Contents
A. Evidence and the Law... 101 B. The Crime, the Crime Event, and the Crime Scene... 102 C. What is Evidence? ... 103 1. The Search for Evidence ... 105 2. The State of the Evidence ... 108 D. Evidence and Reference ... 109 E. Summary ... 110 References ... 111
The eyes see in things only what they look for, and they look only for what is already in the mind.
—Posted in the classroom of the School of Scientific Police at the Palais de Justice in Paris
Luke S. May, Crime’s Nemesis, 1936
The most difficult challenge in the investigative process is the recognition of
relevant physical evidence. Prior to any laboratory analysis, an item must be recognized as evidence in a crime or it will never be examined, much less interpreted. Ideally, the crime defines the relevant evidence. In this chapter we will explore the attributes and circumstances that combine to make something evidence and, in particular, physical evidence. We will also discuss the seem- ingly obvious, but sometimes complex relationship between evidence and ref- erence. We leave a detailed discussion of the crime scene itself to Chapter 8.
A.
Evidence and the Law
As discussed briefly in the Introduction to this book, forensic science has no existence outside of the law. Forensic scientists are invited by the law to assist
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in establishing the elements of a crime. Except for identification evidence, which we will meet again in Chapter 6 we do this only indirectly by providing
circumstantial evidence. To reiterate part of our discussion from Chapter 1, circumstantial evidence is frequently confused with weak evidence. Attorneys, in particular, like to call any evidence that is strong and convincing direct evidence. For example, a quote from a recent news article reads “‘There’s a great deal of direct evidence,’ [the attorney] said, including DNA evidence and a confession [the suspect] made to his father and brother” (Lavie, 1999). While the confession may or may not be true, it is still considered direct evidence. And, however rare the DNA profile is, its presence still does not prove that the suspect committed the murder, nor does it tell us when the material was deposited; it simply indicates the presence of the suspect at some time at a location connected with the crime. Depending on the circumstances of the crime, this may be very convincing, but that does not make it direct evidence. Science may establish a fact that allows for an inference that the element is true, but not the fact of the element. Briefly, an inference consists of two parts, a fact and some assumptions. An inference is not a fact; it relies on facts, but it must also rely on assumptions.
For instance, red wool fibers are found on a body dumped out in the woods. A suspect is apprehended and a red wool sweater found in his closet. The criminalist examines both items and reports to the detective that they are microscopically indistinguishable. The detective infers that the suspect is the perpetrator. Science provides the facts (indistinguishable red wool fibers); the detective provides the many assumptions (the fibers would continue to be indistinguishable by further testing, no one else has a red wool sweater with the same characteristics, the suspect wore the sweater found in his closet when he perpetrated the crime, the fibers were trans- ferred during the commission of the crime, the fibers persisted after the crime, etc., etc., etc.) required to make the (very tenuous) inference that the fibers on the body lead to the suspect as the killer.
We discuss this important concept in more detail in Chapter 8, but for now, we emphasize that science is the only player who can contribute facts about physical evidence. Were it not for our ability to provide this service, we would quickly be uninvited to the party.
B.
The Crime, the Crime Event, and the Crime Scene
A crime is a violation of a statutory law. Therefore, a crime scene might be defined as any location or item connected with the crime. We distinguish this from the crime event which is the actual commission of the crime. 8127/frame/ch05 Page 102 Friday, July 21, 2000 11:48 AM
Recognition of evidence may occur at any time after the crime occurs. However, the interval between the crime event and securing the scene, and any further time that passes before evidence is detected, affects our ability both to recognize evidence as such and to relate it to the crime event. The simple passing of time obscures our ability to determine by whom, or in connection with what, the material was deposited in some particular location. The possibility of connecting multiple scenes also declines with time.
Additionally, the evidence item acquires a history between the crime event and the recognition of it as evidence. It is precisely during this time period that the sample is subjected to unknown and unknowable factors, creating an unbreachable void in our understanding of evidence. It defines the uncontrolled nature of case samples.
C.
What Is Evidence?
Interestingly, in the United States, at least, the law defines evidence only by its relevance. Relevant evidence is admissible; irrelevant evidence is inadmis- sible. The following are excerpts from Article I and Article IV of the U.S. Federal Rules of Evidence*(1999) (Figure 5.2).
Thus, it is left to science to define the conceptual nature of physical evidence. The material attributes of physical evidence require simply that it be detectable. This may be accomplished by human senses or enhanced by optical, physical, or chemical means. Physical evidence may be examined, compared, or analyzed by those same means. Conceptually, evidence must be associated with a crime. As interesting as it may be to examine your office conference room for fingerprints or test unknown stains in the parking lot for blood, these samples would not become evidence unless someone stole the boss’s favorite coffee mug or a murder was committed outside the build- ing. Incorporating the legal requirement, evidence must therefore provide factual information about the crime, establishing its relevance to a criminal proceeding.
Let’s take a simple example. A man walking through a park is assaulted, and a fist fight ensues. The assailant bloodies the nose of the victim. The victim reports the assault to the police, who eventually apprehend a suspect. The clothing of both are collected, and the police enlist the aid of the scientist to determine whether the suspect is in fact the assailant. The testimony of the victim, the testimony of the suspect, any eyewitness accounts, and the clothing collected from both persons involved might all be considered relevant
* The Uniform Rules of Evidence (1988) are very similar to the Federal Rules of Evidence. They have begun to be adopted by states to accomplish a goal of standardization.
104 Principles and Practice of Criminalistics Figure 5.2 Ar ticle I and Ar ticle IV .
evidence under the law. However, only the clothing comprises physical evidence. It is tangible, so we can examine and analyze it; it is associated with a crime by virtue of being worn by the victim or assailant; its relevance can only be established after it is examined and analyzed. The simple fact of its presence on the participants probably does not help to establish any facts about the crime; the presence of bloodstains, fibers, or damage might.
1. The Search for Evidence
Some types of evidence patently relate to the criminal event. Blood from a stabbing, bullets from a deceased person, or broken glass from a burglary, clearly derive from the crime incident and are readily discerned. The recog- nition of such evidence is usually accomplished simply by looking. But for other types of evidence, the connection to the crime is not necessarily obvi- ous. We each lose hundreds of hairs a day (some of us more than others), so the finding of human hair almost anywhere is predictable. The finding of human hairs at a crime scene is therefore expected, whether or not the hairs bear any relation to the crime. The presence of hairs that appear to have been forcibly removed immediately suggests increased relevance to the crime; bloody clumps of hair increase that relevance exponentially.
The case of People v. Axell involved the stabbing murder of a convenience store clerk by an unknown female. The victim was found clutching 10 anagen* hairs in his fist, a circumstance consistent with the victim having pulled the hair from someone’s head during a struggle. Because anagen hairs must be forcibly removed from the scalp, it is reasonable to infer that a clump of hair clutched in the hand of a murder victim is related to the assault that killed him. DNA analysis of the hairs by RFLP** showed genetic concor- dance between the evidence hairs and a reference sample from Linda Axell. While the DNA analysis was vital to increasing the strength of the evidence by virtually individualizing the hairs to Linda Axell, their significance in the context of the crime was determined wholly by circumstance. The hairs were determined to be relevant to the homicide by virtue of their location (his hand) and by their apparent state as pulled hairs. If they weren’t, the results of the DNA analysis would have been less significant in the context of the crime (and probably impossible at the time!).***
Remember that the law asks the relevant question. Whether it is asked by a detective, a criminalist, or an attorney, the elements of a crime must be
* Anagen hairs are in a growth phase and would not be expected to fall out easily. ** Restriction fragment length polymorphism, the first DNA technique to be widely used and accepted.
*** RFLP analysis requires follicular material to be present on the hair root. Fallen telogen hairs are usually not amenable to RFLP testing.
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proved; this defines the purpose of the investigation and, by extension, the examination of the crime scene. The legal question also impacts what will ultimately be considered evidence. That we can ascertain all the evidence that exists just by looking runs counter to the entire evolution of detective work. Just looking does not work; looking with purpose, or searching, is central to a competent investigation. To search effectively, the investigator must discern what evidence might reasonably be relevant to the crime or he will waste inordinate amounts of time searching aimlessly.
Whether we are aware of it or not, we constantly form provisional hypotheses about life as well as about crimes. I assume that the train will arrive at the station at exactly at 8:14 A.M. Therefore, if I get there at exactly
8:13, I will even have a chance to grab a cup coffee before boarding the train. My hypothesis is based on the fact that the train always arrives on time or late, and on my assumption that this pattern will continue today. Similarly, I assume that I will find my keys hanging on the hook next to the door when I go to leave the house. This is based on the fact that I usually place the keys there upon entering, and the assumption that I did that last time I entered, and that neither I nor anyone else has moved them in the meantime. Without these provisional hypotheses, we would not be able to function in life; each decision would require starting from neutral, with all possibilities equally probable. We need not try to eliminate preconception, but to identify our hypotheses and the assumptions on which they are based. We can then use this information to guide our actions, and we can change our hypotheses when new information is presented to us.
When presented with the aftermath of a criminal event, the goal of law enforcement is to solve the crime and charge the guilty party in the most efficient and expeditious way possible. Thus, the investigator in charge of the scene must determine what crime has been committed, what are the legal elements, and what evidence he expects to find. Immediately following rec- ognition of the crime, the investigator will learn information that will lead to a preliminary hypothesis. The challenge presented to an investigator is to determine what evidence exists. At the same time, he must consider the ques- tion of what evidence should exist given that this crime occurred. These complementary notions are based on the expectation of the investigator (or the criminalist, if given access to the scene). To harbor an expectation, we require information about the crime. In other words, we must anticipate what physical manifestations we expect as a consequence of that event.
For instance, the crime is murder, the modus operandi is similar to a string of other crimes, and it occurred at the known headquarters of a local gang. It is not unusual to develop a prime suspect quite early in the investi- gation based on information that has accumulated in police files. The inves- tigator will then direct the search for evidence assuming that this crime was
committed by this person. While a provisional hypothesis is necessary to process a crime scene and begin the search for evidence, the investigator must also be willing to update his hypothesis based on new information. At some point, we become convinced that no piece of information will change our hypothesis about what happened.
The scientist has tried to define her role apart from the law and usually feels some obligation to maintain the same objectivity that characterizes her other scientific work. This does not imply that she holds no preconceptions; however, she may approach the scene with competing hypotheses rather than a single hypothesis. For instance, what evidence do I expect to find assuming either (1) this crime was committed by this person or (2) this crime was committed by some other person. If it is accepted that we can and should look with purpose, then we must ensure that one of the purposes is to search for inculpatory evidence (evidence that supports our primary hypothesis) as well as exculpatory evidence (evidence that supports some other hypothesis).
The environment in which evidence is found must be taken into consid- eration before we determine its relevance to the crime. Like hair, fibers are ubiquitous in our environment, and their presence at a crime scene is expected. Simply detecting a particular fiber, or even finding matching fibers in reference and evidence samples does not a priori imply their connection to the crime. The case of People v. Morin (Commission on Proceedings Involving Guy Paul Morin, 1998), illustrates how easily one can become misled by failing to consider the circumstances of the case. Fibers prove particularly difficult in this respect and we must take great care to understand their relevance to a crime clearly and completely.
In Morin, fibers were collected from the body of a murdered girl and also
from the suspect’s car and house. Thousands of fibers were collected and a few were selected that appeared to be shared between the body (a known crime scene), the car (a suspected crime scene), and the house (a reference environment for the suspect). Only after two trials and a judicial inquiry was it clarified that the two families were neighbors, were occasionally in each other’s houses and cars, and used the same laundromat. The judge conducting the inquiry opined that the fibers should never have been col- lected as their value from the start was worthless due to the shared envi- ronments of the victim and suspect. That most of the matching fibers were later suspected to have originated from contamination in the laboratory is another matter.
We emphasize that searching for evidence involves a different thought process than interpreting evidence. The former is deductive — we formulate a theory based on previous experiences and information and use this working hypothesis to help us know where to look. The philosophy and mind-set of
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searching for evidence may upset some of you as it specifically transposes the conditional (we condition on the hypothesis rather than the evidence) and thus flies in the face of Bayesian thinking. But to interpret evidence, we must have some, and successful searching requires presumptions, expecta- tions, and preliminary hypotheses. In contrast, interpreting evidence uses inductive reasoning — we synthesize all the evidence, including the results of examinations and analyses — and develop a theory of the case. We suggest that the Bayesian logic is most appropriate and helpful in the interpretation
of evidence. We discuss this topic in gory detail in Chapters 6 and 7.
2. The State of the Evidence
A crime scene and all of the evidence in it are subject to the effects of time and environment. We will discuss the dynamic crime scene and its implica- tions for collection and preservation of evidence in Chapter 8. For now, we simply relate the implications of change over time to the recognition of evidence at a crime scene.
As objects were not fixed in some state before the crime event, neither do they become frozen in time for the convenience of the investigator and the analyst. In Chapter 4 we discussed how divisible matter and transfer work to generate evidence. The moment material becomes evidence as silent witness to a crime, environmental influences commence to change it from its original form. The evidence continues to change and evolve subject to biotic and nonbiotic forces; it does not discriminate between intent and happenstance. The changes to biological evidence are most readily apparent. A blood- stain deposited on a painted wall inside a house will initially be wet and quite red. Subject to biochemical reactions within the blood itself and to a gaseous environment, it will clot and dry. Further exposure to air will provoke oxi- dation of the heme molecules in the blood, and the stain will turn darker and darker red, finally acquiring a brownish tinge. The same bloodstain deposited on a sidewalk will additionally be subjected to the diurnal influ- ences of the sun and relatively large temperature fluctuations. It may go from red to black in a short period of time. Rain or a stray step may disintegrate the stain altogether, leaving no trace that it ever existed. At what point do we fail to recognize it as evidence? At what point do we fail to recognize it as blood? At what point do we fail to recognize it at all?
Nonbiological physical evidence also exhibits change due to time and physical influences. For instance, a bullet can change dramatically between its expulsion from the barrel of a gun and its excavation from a wall by a firearms expert. The bullet may have passed through a body, possibly acquir- ing biological material or damage as a result of deflecting off bone. Its entrance into the wall may further alter its overall shape and potentially 8127/frame/ch05 Page 108 Friday, July 21, 2000 11:48 AM
obliterate microstriae. Alternatively, the bullet may explode or be shattered into many small pieces, or it may become lodged in the body and never exit. Both the bullet and any casings may accumulate debris if either they or the scene remain undiscovered for some time. These alterations can confound our attempts to recognize the items and relate them directly to the crime.