Trace Evidence

• Author: D. P. Lyle
• Pages: 293-312

Trace Evidence 293

CHAPTER 16

TRACE EVIDENCE

The Locard Exchange P rinciple, introduced and discussed in Chapter 3,

states that whenever two objects contact each other, a transfer of materials

occurs. Nowhere is this principle more clearly demonstrated than in the

location, collection, and analysis of trace evidence. Trace evidence can best

be defined as any small bit of evidence that the perpetrator doesn’t see and

doesn’t realize he is leaving behind or carrying away. It clings to clothing,

latches onto shoe soles, nestles into hair, and settles into nooks and cran-

nies. It is often very durable and survives for months or years.

This transferability is the main reason that crime scenes must be

secured and access must be controlled. Each person who enters the scene

will add and/or take away trace materia ls, and this contamination might

render some evidence items useless. Obviously, if the first officers on the

scene do not yet know a crime has occurred or if they must disar m and

apprehend a suspect or assist a victim, some contamination will oc cur. The

same is true for medical and rescue personnel who must help the injured

and firemen who must extinguish a blaze. Such contaminations make the

criminalists’ job much more difficult.

The major use of trace evidence is to create an association or link

between suspects, places, and objects. In m any situations, it is the only evi-

dence that makes this critical connection.

294 Trace Evidence

T race evidence is predominantly class rather than individualizing

evidence (Chapter 3). It can exclude a suspect but can rarely absolutely

implicate him. For example, if a blonde hair is found at a crime scene and

the suspect has black hair, he is eliminated as the sourc e of the hair and

the police must develop another suspect. But, if the suspect’s hair matches

that found at the crime scene, he remains a suspect. The hair might be his

or it might be from someone else with similar hair.

Types of Trace Evidence

The bulk of trace evidence investigation involves hair, fibers, glass, paint,

soil, and plant materials. Each of these can be brought to or car ried away

from the crime scene by the perpetrator without his knowledge and can

serve to connect him to the scene.

Hair

Hair is small a nd easily shed, clings to clothing and other materials, a nd

goes unnoticed. It is hardy and survives for a long period of time, even

many years after a body has u ndergone putrefaction. Nuclear DNA is found

in hair that has attached bulbs (follicles) and mitochondrial DNA is present

in the hair shaft (Chapter 11). Many toxins, particula rly heavy metals such

as arsenic, can be found in hai r (Chapter 12).

Hairs, b oth human and animal, a re frequently found at crime scenes.

In analyzing hai r evidence, the ME’s first task is to determine whether

the hair is from the victim, the perpetrator, the family dog, or from some

unknown source. After this is c ompleted, hair analysis predominantly

deals with its structure and chemical cha racteristics, which the examiner

will use to match ha irs from known (the suspect) and unknown (the crime

scene) sources. It is essential to note that hair varies not only from person

to person but a lso from one area of a n individual’s body and another. A per -

son’s head, pubic, and axilla ry (armpit) hair are often very different. Not in

DNA or toxicological terms, but in structure.

The forensic use of hair ana lysis is one of the earliest forensic tech-

niques, with the first paper on its use published in France in 1857. By the

turn of the century the microscopic examination of hai r was widely used

and in 1931 Professor John Glaister (1856–1932) published his landmark

book on the subject, Hairs of the Mammalia from the Medico-legal Aspect.