifestyle. Imagine the suspect owns a cat. At the scene of the crime, cat hair is found only near the vic- tim and in no other part of the residence. Law enforcement officers investigate and find out that the victim was allergic to cats. The hair could then add value to the case by helping tie a suspect to the crime scene.

The cuticle can also be used to determine if hair has been dyed. Hair dyes will coat the cuticle cells but will not be taken up by the second layer of hair. This means that if an analyst looks at a hair sample and the outer layer appears pigmented, or painted, then the hair was dyed. However, if enough time passes between a crime and the collection of hair samples, the dye may have worn away. This should be noted in the results, if the analyst found other similar characteristics between suspect and evidence hair.

The second layer of the hair is known as the cortex. This layer is composed of roughly hour- glass-shaped cells called cortical cells. These cells contain the pigments that give hair its color. There are thought to be two colors of pigment that give hair its color. One is a dark brown pig- ment, and the other is a yellowish-colored pigment. Though it has not been proven, there may be a third pigment involved that could be reddish in color (Preedy, 2012). The combination of

a) Human hair b) Cat hair

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Section 6.2Hair

pigments in different proportions results in the hair colors that occur in nature. White hair is simply devoid of pigment.

The final layer present is the medulla. In animals, this can be seen as a shaft running the length of the hair. In humans, the medulla can run the length of the hair; be broken into reg- ular-length pieces, known as an interrupted medulla; be broken into irregular-length pieces, known as a fragmented medulla; or may even be absent. Using a micrometer, which is a mea- suring device integrated into a microscope, a simple measurement and reference to known values can tell whether a hair is probably human or animal. This is known as the medullary index. To calculate this number, a scientist will measure the diameter of the hair and the diam- eter of the medulla. The medullary diameter is divided by the hair diameter to come up with a fraction. If the fraction is less than ⅓, it is most likely a human hair. If the fraction is greater than ½, it is likely to be animal hair. The layers of hair can be seen in Figure 6.2.

Figure 6.2: Layers of hair

Hair has three layers, with the cuticle as the outermost layer, the cortex as the middle layer, and the medulla as the deepest layer.

Adapted from ttsz/iStock/Thinkstock

Figure 6.3 shows an entire strand of hair and the three areas along its length. The first, wid- est section of the hair is the root. It is located below the surface of the scalp in the hair follicle and is where all growth takes place. The root is connected to the follicle through capillaries entering the bulb. Nutrients are fed into the root through the capillaries. As was mentioned in Chapter 5, hair can be tested to determine drug use. The drugs get into the hair through these capillaries. Once in the hair, they are bound to proteins within the hair and remain trapped until that section of hair is cut.

CuticleCuticle

CortexCortex

MedullaMedulla

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Section 6.2Hair

Figure 6.3: Components of a hair strand

The hair shaft is the visible part of a hair strand, while the root is under the skin, within the follicle.

Adapted from ttsz/iStock/Thinkstock

The shaft of the hair takes up most of the length and is the visible portion of the hair above the scalp. The tip of the hair is the terminating end and the narrowest part of the entire hair. Even the tip of the hair can be used to assist in analysis. Should there be color variations, the tip can be used to determine if a hair was bleached and the real color is coming back. If time has passed since the hair was colored, the tip end will typically remain colored, while the newer growth, toward the scalp, will be the person’s natural color.

Head hair usually grows at a rate of about 1 cm, or about ½ an inch, per month. Other body hair grows at different rates (Alaiti, 2011). Each hair goes through a growth cycle, and this growth cycle normally ends in the hair being shed. When hair is actively growing, the fol- licular cells are tightly against the root of the hair. The capillaries feeding the root are firmly attached, and the hair is hard to remove. During this anagen phase, growth is almost continu- ous. Periodically, the growth of the hair starts to slow. The root begins to shrink in on itself, and the hair becomes loose in the follicle as the follicular cells shrink away from the root. This is the catagen phase of hair growth and development. It normally lasts from 2 to 3 weeks, until growth completely stops. When growth has ceased completely, the hair enters the telo- gen phase. At this phase, the root of the hair has completely pulled away from the follicle, and the hair is loose in the follicle. It can easily slide out of the follicle and be shed. This shedding of the loose hair will normally trigger the resumption of a new anagen phase, and the hair will be replaced. Therefore, observation of the root end of a hair can be very telling. If the root end

Hair shaftHair shaft

Hair rootHair root

Follicle

CapillariesCapillaries

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Section 6.2Hair

is shriveled, it can be deduced that the hair fell out on its own. If the root is fully rounded with follicular cells attached to the base of the hair, it can be deduced that the hair could have been pulled or forcibly removed. Hair with the root attached is important for analysis and must be included in sample collection.

Collection and Preservation Hair can be an important piece of evidence and should be collected carefully, as even one strand with a root can change the outcome of a case. As with every other type of trace, there are two types of hairs that must be collected: evidence hairs and standards. You could also call these “unknowns” and “knowns.”

Evidence Hairs The first type of hair that should be collected is evidence hair, which is taken from the crime scene. These specimens are of unknown origin. Evidence hairs can be collected in different ways, depending on the source of evidence. Many hairs are a part of sexual assault cases. In those cases, evidence hairs might be found on the victim, a suspect, his or her clothing, or even the location where the assault took place. If evidence is located on a bed, the entire bedding should be collected. The analyst will search the sheets and find not only hairs but also possible fibers and body fluids. The victim’s clothing can be collected at a hospital or clinic. The victim should gently be instructed by medical personnel to disrobe while standing on clean, white butcher paper. Often, two layers of paper are used to prevent contamination from the floor or carpet. The clothing should be packaged separately in paper bags. These will allow air to circulate, preventing the degradation of any body fluids and stains that may be present along with the hair or fiber evidence.

Evidence can be found on and collected from the victim as well. Often, the suspect’s head and pubic hairs can be found on the victim’s body, head, or pubic region. Standard sexual assault kits have historically contained paper envelopes for hair evidence and clean combs for use in collection. One comb is for the pubic region and one for the head. Any loose hairs found may belong to the suspect. Any of these hairs found while combing should be packaged in the appropriate envelope and sealed to be sent to the laboratory. After combing and collecting the potential evidence hairs, knowns must be collected. Some protocols ask for the known hairs (from either a victim or suspect) to be forcibly pulled using a tweezers, while others specify that they can be cut, close to the skin. These pulled or cut strands are obtained and packaged in the prelabeled, sealable paper envelopes that sexual assault evidence-collection kits pro- vide. These sealed envelopes are in turn sealed back into the kit box for additional security. (More about these kits in Chapter 10.)

Extreme care must be taken in the evidence-collection process so that victim and suspect clothing are not in the same area at the same time. Investigators may take the precaution of not collecting this type of evidence from a victim and a suspect in the same location for this reason. Mixing clothing of a victim or a suspect together in the same container is bad practice. Hairs can be transferred from one surface to another. So a hair that was attached to under- wear, for example, could end up attached to outerwear by the time the analyst looks at it. The location of the hair might suggest different interpretations about how it got there.

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Section 6.2Hair

Standards When comparing hairs, an analyst must have standards (knowns or exemplars). These come from the victim and the suspect, if one has been identified. The analyst may also call for elimi- nation hair standards. These are sometimes called alternate knowns. They are collected from family members or roommates living in the same home as the victim or from others who may have had occasion to shed hair in the same location. They are necessary because those who have free access to the dwelling will shed hairs that may be collected by the evidence technicians. Standard hairs must originate from the same area of the body as the evidence hairs. This is because head hairs are different from pubic hairs, which in turn differ from facial hairs and body hairs. Generally about 25 to 50 known hairs, which represent the scope of characteristics in the individual, will be needed for comparison. These should all be pulled hairs so that their origin is known and the evidence technicians will have roots to aid in com- parison, although some collection protocols say that hairs cut close to the skin are acceptable. Additionally, they should be pulled from different parts of the area being sampled in order to get a representative sample over the whole head, pubic region, or other area. For example, hairs pulled from different portions of one’s head might contain different subpopulations of hair. As we are about to discuss, the analyst needs to have a true picture of the variation in a person’s known hairs before making any comparison to the evidence hairs. You might be wondering: Why 25 to 50 hairs? Why not 10, or 20, or 200? There is no good answer to this question. For years, hair analysts did not completely agree among themselves on the number of known hairs that should be collected for comparison. The latest recommendations from the Scientific Working Group for Materials Analysis (2005) call for 50 strands of head hair and 25 strands of hair from any other body region and as of mid-2018 were being reviewed by the Materials (Trace) Subcommittee of NIST. The logic behind this thinking is that enough known hairs must be collected to reveal any intra-individual variation so that this can be taken into account in the comparison. Head hairs from the same individual show more variability than do hairs from other body regions.

Analysis and Comparison Most analysis of hairs is completed using microscopy, in which the structural characteristics of the hair are analyzed under a microscope. The analysis consists of a comparison between known and evidentiary hairs. This type of analysis will only provide results that are of a class nature rather than individuality, and this type of evidence was routinely considered cir- cumstantial in nature until nuclear DNA analysis became available. Information that can be gleaned through microscopy includes the possible race of the contributor, area of the body the hair originated from, color, length, diameter, whether the hair was dyed, amount of curl, and presence and type of medulla. In all, a microscopist trained in the comparison of hair can look at nearly 60 characteristics. They can be directly compared to the hair of a suspect or victim for similarity. After analysis, an assessment is made that estimates how common the char- acteristics might be and how many people in the population are expected to share the char- acteristics, assuming there were no discrepancies between the knowns and the unknowns. Any discrepancy would show that the knowns were excluded as a source of the unknowns. In nonexclusion cases, the estimates are based on population studies of people’s hair. Generally, there is a sizable likelihood that characteristics can be shared. An analyst cannot ever say a hair specifically came from an individual based on microscopical comparison analysis. The characteristics can, however, be used to exclude possible suspects as possible depositors. In addition, an analyst can list the shared characteristics and let the jury know the estimated