Today, 9/18

1 – Short lecture (take out notes)

2 – Start project

Chapter Three: Evidence

Two broad types 1) testimonial (a.k.a. eye-witness) 2) physical

Which do you think has more reliability and why?

PHYSICAL EVIDENCE

Chapter 3

Physical Evidence Two major categories:

individual characteristics class characteristics

Individual characteristics: allow techs to attribute evidence to a common source with a high degree of certainty

Class characteristics: the properties of the evidence can only be associated with a certain group and never with a single source

Examples

Individual characteristics: two finger prints having same whorl

pattern broken pieces of glass that fit

together like a puzzle bullet striations

Examples continued

Class characteristics Blood type Tire prints that do not have wear

patterns yet Fiber found in most car fabric

Identification

Determine the substance via chemical or physical properties

Must be able to exclude all other substances

Might take one test or might need multiple tests

Identification

The purpose of identification is to determine the physical or chemical identity with as near absolute certainty as existing analytical techniques will permit.

1st - adoption of testing procedures that give characteristic results for specific standard materials.

Results have been established, they may be permanently recorded and used repeatedly to prove the identity of suspect materials.

2nd - the #and type of tests needed to identify a substance be sufficient to exclude all other substances.

Common Types of Identification

Chemical composition of an illicit drug. Gasoline in residues recovered from the

debris of a fire, or it may have to identify the nature of explosive residues—for example, dynamite or TNT.

Blood, semen, hair, or wood are also very common and, as a matter of routine, include a determination for species origin.

Comparison

Uses standard/reference samples Determines common origin of

evidence

Crossing Over Crossing over the line from class to

individual does not end the discussions. How many striations are necessary to

individualize a mark to a single tool and no other? How many color layers individualize a paint chip

to a single car? How many ridge characteristics individualize a

fingerprint? How many handwriting characteristics tie a

person to a signature? These are all questions that defy simple

answers and are the basis of arguments.

Types of physical evidence

Blood, semen, saliva: found on clothes, cigarette butts etc

Documents: includes paper, ink, writing, charred or burned documents

Drugs: anything in violation of laws Explosives: anything with a charge

as well as residue from explosions

Hair Impressions: includes foot prints,

tire tracks and even tool markings Manufactured items: firearms,

ammunition, fibers, paint, glass, etc

Fibers: natural

Each major category of physical evidence will be a separate unit

Chapter Three activities will include evidence collection, probability calculations and using facial reproduction software.

Significance of Physical Evidence Do you think that there are tests for

every thing? Probability plays a major role in

significance Simply defined, probability is the

frequency of occurrence of an event. Chances are greater that the

evidence will only reveal class characteristics

Product Rule Determines frequency a certain

combination of characteristics occurs in a population

Multiply the frequency of each individual characteristic

Blood factors, A 26%, EsD 85%, PGM 2%. .26 x .85 x .02 = .44 percent, so less then 1 in 200 people.

Value of Physical Evidence

Class physical evidence has the ability to corroborate events.

The value/significance accorded physical evidence is left entirely to the jury of usually lay people.

Forensic Databases

• The Integrated Automated Fingerprint Identification System (IAFIS), a national fingerprint and criminal history system maintained by the FBI.

• TheCombined DNA Index System (CODIS) enables federal, state, and local crime laboratories to electronically exchange and compare DNA profiles.

• The National Integrated Ballistics Information Network (NIBIN) allows firearm analysts to acquire, digitize, and compare markings made by a firearm on bullets and cartridge casings.

• The International Forensic Automotive Paint Data Query (PDQ) database contains chemical and color information pertaining to original automotive paints.

• SICAR (shoeprint image capture and retrieval) is a shoeprint database.

Reconstruction

Supports likely sequence of events of the crime

Must contain observation, evaluated physical evidence and witness statements

Done with cooperation between police, criminalists and medical examiner

Figure 3–9  Crime-scene reconstruction relies on the combined efforts of medical examiners, criminalists, and law enforcement personnel to recover physical evidence and to sort out the events surrounding the occurrence of a crime.

The Role of Physical Evidence The physical evidence left behind at a crime

scene plays a crucial role in reconstructing the events that took place surrounding the crime.

Although the evidence alone does not describe everything that happened, it can support or contradict accounts given by witnesses and/or suspects.

Information obtained from physical evidence can also generate leads and confirm the reconstruction of a crime to a jury.

The collection and documentation of physical evidence is the foundation of a reconstruction.

Figure 3–8  A laser beam is used to determine the search area for the position of a shooter who has fired a bullet through a window and wounded a victim. The bullet path is determined by lining up the victim’s bullet wound with the bullet hole present in the glass pane.