8127/frame/fm

 
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
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Recognition of Physical Evidence
109
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.
Similarly, a piece of colored paper left at a crime scene may become
bleached by exposure to the sun, altering its color; it may get wet and dry
again, altering its texture; it may blow away with the wind, challenging us to
both find it and recognize it as evidence. If we are searching for the paper
using the other half of an extortion letter as a reference, the writing on the
paper may have become faded and smudged. If it was written in “invisible
ink,” it may not be visible at all until we can find it and examine it with that
purpose in mind.
All of these circumstances challenge our ability to recognize and recover
evidence at the scene. They also potentially confound our ability to eventually
connect evidence and reference samples. We discuss the criteria we use to
conclude that two items share the same source in Chapter 6. In particular we
address 
 
explainable differences
, one of the most basic tenets of forensic science,
and without which we could not function as forensic analysts. We also discuss
implications of the 
 
absence of evidence
in Chapter 7.