FORENSIC SCIENCE
1. Collection of Physical Evidence
2. Analysis of Physical Evidence
3. Provision of Expert Testimony
1. Collection of Physical Evidence
If the investigator cannot recognize physical
evidence or cannot properly preserve it for
laboratory examination, no amount of
sophisticated laboratory instrumentation or
technical expertise can salvage the situation.
Physical Evidence can achieve its optimum value in
criminal investigations only when its collection
is performed with a selectivity governed by the
collector’s thorough knowledge of the crime
lab’s techniques, capabilities, and limitations
2. Analysis of Physical Evidence
A forensic scientist must be skilled in
applying the principles and techniques of the
physical and natural sciences to the analysis
of the many types of evidence that may be
recovered during a criminal investigations.
3. Provision of Expert Testimony
The forensic scientist must also be aware of
the demands and constraints that are
imposed by the judicial system. The
procedures and techniques that are used in
the laboratory must not only rest on a firm
scientific foundation but also satisfy the
criteria of admissibility that have been
established by the courts.
Expert witness – an individual whom the court
determines possesses knowledge relevant to
the trial is not expected of the average
layperson
7 Steps: Processing a Crime Scene
1.
2.
3.
4.
5.
6.
7.
Secure and isolate the crime scene
Record the scene: photograph, sketch, take notes
Conduct a systematic search for evidence
Collect and package evidence
Maintain chain of custody
Obtain controls
Submit evidence to laboratory
Secure and Isolate the Crime Scene
• 1str responding officer: preserve and protect the area
• First priority: obtain medical assistance and arrest
perpetrator
• Exclude all unauthorized personnel and isolate area;
every person that enters the scene has potential to
destroy evidence
• Determine boundaries and establish the perpetrator’s
path of entry and exit; crime scene size will depend of
locale of scene, size of area, and victims and suspects
Record the Scene
• There is a limited amount of time to permanently
record the crime scene in its untouched state
• Three methods of recording a crime scene:
– Photography
– Sketches
– Notes
Record the Crime Scene
• Photography
– Unless there are injured parties, objects must not be moved until
they have been photographed from all angles. If objects are
moved, photographs may not be admissible in court – unless
recorded in notes.
– Items to be photographed:
•
•
•
•
Area in which the crime took place and all adjacent areas
Points of entry and exit from various angles
If indoors, the entire room and wall area as well as adjacent rooms
A body’s position and close up depicting injuries and near by weapons;
once the body is removed, the area under the body should be
photographed
• Physical evidence position and location as well close up (a ruler or other
measuring devise may be used as a point of reference)
• Videotaping with sound is also a popular method of
recording a crime scene
Record the Scene
• Notes
– Detailed written description of the scene with
location of items of physical evidence
– Time of discovery of evidence, by whom, how and by
whom it was packaged and marked, and the
disposition of the item after it was collected
– Notes may be the only written record to refresh the
memory of the investigators
– Tape-recording is commonly done
Conduct a Systematic Search for Evidence
• Lead investigator will subdivide the scene into
a grid or a spiral
• Areas searched must include all probable points of entry and
exit
• Search for evidence will depend on type of crime:
– Homicide -Burglary -Vehicular -Kidnapping
• Physical evidence can be a/t
from massive objects to microscopic traces
• Some objects may only be traceable in laboratory settings
therefore the collection of possible carriers of trace evidence is
important
– Minute traces of blood on clothing
– Hair or fibers in vacuum sweepings
•
The search continues in the autopsy of
a deceased victim
•
Medical examiner determines cause
and manner of death
•
The following should be collected and
sent to the forensic lab:
1.
2.
3.
4.
5.
6.
7.
Victim’s clothing
Fingernail scrapings
Head and pubic hair
Blood
Vaginal, anal and oral swabs (in sex
related crimes)
Recovered bullets from the body
Hand swabs from shooting victims
(gunshot residue)
Virtual Autopsy http://www.hbo.com/autopsy/swf/casebook2/casebook2.html
Collect and Package Physical Evidence
• Physical evidence must be handled and processed in a way
that prevents any change from the crime scene and the time
it is received by the crime laboratory
• Changes arise through contamination, breakage, evaporation,
accidental scratching or bending, or loss through improper or
careless packaging
• Each different item or similar items collected at different
locations must be placed in separate
containers to prevent damage and
cross-contamination.
Examples of packaging containers and techniques:
• Plastic pill bottles for hairs, glass, fibers, and
other types of small evidence
• Manila envelopes or screw-top glass vials for
trace evidence (never ordinary envelopes
because of possible leaking)
• To avoid accumulation of moisture and mold,
bloodstained materials should be packaged in
manila envelopes or paper bags
• Clothing should be air dried and placed in paper
bags to allow air flow
• Charred debris should be sealed in an airtight
container to prevent evaporation
Maintain Chain of Custody
• Chain of custody – continuity of possession must be
established whenever evidence is presented in court
• Standard procedures include
– Recording the location of the evidence
– Marking it for ID (collector’s initials and date)
– Properly completing evidence submission forms for lab
analysis
– Accounting for every person that handles or examines the
evidence.
Standard/Reference Sample
• Examination often requires comparison with a known
standard or reference sample
– Hit and Run might require paint sample from car to compare
to paint at scene
– Bloodstained evidence must be compared to blood samples
or buccal swaps from all people at the crime scene
• Evidence may also be compared to substrate controls –
materials adjacent to or close to areas where the
evidence has been deposited
– If a burned area is suspected to be covered in gasoline, it
should be compared to a similar area not suspected
– Bloodstains on garments should be compared to area without
stains
Submit
In-Class
Evidence
Crime
to the
Scene
Laboratory
Assignment:
• Evidence is submitted by personal
delivery or mail shipment
depending on the location of the
• Respond to the crime scene
lab
• Evidence submission forms
• Collect evidence, record etc.
should be completed which
provides a brief •history
Turnofinthea detailed written report
case, the evidence submitted,
of the
and the type of analysis
thatcrime scene investigated
should be preformed
• Analyst not strictly bound to
specific test requested if new
evidence is found or to search for
trace evidence
Assignment #1
 Read p. 26-33
 Write a summary of
 Crime
 Evidence
 Forensic Techniques Used
 Trial
 Findings
 Outcomes of the trial for forensic
science