criminalistics chapter 2 and chapter 3 the crime scene and physical evidence

Forensic ScienceCriminalisticsChapter 2 and Chapter 3The Crime Scene and Physical Evidence

Goal of any crime scene investigation….

Recognize, document and collect evidence at the scene of crime.

This should lead to a conclusion of some sort- an arrest of a suspect, determination of motive, etc.

Principle of Exchange

Whenever two people or things come into contact with each other, a physical transfer occurs. Trace evidence:

transferred materials left at a crime scene.

Locard’s Principle

Physical Evidence: any object or material that is relevant in a crime. Should provide a

link between a crime and the victim or perpetrator.

Ex. Hair, fibers, piece of crashed automobile, etc.

What is evidence?

Direct Evidence: Evidence in the form of a statement and made under oath. Ex. Eyewitness

testimony. (Pointing to the defendant in the courtroom and saying, “He did it!”)

Police car dash cameras

Common Types of Evidence:

Drugs and toxic substances

Paints Gunshot residues Firearms and ammunition Impressions (shoe prints,

bite marks, etc) Petroleum products Alcohols Rubber material Resins, plastics Explosive residues Documents

  Fibers Soil, glass Blood Body fluids Fingerprints Hair Tissues Pollen Wood materials Feathers Bones

If evidence does not prove a fact it is called indirect evidence.

Circumstantial evidence implies a fact or event. The greater the volume of circumstantial evidence there is, the greater weight it carries.

So, what good is evidence?

Physical Evidence should be able to: Prove that a crime has been committed Corroborate testimony Link a suspect with a victim or with a

crime scene Establish the identity of persons

associated with a crime Allow reconstruction of events of a crime

Rules of Evidence

Must be relevant which means it must prove something, and must address the particular crime.

Evidence is admissible only if it is reliable and the presenter of evidence is credible and competent.

While we know that using the Frye Standard, testimony and scientific procedures must be “generally accepted”, here are some guidelines for judgement:

The scientific theory or technique must be testable

The theory or technique must be subject to peer review and publication.

Rate of error or potential errors must be stated.

The technique must follow standards. Consideration must be given as to

whether the theory or technique has attracted widespread acceptance within a relevant scientific community.

Individual vs. Class Evidence Individual Evidence:

Evidence that can be associated with a common source with a high degree of probability

Examples: Matching ridge

characteristics of two fingerprints

Matching striations on bullets

Matching irregular and random wear patterns on footwear

Class Evidence: Properties of

evidence that can only be associated with a group and never with a single source

Examples: Brands of items or

manufacturers.

Problems or Exceptions?

Disappointments when the lab is unable to find an exact match.

Weakness Mostly forensic scientist must rely on

personal experience to interpret significance of comparison

Recognition:

Must recognize that an item is evidence of a crime.

Must be able to separate those pieces with evidentiary value from unrelated materials.

Identification

Identify class characteristics. Ex. Determine if

hair is animal or human, brown or blond, etc.

Comparison

Characteristics of evidence must be compared to a known sample. Ex. Strand of hair

found at a crime scene and you take a strand of hair from a suspect. The hair from the suspect is the known sample for comparison purposes.

Individualization

Can be matched to a unique source. Ex. Striation on a

bullet can be matched to the firearm.

Reconstruction

Team event Reconstruct events

that occurred Prior to crime During crime Subsequent to crime

Collaborative effort including Law enforcement,

criminalist, Medical examiners

The Crime Scene Investigation Team

Police officers: first to arrive A District Attorney

may accompany to determine if a search warrant is needed

Crime-scene Investigators Document scene

in detail and collect physical evidence

Medical Examiners

Also known as a coroner

Determine the cause of death in the case of a homicide

Detectives

Look for leads by interviewing witnesses and talking to crime scene investigators about the evidence.

Specialists

Experts in a particular field pertaining to the evidence

Ex. Botanist, entomologist, anthropologist.

1. Securing the crime scene

Duties of the Arriving Officer:

Get medical attention for suspect or victim

Arrest perpetrator if remaining on premises

Secure the Crime Scene

Securing and Isolating a Crime Scene

The Arriving Officer must prevent unauthorized or unneeded access: Neighbors/Onlookers Members of the Media

Every individual entering a crime scene can potentially destroy important evidence

2. Separating the witnesses

Want to compare the accounts of each witness

Don’t want them to work together to create a story

3. Scanning the crime scene

Need to determine the primary crime scene and a possible secondary crime scene. Primary crime scene: where the crime

occurred Secondary crime scene: suspects

house, where the body was found, etc.

The Lead Investigator will: Determine the boundaries of the scene Document and photograph obvious

evidence Develop a strategy for a systematic

examination and documentation of entire scene

4. Seeing the scene

Methods of crime scene recording Photography Videography

Crime Scene Photography

The crime scene must be unaltered and in the original state found by investigators

Must be photographed from all necessary angles

Evidence must not be moved prior to photography: if it has been moved, it cannot be reintroduced into the scene for photo purposes

Crime Scene Photography

Photos must be taken of the entire room and all rooms adjacent to initial crime scene

Close-up photos and panoramic photos are taken and sometimes include a ruler for scale

All pieces of physical evidence are photographed prior to removal

Crime Scene Videography

Increasingly popular due to affordable cost

Long-shots and close-ups are taken of scene

Narrations are often included as a method of note-taking

5. Sketching the scene

Rough Sketch: An accurate

depiction of dimensions of the scene, showing the location of all objects that may have implications to the crime.

Done at the scene and used for finished sketch later

Rough Sketches

Objects are depicted from two fixed locations at the scene

Accurate measurements must be taken with a tape measure

Sketch evidence by using letters or numbers and a legend at the bottom

Sketch should always depict the NORTH direction

Finished Sketch

A precise depiction of the crime scene, drawn to scale

Sometimes drawn with the aid of a computer CAD: Computer-

aided drafting

Notetaking at a Crime Scene

Constant activity at a crime scene that includes the following: A written description of the scene in a

narrative format Location of physical evidence items

recovered▪ Time evidence discovered and by whom▪ The condition of the evidence recovered