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 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 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 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 or Individual?
DNA Evidence