Crime Scene InvestigationChapter 2

THE CRIME SCENE

• CSI’s carefully and systematically sift through a crime scene in order to learn:– How and when the crime was committed– Who committed it and why– What items may have been removed from a crime

scene

• The methods used by anthropologists, archaeologists, and historians are similar to those used at a crime scene

THE CRIME SCENE (2)• There must be a plan for systematically searching

the site

• Safety of the searchers must be considered

• Only highly qualified and trained people should conduct the search

• Contamination must be minimized

• Time may be of the essence

• The scene must be thoroughly documented

• There must be a chain of custody for each piece of evidence

A Crime Occurs & Is Discovered

• Crimes can be discovered in one of three ways– A witness sees it and reports it to the police– A victim of the crime reports it to the police– The police discover the crime in progress

• Police may also “discover” a crime with a sting operation– A scenario whereby criminals are encouraged to

commit crimes that they probably would have done anyway

The First Officer at the Crime Scene

• Determine if the perpetrator is still there– Hot search (immediate) or cold search (interview

witnesses)

• Tend to the injured

• Secure the scene to prevent contamination

• Do not walk through the scene and search for evidence

• Note any obvious safety hazards and warn others

Crime Scene Investigation

• After a crime scene has been discovered and protected, CSI’s will arrive

• If there is a dead body, a forensic pathologist will– Certify the death– Determine the postmortem interval (PMI)– Take photographs and collect trace evidence

• The CSI unit takes charge of the scene• Each member has a defined role:

– Sketcher, photographer, searcher, documenter, fingerprinter, blood spatter analysis, etc.

Preliminary Scene Examination

• The first duty of the CSI at a crime scene is to conduct a preliminary examination– Safety hazards must be addressed and remediated– Boundaries of the crime scene must be

ascertained

• Perpetrators often carry evidence away and there may one or more secondary crime scenes

Systematic Search of the Scene

• Must be carried out in a way that minimizes alteration of the scene

• Photograph as early as possible

• Determine the search pattern that will be used– If it is a room, a spiral or back-and-fourth pattern

may be used– Outdoor scenes are often divided into grids

• Avoid covering the same ground more than once to minimize contamination

Recording the Crime Scene

• Two basic methods– Freehand sketch including measurements of various

objects – later translated into a scale drawing– Still photography including many pictures at different

distances and angles under various lighting conditions

• Hand drawn sketches are still used, but computers are often used to create scale drawings

• Crime scenes are often videotaped

Collection of Evidence• Evidence is of little or no value if it is

contaminated or degraded

• At some crime scenes (especially homicides) forensic scientists work with CSI’s to process evidence

•There are three major steps in the process of evidence collection

–Recognition

–Collection

–Packaging/preservation

Recognition of Evidence

• How do investigators know what is evidence and what is not?

• Homicides, burglaries, sexual assaults and other crimes usually have some characteristic types of evidence– Weapons, blood, fibers, hair, fingerprints, tools,

glass, soil, body fluids

• There is no such thing as too much evidence

• The exact location of the evidence must be recorded

Collection• As much evidence as possible should be

collected

• Lack of sample size may limit testing– The defendant is given a chance to have their own

tests performed, so if it is used up, it is excluded from trial

• Probative value (significance of evidence) is greatly enhanced if it can be linked to a known material or object

Packaging & Preserving Evidence

• There are physical, scientific, and legal requirements that determine how evidence should be packaged

• All evidence must be authenticated– Chain of custody– Uniquely identified to avoid confusion with other

evidence

• Tamper-evident packaging must be used

Preserving Evidence

• Packaging must be designed to preserve the evidence

• It may be weeks or months before the evidence is analyzed

• Different types of evidence require unique packaging to preserve it– Living plants, wet blood or body fluids, wet paint,

trace evidence, powder, fire residue

Reconstruction

• Once evidence is collected and analyzed, investigators will attempt to reconstruct the crime– Identities of the victims– Identities of the perpetrators– The sequence of events that took place

• The focus is to link the suspect to the crime and build a case that will stand up in court beyond a reasonable doubt