Physical Evidence

Objects that can establish that a

crime has been committed

Common Types of Physical

Evidence

Blood, Semen, and Saliva these are subject to serological and biochemical analysis

Documents – any handwriting, typewriting, ink, indented writing, obliterations, burned or charred documents

Drugs

Explosives – any device with an explosive charge and any item that may contain the explosive residue

Fibers – any fiber that may show cross-transfer between object and/or persons

Fingerprints – latent and visible

Common Types (cont.)

Firearms and Ammunition

Glass – particles that may show cross-transfer or panes that may have bullet holes

Hair – animal or human hair showing cross-transfer

Impressions – tire markings, shoe prints, depressions in soil, glove or fabric impressions, bite marks in skin or food

Organs and Physiological Fluids – body organs and fluids are tested for drugs and poisons

Paint – particles showing cross-transfer (auto accidents)

Common Types (cont.)

Petroleum Products – gas residues from arson

scenes, grease or oil stains

Plastic bags – usually for drug residues

Plastic, Rubber or other Polymers- cross-

transfer

Powder Residues – firearm discharge residues

Serial Numbers – stolen property submitted to

lab for serial number restoration

Soil and Minerals – cross-transfer (esp. soil

embedded in shoes)

Common Types (cont.)

Tool Marks – any object suspected of having an impression of another tool in it (ex. – crowbar may leave marks on a windowsill in a break-in)

Vehicle Lights – examination of vehicle lights can determine whether they were on or off at the time of impact

Wood or other Vegetative Matter – cross-transfer

Identification of a Substance

Identification – process of determining

a substance’s physical or chemical

identity

Identification (cont.)

Drug analysis, species determination, and

explosive residue analysis are typical tests

performed to determine identity

Common standard tests are used to determine

identity. There are standard tests for most

drugs, blood species of origin and explosive

materials.

Some materials can be determined with one

test while others may require several tests

Comparison

Comparison--process of determining

whether two or more objects have a

common origin

Comparison Con’t

Scientists will compare the control and suspect

specimen to determine if they are the same

This type of analysis can assist in placing a

suspect at the scene

Two-step Procedure:

combos of select properties of the control and

suspect specimens are chosen for comparison

once the examination is completed, the forensic

scientist will render a conclusion as to how closely

related the two specimens are

Comparison Con’t

If any of the selected properties are not alike,

the scientist will declare that the two items are

not from the same source. However, if all of

the selected properties match, they still may

not be able to conclude that the two specimens

are from the same source with 100% accuracy.

Therefore, usually only the probability that two

specimens came from the same source is

given

Individual Characteristics

Individual Characteristics –

Properties of evidence that can be

attributed to a common source with an

extremely high degree of certainty

Individual Characteristics

Individual Characteristics

Individual Characteristics Con’t

Examples: DNA, fingerprint ridges, grooves/striations on a bullet or toolmark, irregular and random wear patterns on tire or shoe impressions, handwriting characteristics, fitting together pieces of broken objects, matching the striations in sequentially made plastic bags, etc.

** The probability of two people having the same fingerprints is 0.0000000000000000000000000000000000000000000000000000000000006.

Class Characteristics

Class Characteristics – Properties of

evidence that can only be associated

with a group and never with a single

source

Class Characteristics

Class Characteristics Con’t

Examples: new factory painted cars with

only two layers of paint, blood to a

certain degree

Uses of Class Characteristics are usually

to corroborate the events with data that

are free of human bias

Class v Individual Characteristics

Class or Individual?

DNA Evidence

Class or Individual?

Boot Print

Class or Individual?

Fiber Evidence

Class or Individual?

Fingerprint

Class or Individual?

Hair

Class or Individual?

Shotshell Casings