Introduction to Forensic Science Physical Evidence.

Introduction to Forensic Science

Physical Evidence

Value of Physical Evidence

• Vernon J. Gerberth, in Practical Homicide Investigation, points out that physical evidence is useful in:– Helping to establish the scope of the

crime scene – Placing a perpetrator at a scene – Connecting a suspect with a weapon – Supporting witness statements – Connecting crime scene areas

(abduction, vehicle used, dump site)

Physical Evidence is Varied

• 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• Soil and minerals• Tool marks• Vehicle lights• Wood and other

vegetative matter

PRENTICE HALL ©2007 Pearson Education, Inc. Upper Saddle River, NJ 07458

CRIMINALISTICS An Introduction to Forensic Science, 9/EBy Richard Saferstein

Trace Evidence

• No matter how much someone tries to clean up a crime scene, something is generally left behind.

• 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.



Class and Individual Characteristic 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

footprintsPRENTICE HALL ©2007 Pearson Education, Inc. Upper Saddle River, NJ 07458


Categories of Evidence:

• Biological/Physiological– Blood, semen, saliva and other bodily fluids.

• Chemical– Narcotics, prescription drugs, powders

associated with firearms.

• Physical– Fingerprints, footprints, impressions,

toolmarks, tiremarks, firearms evidence.

• Non-specific/miscellaneous– Photography, dye marks, voice analysis.

Six Types of Evidence

• Trace evidence• Transfer evidence• Indented or impression evidence• Striated evidence• Geometric evidence• Chemical evidence

• All of these can be class or individual evidence

Trace 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.



Locard’s Principle

• Every person who is physically involved in a crime leaves some minute trace of his or her presence, and often takes something away.

• This is Dr. Edmond Locard's principle of contact, proposed when he began his forensic laboratory in 1910.



Transfer 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).

Fiber Analysis

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

• Cross transfers of fiber 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 that the fibers can corroborate or something unique to the fibers that set them apart.

Fiber Analysis

• A better case can be made with class evidence when the evidence has either a greater number of identifying features or a greater number of different types of evidence.

• For example finding a mixture of white dog and yellow dog and black cat hairs on a thief from a household with those color pets is stronger than just one type of hair.

Fiber Analysis

• Fibers are gathered at a crime scene with tweezers, tape, or a vacuum.

• They generally come from clothing, drapery, wigs, carpeting, furniture, and blankets.

• For analysis, they are first determined to be natural, manufactured, or a mix of both.

Fiber Analysis

• Natural fibers come from plants (cotton) or animals (wool).

• Manufactured fibers are synthetics like rayon, acetate, and polyester, which are made from long chains of molecules called polymers.

• To determine the shape and color of fibers from any of these fabrics, a microscopic examination is made.

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.

Fibers are Usually Class Evidence

• 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.

But Can be Individual 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.

Soil 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

Indented or Impression 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

Fiber Impressions

• 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.

Collecting Fiber and Hair Evidence

• Recovery of evidence should be the most direct but least intrusive technique practical. This could include picking, scraping, or vacuuming.

• Wrap clothing or items to protect adhering fibers if collection is to be done by laboratory personnel.

Collecting Fiber and Hair Evidence

• Since fiber evidence is generally small in nature, care should be taken to prevent loss or contamination.

• Several methods could be used in the collecting of fiber evidence: – visual searches, – alternate light sources – searches with additional magnification.

Striated Evidence

• Marks left on objects that rub together, when one object is harder than the other.

• Striations on bullets are a good example.

• Photomicrograph of two bullets fired from the same gun.

Geometric Evidence

• Pieces of Broken Evidence – Headlights with hit

and run accidents.

Chemical Evidence

• Narcotics• Flammables• Alcohol• Chemicals associated with

clandestine labs.

Factors to Consider for Trace Evidence Examination

• 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

Trace Evidence• The quality of trace evidence examination depends

on the information obtained from the crime scene and the many factors that affect the trace material.

• Using trace evidence an investigator can link a suspect to a crime.

• There are many modes of collection of trace evidence. Some of the more popular collection methods are vacuuming, lifting with tape, and gathering by hand.

• Most trace evidence investigations involve the use of the microscope and a spectrophotometer.

• As with hair identification and firearm identification, a comparison microscope is a very helpful tool in characterizing the evidence.

Trace Evidence can be Fragile and Easily Lost

• E.g. footprints in the dust on a floor or in the snow can be easily lost.

• Footwear prints and impressions should be photographed: – As part of the general scene– Also photograph with a scale

• Dental Stone is used in casting impressions

Individual vs. Class Evidence

• 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.



Probability

• 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.



Individual Evidence

• Examples: – The matching ridge characteristics of two

fingerprints– The comparison of random striation markings

on bullets or tool marks– The comparison of irregular and random wear

patterns in tire or footwear impressions– 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



Class 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 certainty.



Class Evidence

• A current weaknesses 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?

• There is little statistical data available from which to derive this probability in a world with so many mass-produced products.



Class Evidence

• One of the primary endeavors of forensic scientists must be to create and update statistical databases to evaluate the significance of class physical evidence.

• Most physical evidence 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.



Class Evidence

• The chances are low of finding two indistinguishable items of evidence at a crime scene that originated from different sources.

• In 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.

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



Crossing over the line from class to individual evidence is

difficult.• 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. PRENTICE HALL

©2007 Pearson Education, Inc. Upper Saddle River, NJ 07458


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.

• 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.



Forensic Databases• The Integrated Automated Fingerprint

Identification System (IAFIS), a national fingerprint and criminal history system maintained by the FBI.

• The Combined 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. PRENTICE HALL

©2007 Pearson Education, Inc. Upper Saddle River, NJ 07458


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.



Reconstruction and Physical Evidence

• 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.



Case Study: Sam Sheppard

• 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.