Physical Evidence

Chapter 3

Physical evidence is any evidence introduced in a trial in the form of a physical object.

It can include part or all of any object.A piece of evidence is NOT physical evidence

if it merely conveys information that would be conveyed by the physical evidence.

Physical Evidence Defined

Vernon J. Gerberth points out that physical evidence is useful in:

- Helping to establish the scope of the crime scene.- Placing a perpetrator at the scene.- Connecting a suspect to a weapon.- Supporting witness statements.- Connecting crime scene areas (abduction, vehicle

used, and dump site).

Value of Physical Evidence

Physical Evidence is Varied- Blood, saliva, semen- Documents- Drugs- Explosives- Fibers- Fingerprints- Firearms and ammunition- Glass- Hair- Impressions- Organs and physiological

fluids

- Paint- Petroleum products- Plastic, rubber, and other

polymers- Powder residues- Soil and minerals- Tool marks- Vehicle lights- Wood and other vegetative

matter

No matter how much someone tries to clean up a crime scene, something is generally left behind. Whose principle is this?

It may not always be detected, but it’s difficult to take any kind of violent action without shedding something.

Trace evidence, though often insufficient on its own to make a case, may corroborate other evidence or even prompt a confession.

Trace Evidence

Class: A group of objects or persons with characteristic physical evidence common to it.

-Examples include soil and hair.Individual characteristics can be

identified as having originated with a particular person or source.

-Establishes individuality-Examples include fingerprints and

footprints

Class and Individual Characteristic Evidence

Biological/Physiological

-Blood, semen, saliva, and other bodily fluids.Chemical

-Narcotics, prescription drugs, powders associated with firearms.

Physical-Fingerprints, footprints, impressions, tool

marks, tire marks, firearms evidence.

Non-Specific/Miscellaneous-Photography, dye marks, voice analysis.

Categories of Evidence

Trace evidenceTransfer evidenceIndented or impression evidenceStriated evidenceGeometric evidenceChemical evidence

All of these can be class or individual evidence.

Six Main Types of Evidence

Trace evidence is material found at a crime scene or accident scene in small but measurable amounts.

This is important as it can definitively link an individual or object to the scene.

Trace Evidence

Because trace evidence can be any number of things, from a fingerprint to DNA to plant debris, there are numerous different methods used for analysis.

For some objects, there is a large database available for comparisons (fingerprints, DNA), while the science of others has not advanced that far.

Trace Evidence

Transfer evidence is defined as any evidential substance or particle such as blood, fluids, hairs, fibers, paint, and skin that is exchanged between an assailant and the victim or the scene of the crime.

-From the criminal to the victim.-From the victim to the criminal.-Into or out of the crime scene.-When forcible contact occurs between persons, vehicles, or objects (e.g. paint in a car crash).

Transfer Evidence

Most investigations center on fiber or hair, which is easier to see than pollen or dirt.

Cross transfers of fiber can often occur in cases in which there is person-to-person contact.

Investigators hope that fiber traceable back to the offender can be found at the crime scene, as well as vice versa.

Success in solving the crime often hinges on the ability to narrow the sources for the type of fiber found.

Fiber Analysis

The problem with fiber evidence is that fibers are not usually unique.

They cannot pinpoint an offender in any definitive manner.

There must be other factors involved, such as evidence tat the fibers can corroborate or something unique to the fibers that set them apart.

Fiber Analysis

Generally, the analyst gets only a limited number of fibers to work with—sometimes only one.

Whatever has been gathered from the crime scene is then compared against fibers from a suspect source, such as a car or home, and the fibers are laid side by side for visual inspection through a microscope.

Fiber Analysis

Comparisons of fibers of the same type of material include the presence of dyes and a number of microscopic features.

Comparisons can be made with fibers from a fabric to individual fibers or from fibers removed from another piece of fabric.

Fibers are Usually Class Evidence

The weave pattern of fabrics can also be compared.

Sometimes, when a piece has been torn from a fabric, that piece can be matched up with weave pattern and irregularities of a mutual tear edge.

Cordage (ropes, string, etc.) tears and cuts can be matched if the ends are not too frayed or distorted.

But Can be Individual Evidence

Soil evidence is important when the suspect drives/walks on unpaved areas.

It is picked up by:-Tire treads-Shoe bottoms-Pants cuffs

It may also be located in:-Subject’s vehicle-Articles in a suspect’s trunk

Soil Evidence

Footprints or shoe prints.Writing surfaces, like a pad of paper with the

top sheet removed.Tool marks.Tire marks.These can be class or individual evidence.

Indented or Impression Evidence

Fabric impressions may be found in a number of situations.

The impression of a victim’s garment may remain on the oil pan or on another flat surface of a vehicle in an accident investigation.

Fiber Impressions

The commonness of the material.The mode of transfer.Quantity found.Possible contamination from other sources.Whether someone attempted to remove the

material.Certainty of identification of the material.How the material was collected, preserved,

and retained.Background information of the case.

Factors to Consider for Trace Evidence Examination

Individual Characteristics-Evidence that can be associated to a common source with an extremely high degree of probability is said to possess individual characteristics.

Class Characteristics-Evidence associated only with one

group is said to have class characteristics.

Individual vs. Class Evidence

To understand the accuracy of a comparison, one must appreciate the role that probability has in ascertaining the origins of two or more specimens.

Simply defined, probability is the frequency of occurrence of an event.

In flipping a coin, probability is easy to establish.

The exact probability of some things are almost impossible to define – of transferring dog hair and blue jeans fiber for example.

Probability

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.

Probability

Examples:- The matching ridge characteristics of two fingerprints.- The comparison of random striation markings on bullets or tool marks.- The comparison of handwriting characteristics.- The fitting together of the irregular edges of broken objects in the manner of a jigsaw puzzle.- Matching sequentially made plastic bags by striation marks running across the bags.

Individual Evidence

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

Here again, probability is a determining factor.

However, the high diversity of class evidence often makes it very significant.

However, a lab may be unable to classify evidence to a common origin with a high degree of certainity.

Class Evidence

A current weakness of forensic science is the inability 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?

Class Evidence

Most physical evidence cannot be linked directly 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.

Class Evidence

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 legal arguments.

Crossing Over the Line From Class to Individual Evidence is Difficult

The final value of physical evidence to a case is determined in the courtroom.

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.

Just as important, 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 subject.

Using Physical Evidence

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.

Reconstruction

The physical evidence plays a crucial role in reconstructing the events that took place.

Evidence should support the theory of the crime.

Evidence may 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.

Reconstruction and Physical Evidence

To determine if the weapon could have been a flashlight or if the assailant was right or left handed, experts did reconstructions of the crime scene.

Case Study: Sam Sheppard