1. Science of Crime Scenes
Chapter 5.0
Science of Crime Scenes

2. Searching for Evidence: Recovery
The primary purpose of a crime scene search is to recognize, locate, and collect any evidence potentially relevant to the case
It is commonsense to claim that the crime scene is at the root of forensic science
Lack of physical evidence could jeopardize the whole criminal procedure or at least decrease the relevance of the various investigative hypotheses
Science of Crime Scenes

3. “Absence of evidence is not evidence of absence”
There are several possible reasons for a lack of physical evidence
The perpetrator has committed the “perfect crime” involuntarily.
This hypothesis can be rejected out of hand, as it would violate Locard’s Exchange Principle
The perpetrator has taken protective countermeasures to conceal the presence of his or her own traces at the scene
In that case, “absence of evidence” is an indication of his or her actions
The CSIs did not detect any critical evidence for various reasons
The most plausible reason
Science of Crime Scenes

4. Evidence can be anything
Pollen grains
World Trade Center Towers, NYC
Science of Crime Scenes

5. Science of Crime Scenes
What is a “trace”?
A trace is the residue or the vestige of an object or an action
A trace’s form can be thought of as occurring in a variety of forms, described variously as marks, signs, or clues
Traces may be in such a small quantity that the CSI cannot choose the “best” one
Other times, however, there may be too many and the problem then becomes one of choosing or sampling, in statistical terms
Regardless, there is a low degree of freedom in selecting pieces of evidence
Science of Crime Scenes

6. Inherent weaknesses of scant evidence
How was this trace found?
Has intentional deception or staging been eliminated?
Were other traces searched for nearby?
Could it have been left by an action other than the criminal one(s)?
Are there other alternatives than the perpetrator as the source of this trace?
Does the other evidence support the relevancy of this trace?
Science of Crime Scenes

7. What is collected depends on
The specific case at hand
The protocols adopted in the laboratory
Personnel
Time
Weather
Supplies
Science of Crime Scenes

8. Which Evidence Is Useful?
Confirm that a crime was committed
Describe the circumstances in place at the time of the crime
Establish what happened (reconstruction)
Link the victim(s) to the scene of the crime
Link the perpetrator(s) to the scene of the crime
Link the victim(s) to the perpetrator(s)
Link different scenes of crimes to each other (through operative forensic intelligence)
Identify the author of a crime, but also link it to another criminal event.
Science of Crime Scenes

9. Collecting everything is not possible or desirable
Theoretically, it is senseless: collecting everything also means collecting items unrelated to the forensic investigation
Practically, the entire scene cannot be fully collected
Operationally, the forensic facilities in charge of analyzing various items of evidence would be rapidly overwhelmed with the vast majority of collected items having no probative value
Remember “Garbage Truck Forensics”?
Science of Crime Scenes

10. Stop for a second and realize that collecting is not submitting
Science of Crime Scenes

11. What supports or refutes the hypotheses?
Science of Crime Scenes

12. Practical Search: Focal and Ancillary Points
But where should a systematic search begin?
How far may I track the path used by the suspect to and from the scene?
How did the suspect enter and exit the scene itself?
What kind of materials is the scene composed of?
Was evidence moved at the crime scene?
How was the victim handled?
Main areas should be rapidly identified as focal points and ancillary ones for trace searches.
Science of Crime Scenes

13. Science of Crime Scenes
Focal points
Areas having the highest probability of containing relevant pieces of evidence
Points of entry and exit, the paths suspected of having been used by the perpetrator, and the points of contact with the victim or the crime scene
Which drawer of the kitchen cupboard was opened?
Which room hid the stolen goods?
Where was the safe?
Which areas have been obviously damaged, looted, visited? Why?
Which specific location of this area was of interest?
Did the weapon come from the victim’s house?
Did the perpetrator stay long enough at the scene or nearby to have eaten, showered, or performed other activities?
Pathways between these different areas should also be preserved as ancillary points.
Science of Crime Scenes

14. Avoiding contamination
Anyone entering the scene should wear disposable coveralls that cover hair, gloves, and shoes.
All personal protective equipment should be collected when leaving the scene to prevent the transfer of materials.
A unique path of entry to the scene for the CSIs and other professionals should be identified, different from the most probable pathway used by the perpetrator.
Unfortunately, this entry path is frequently decided by patrol officers, first responders, or SWAT teams.
Identities of any stakeholders at the scene should be recorded to collect reference samples for future shoeprints, fingerprints, DNA, and other traces later found in the evidence for elimination purposes.
Science of Crime Scenes