Chapter 3: Physical Evidence

Chapter 3:

Physical Evidence

“You can learn a lot by just watching.”

—Yogi Berra, former New York

Yankees catcher and sage

2Chapter 3

What is evidence?

State 5 examples.

Please Do Now

3Chapter 3

Locard’s Exchange Principle

Edmond Locard (1877-1966)

“Locard's Exchange Principle” states that whenever two objects come into contact there is a transference of material between them.

4Chapter 3

Types of Evidence

Two general types: Testimonial—a statement made under oath; also

known as direct evidence or Prima Facie evidence

Physical—any object or material that is relevant in a crime; also known as indirect evidence. Examples are hair, fiber, fingerprints, documents, blood, soil, drugs, tool marks, impressions, glass.

Note: This chapter will focus on physical evidence

5Chapter 3

Reliability of EyewitnessFactors:

Nature of the offense and the situation in which the crime is observed

Characteristics of the witness Manner in which the information is retrieved

Additional factors: Witness’s prior relationship with the accused Length of time between the offense and the

identification Any prior identification or failure to identify the

defendant Any prior identification of a person other than the

defendant by the eyewitness

6Chapter 3

Eyewitness

A police composite may be developed from the witness testimony by a computer program or forensic artist.

“Perception is reality.”

As a result of the influences in eyewitness memory, physical evidence becomes critical. Faces—a composite program

by InterQuest

7Chapter 3

Value of Physical Evidence

Generally more reliable than testimonial Can prove that a crime has been committed Can corroborate or refute testimony Can link a suspect with a victim or with a crime

scene Can establish the identity of persons associated

with a crime Can allow reconstruction of events of a crime

8Chapter 3

Physical Evidence

It would be impossible to list all the objects that could conceivably be of importance to a crime.

Almost anything can be Physical Evidence. Although you cannot rely on a list of categories, it is

useful to discuss some of the most common types of physical evidence.

The purpose of recognizing physical evidence is so that it can be collected and analyzed.

It is difficult to ascertain the weight a given piece of evidence will have in a case as ultimately the weight will be decided by a jury.

9Chapter 3

Common Types of Physical Evidence

Blood, semen, and saliva Documents Drugs Explosives Fibers Fingerprints Firearms and

ammunition Glass Hair Impressions Organs and physiological

fluids

Paint Petroleum products Plastic bags Plastic, rubber, and

other polymers Powder residues Serial numbers Soil and minerals Tool marks Vehicle lights Wood and other

vegetative matter

10Chapter 3

Physical Evidence

Blood, semen and saliva Documents

11Chapter 3

Physical Evidence

Drugs Explosives

12Chapter 3

Physical Evidence

Fibers Fingerprints

Silkfibers

13Chapter 3

Physical Evidence

Firearms and

ammunition

Glass

Refractive index

14Chapter 3

Physical Evidence

Hair Impressions

Dyed human hair

Hair shaft comparison

Impression foam

15Chapter 3

Physical Evidence

Organs and physiological fluids

Paint

Car paint layers 300X

16Chapter 3

Physical Evidence

Petroleum products Plastic bags

17Chapter 3

Physical Evidence

Plastic, rubber and

other polymers

Powder residues

Powder residue test

18Chapter 3

Physical Evidence

Serial numbers Soil and minerals

Serial number restoration

19Chapter 3

Physical Evidence

Tool marks Vehicle lights

Honda Tail lights

20Chapter 3

Physical Evidence

Wood and other vegetative matter

Mixed pollen

Diatoms

21Chapter 3

Common Types of Physical Evidence

Blood, semen, and saliva Documents Drugs Explosives Fibers Fingerprints Firearms and

ammunition Glass Hair Impressions Organs and physiological

fluids

Paint Petroleum products Plastic bags Plastic, rubber, and

other polymers Powder residues Serial numbers Soil and minerals Tool marks Vehicle lights Wood and other

vegetative matter

22Chapter 3

Using Physical Evidence

As the number of different objects linking an individual to a crime scene increases, so does the likelihood of that individual’s involvement with the crime

Also a person may be exonerated or excluded from suspicion if physical evidence collected at a crime scene is found to be different from standard/reference samples collected from that suspect

23Chapter 3

Why Examine Physical Evidence?

The examination of physical evidence by a forensic scientist is usually undertaken for identification or comparison purposes.

Identification has, as its purpose, the determination of the physical or chemical identity of a substance with as near absolute certainty as existing analytical techniques will permit.

A comparison analysis subjects a suspect specimen and a standard/reference specimen to the same tests and examinations for the ultimate purpose of determining whether or not they have a common origin.

24Chapter 3

Identification

to determine the physical or chemical identity with as near absolute certainty as existing analytical techniques will permit first requires the adoption of testing procedures that

give characteristic results for specific standard materials Once these test results have been established, they may be

permanently recorded and used repeatedly to prove the identity of suspect materials

Second, the number and type of tests needed to identify a substance be sufficient to exclude all other substances

25Chapter 3

Common Types of Identification

The crime lab is often requested to identify:

1. Chemical composition of an illicit drug

2. Gasoline in residues recovered from the debris of a fire, or the nature of explosive residues— for example, dynamite or TNT

3. Blood, semen, hair or wood - including a determination for species origin

26Chapter 3

Comparison

A comparative analysis has the important role of determining whether or not a suspect specimen and a standard/reference specimen have a common origin.

Both the standard/reference and the suspect specimen are subject to the same tests.

27Chapter 3

Forensic comparison

A two step procedure:1. First, combinations of select properties

are chosen from the suspect and the standard/reference specimen for comparison.

2. Second, once the examination has been completed, the forensic scientist must draw a conclusion about the origins of the specimens

28Chapter 3

Role of Probability in Forensic Comparison

To comprehend the evidential value of a comparison, one must appreciate the role that probability has in ascertaining the origins of two or more specimens

Probability: the frequency of occurrence of an event

Easy to establish the probability of flipping a coin Exact probability is impossible to define with many

analytical processes

29Chapter 3

ProbabilityFor example:Blood Factors Frequency A 26% EsD 85% PGM 2+2— 2%

What is the probability of someone having all three factors?

0.26 X 0.85 X 0.02 = 0.00442 which is Using the product rule:

ONLY 0.442% or 1OO or 1 out of 226 people .442

30Chapter 3

Classifying Characteristics

Individual Characteristics Properties of evidence that can be

attributed to a common source with an extremely high degree of certainty

Class Characteristics Properties of evidence that can only be

associated with a group and never with a single source

31Chapter 3

Individual Characteristics

In all cases, it is not possible to state with mathematical exactness the probability that the specimens are of common origin.

It can only be concluded that this

probability is so high as to defy

mathematical calculations or human

comprehension.

32Chapter 3

Evidence Characteristics

Class characteristics—Evidence common to a group of objects or persons

Individual characteristics—Evidence can be identified with a particular person or a single source with an extremely high degree of probability

Blood DNA TypingFingerprints

33Chapter 3

Examples of Individual Characteristics

1. the matching ridge characteristics of two fingerprints

2. the comparison of random striation markings on bullets or tool marks

3. the comparison of irregular and random wear patterns in tire or footwear impressions

4. the comparison of handwriting characteristics

5. the fitting together of the irregular edges of broken objects in the manner of a jigsaw puzzle

6. matching sequentially made plastic bags by striation marks running across the bags

34Chapter 3

Class vs Individual Evidence

Which examples do you think could be individual evidence?

35Chapter 3


The large piece of glass fits to the bottle—it is individual evidence

These fibers are class evidence—there are millions like them.

36Chapter 3


it is individual evidence— the dental work on the teeth can be used to identify the person

This tape is class evidence—but it could be individual evidence if the striations of the tear match.

37Chapter 3


This tire track is class

evidence, as it can be

linked to a class of tire

—but it could be individual

evidence if there are

unique impressions.

38Chapter 3

Class Characteristics

Frequently forensic scientists cannot relate physical evidence to a common origin with a high degree of certainty

Evidence is said to possess class characteristics when it can be associated only with a group and never with a single source.

Again, probability is a determining factor. Nevertheless, the high diversity of class evidence

in our environment makes their comparison very significant in the context of a criminal investigation.

39Chapter 3

Class Evidence A weaknesses of forensic science is the inability of

the examiner to assign exact or even approximate probability values to the comparison of most class physical evidence.

For example, what is the probability that a nylon fiber originated from a particular sweater, or that a paint chip came from a suspect car in a hit and run?

There are very few statistical data available from which to derive this information, and in a mass-produced world, gathering this kind of data is increasingly elusive.

40Chapter 3

Class Evidence One of the primary endeavors of forensic

scientists must be to create and update statistical databases for evaluating the significance of class physical evidence.

Most items of physical evidence retrieved at crime scenes cannot be linked definitively to a single person or object.

The value of class physical evidence lies in its ability to provide corroboration of events with data that are, as nearly as possible, free of human error and bias.

41Chapter 3

Class Evidence

The chances are low of encountering two indistinguishable items of physical evidence at a crime scene that actually originated from different sources.

When one is dealing with more than one type of class evidence, their collective presence may lead to an extremely high certainty that they originated from the same source.

Finally, the contribution of physical evidence is ultimately determined in the courtroom.

42Chapter 3

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.

43Chapter 3

Natural vs. Evidential Limits

There are practical limits to the properties and characteristics the forensic scientist can select for comparison. Modern analytical techniques have become so

sophisticated and sensitive that natural variations in objects become almost infinite.

Carrying natural variations to the extreme, no two things in this world are alike in every detail.

Evidential variations are not the same as natural variations.

Distinguishing variations of evidential use from natural variations is not always an easy task.

44Chapter 3

Forensic Databases

The ultimate contribution a criminalist can make is

The crime lab is on the forefront of the

investigation seeking to identify perpetrators computerized databases link all 50 states and

those that link police agencies throughout the world

to link a suspect to a crime through comparative analysis

45Chapter 3

Forensic Databases Integrated Automated Fingerprint

Identification System (IAFIS) a national fingerprint and criminal history

system maintained by the FBI

60 million prints on file

46Chapter 3

Forensic Databases Combined DNA Index System (CODIS)

enables federal, state, and local crime laboratories to electronically exchange and compare DNA profiles - maintained by the FBI

47Chapter 3

Forensic Databases National Integrated Ballistics Information

Network (NIBIN) Allows firearm analysts to acquire, digitize, and

compare markings made by a firearm on bullets and cartridge casings

maintained by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF)

48Chapter 3

Forensic Databases

International Forensic Automotive Paint Data Query (PDQ) contains chemical and color information

pertaining to original automotive paints maintained by the Forensic Laboratory Services of

the Royal Canadian Mounted Police

Cross section of Automobile paint

49Chapter 3

Forensic Databases

Shoeprint image capture and retrieval (SICAR) shoeprint database computer retrieval system - not maintained by a government agency

50Chapter 3

Reconstruction

The method used to support a likely sequence of events by the observation and evaluation of physical evidence, as well as statements made by those involved with the incident, is referred to as reconstruction.

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.

51Chapter 3

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.

52Chapter 3

Reconstruction

Physical Evidence is used to answer questions about:

what took place how the victim was killed number of people involved sequence of events

A forensic scientist will compare the questioned or unknown sample with a sample of known origin.

53Chapter 3

Summing It Up

Reconstruction is a team effort that involves putting together many different pieces of a puzzle.

The right connections have to be made among all the parts involved so as to portray the relationship among the victim, the suspect, and the crime scene.

If successful, reconstruction can play a vital role in aiding a jury to arrive at an appropriate verdict.

The recognition, collection, and analysis of physical evidence is the foundation to successful reconstruction, but only part of the process.

54Chapter 3

55Chapter 3

Types of Physical Evidence

Transient Evidence—temporary; easily changed or lost; usually observed by the first officer at the scene

Pattern Evidence—produced by direct contact between a person and an object or between two objects

Conditional Evidence—produced by a specific event or action; important in crime scene reconstruction and in determining the set of circumstances or sequence within a particular event

Transfer Evidence—produced by contact between person(s) or object(s), or between person(s) and person(s)

Associative Evidence—items that may associate a victim or suspect with a scene or each other; ie, personal belongings

—Lee and Labriola in Famous Cases, 2001

56Chapter 3

Examples of Transient Evidence

Odor—putrefaction, perfume, gasoline, urine, burning, explosives, cigarette or cigar smoke

Temperature—surroundings, car hood, coffee, water in a bathtub, cadaver

Imprints and indentations—footprints, teeth marks in perishable foods, tire marks on certain surfaces

Markings

57Chapter 3

Examples of Pattern Evidence

Pattern Evidence—most are in the form of imprints, indentations, striations, markings, fractures or deposits.

Clothing or article distribution

Gun powder residue Material damage Body position Tool marks Modus operandi

Blood spatter Glass fracture Fire burn pattern Furniture position Projectile trajectory Tire marks or skid marks

58Chapter 3

Examples of Conditional Evidence

Light—headlight, lighting conditions

Smoke—color, direction of travel, density, odor

Fire—color and direction of the flames, speed of spread, temperature and condition of fire

Location—of injuries or wounds, of bloodstains, of the victim’s vehicle, of weapons or cartridge cases, of broken glass

Vehicles—doors locked or unlocked, windows opened or closed, radio off or on (station), odometer mileage

Body—position, types of wounds; rigor, livor and algor mortis

Scene—condition of furniture, doors and windows, any disturbance or signs of a struggle

59Chapter 3

Classification ofEvidence by Nature

Biological—blood, semen, saliva, sweat, tears, hair, bone, tissues, urine, feces, animal material, insects, bacterial, fungal, botanical

Chemical—fibers, glass, soil, gunpowder, metal, mineral, narcotics, drugs, paper, ink, cosmetics, paint, plastic, lubricants, fertilizer

Physical—fingerprints, footprints, shoe prints, handwriting, firearms, tire marks, tool marks, typewriting

Miscellaneous—laundry marks, voice analysis, polygraph, photography, stress evaluation, psycholinguistic analysis, vehicle identification

60Chapter 3

Forensic Investigations

Include some or all of these seven major activities

1. Recognition—ability to distinguish important evidence from unrelated material Pattern recognition Physical property observation Information analysis Field testing

2. Preservation—collection and proper preservation of evidence

61Chapter 3

Investigations3. Identification—use of scientific testing

Physical properties Chemical properties Morphological (structural) properties Biological properties Immunological properties

4. Comparison—class characteristics are measured against those of known standards or controls; if all measurements are equal, then the two samples may be considered to have come from the same source or origin.

62Chapter 3

Investigations

5. Individualization—demonstrating that the sample is unique, even among members of the same class

6. Interpretation—gives meaning to all the information

7. Reconstruction—reconstructs the events of the case Inductive and deductive logic Statistical data Pattern analysis Results of laboratory analysis

—Lee, Dr. Henry. Famous Crimes, 2001