CRIMINALISTICS CHAPTER 3

PHYSICAL EVIDENCE

PHYSICAL EVIDENCE

• Impossible to list all items that could be important at crime scene\

• Practicle to list those that scientific examination can yield significant results

• Investigator needs to be familiar with – Recognition – Collection – analyses – – Laboratory procedures and capabilities

COMMON TYPES OF PHYSICAL EVIDENCE

Lists and definitions or examinations

BLOOD, SEMEN & SALIVA

• All suspected saples – Liquid or dried– Animal or human

• Subjected to serological examination (DNA) to determine:– Identity– Origin

DOCUMENTS

• Handwriting or Typewriting– Determine authenticity or Source

• Related Items– Paper– Ink– Indented writing– Obliterations– Burned or Charred documents

DRUGS

• Controlled substances seized in violation of the law– Sale – Distribution– Manufacture– Possession

EXPLOSIVES

• Devices containing explosive charge

• Residue from scene of explosion to examine for suspected explosive charge

FIBERS

• Natural or synthetic fibers

• Transfer may be helpful in establishing relationship between objects/persons

FINGERPRINTS

• Latent or other prints – Establish identity– Place at scene

FIREARMS

• Firearms, Projectiles, cartridges– Identify– Compare

• To other exemplars

• To firearms

• Firearms residue– Present?– Distance & angle

GLASS

• Glass particles or fragments– Transfer evidence

• Window panes with holes – Sequence & origin

• Broken windows – Characteristics

HAIR

• Animal or human

• Origin – Race– Body area

• Comparison

• Root present– DNA

IMPRESSIONS

– Tire or Shoe prints in soft material– Fabric Impressions– Bite marks

PAINT

• Liquid or dried

• Transfer from one surface to another

• Identification

• Comparison

PETROLEUM PRODUCTS

• Arson– Residues identified at fire scene (accelerants)

• Oil or grease stains – Comparisons to place at scene

PLASTIC BAGS

• Identification

• Comparison to known

• Fracture match

POLYMERS

• Plastic, Rubber, etc

• Identification

• Comparison to objects at scene

SERIAL NUMBERS

– Usually falls under firearms– Stolen property

• Defaced serial numbers restored

SOIL AND MINERALS

• Link a person or object to location

• Examples – Soil found on clothing or vehicles– Soil found on tools

TOOL MARKS

• Usually under firearms

• Impressions made by tool

LIGHTS

• Mainly vehicle

• Conducted to determine on/off during crash

WOOD

• Other vegetative material

• Comparison– To link suspect to scene

PHYSICAL EVIDENCE

• IDENTIFICATION

• COMPARISON

MATHEMATICAL PROBABILITY

• Underlies virtually every human action

• Based on idea that the outcome of an event can be logically estimated

• Estimate based on known conditions, past performance , & experience with similar events

• Familiar example (coin toss)

PROBABILITY

Blood as example

Four major types

O 40% 8:20

A 40% 8:20

B 15% 3:20

AB 5% 1:20

Any blood stain must be one of these types

Probability

• Significance– Probability of occurrence of different types– Ex: O is 8 times more common than AB– If O found fewer people eliminated than if AB

is found– AB sample is therefore higher quality evidence

than type O

Probability

• The greater the frequency of occurance the less significance attached

• Must also consider concept of Mutually Exclusive– Events that by nature preclude other events– Not possible for individual to have two types– Therefore if type A found at scene the blood

did not come from person with any other type

PROBABILITY

• Some forms of physical evidence do not lend themselves to statistical evaluation

• Example: Do not know statistical probability of a particular configuration of a toolmark

• Value must be estimated base on experience• Must form an opinion as to value of

evidence

PROBABILITY

• Experience has shown that microscopic relationships between the impression and the tool are so specific to be unique

• The basis of the opinion must be experience that equips the examiner to note the unique points that distinguish the item from a larger body of occurances

IDENTIFICATION

• Definition– The determination of the physical or chemical

identity of a substance with as near absolute certainty as existing analytical techniques will permit

IDENTIFICATION

• EXAMPLES– Suspected drug sample identified to contain

cocaine– Residues from a fire identified to contain

gasoline– Sample found at scene identified as blood

• May determine species

– Vegetable material identified as type of wood

IDENTIFICATION

1.Adoption of standard testing procedures

must give characteristic results for standard testing materials

Testing permanently established

used to establish identity

test results must be same on unknown as on a known sample

IDENTIFICATION

• 2. The number and type of tests must be sufficient to exclude all other substances– Must devise an analytical scheme that will

eliminate all but one substance– If identify white powder as cocaine must

exclude every other drug

IDENTIFICATION

• No simple rules as to what constitutes thorough and foolproof analytical scheme

• Each type of evidence requires different type of tests

• Some may be identified with one test, some may require many

• Little or no control over the quality or quantity of samples submitted

IDENTIFICATION

• The forensic scientist must conclude in many cases:– What point the analyses is concluded– The criteria for positive identification

• Conclusion beyond reasonable doubt for court

• Most disciplines have SOP

COMPARISON

OF KNOWN AND UNKNOWN

COMPARISON

• Definition– An analysis that subjects a suspect speciman

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.

COMPARISON

• Examples– Place a suspect at scene by noting similarities

between hair found at scene to known hair of suspect

• Similar to or consistent with and could have a common origin

COMPARISON

Examples

Place vehicle at scene of hit and run by noting similarities between paint found on victim clothing and known paint from vehicle

Chemical and physical property match

could have common origin

Fracture match

have a common origin

COMPARISON

• Forensic comparison two step process• 1. Combinations of select properties are

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

• How many properties are chosen depends on the type of material

• Overriding consideration must be the ultimate evidential value of the conclusion

COMPARISON

• 2. When examination is completed the examiner must be prepared to render a conclusion with respect to the comparison

• Do they come from same source• If one property does not agree, no common

origin• If all properties compared agree are they

from common origin (not necessarily)

COMPARISON

• Physical match– The most definite point of comparison between

objects– Example – point of screwdriver and shaft from

which it broke– Any item that suggests a rip, tear, or breakage

should lead to a search for its counterpart

INDIVIDUAL CHARACTERISTICS

• Definition– Evidence that can be associated with a common

source with a high degree of probability

• Examples– Matching ridge characteristics of two

fingerprints– Matching striations on bullets– Matching irregular and random wear patterns

on footwear

INDIVIDUAL

• Not possible to state with mathematical exactness the probability that specimens are of common origin. (exception DNA)

• Conclusion that probability is so high as to defy mathematical calculations or human comprehension

• INDIVIDUAL– Example – Fingerprints– Several mathematical models proposed– One model – probability of two individuals with same fingerprint

1 x 10 followed by 60 zeros– Practical – millions of prints compared over 100 years never found

two the same

RARITY

• Exceptional circumstances connected with the place, time or general conditions under which physical evidence is discovered can heighten its quality

• Example: A hairpin found near murdered wonam may not be of as much value as a mans tie clasp

CLASS CHARACTERISTICS

• Definition– Properties of evidence that can only be

associated with a group and never with a single source

• Disappointments when Lab unable to relate evidence to a common source with a high degree of probability

CLASS CHARACTERISTICS

• Weakness– Inability of examiner to assign exact or

approximate probability values to the comparison of most pyhsical evidence

– Scientist attempt to creat or update statistical databases when possible

– Mostly forensic scientist must rely on personal experience to interpret significance of comparison

CLASS CHARACTERISTICS

• Efforts made to find individual evidence– Fingerprints, DNA, Firearms match

• Most physical evidence is class match

• Majority of evidence in case is subjetive– Eye witness, confessions, informant

• Physical evidence provides corroboration

CLASS CHARACTERISTICS

Trying to find define the significance of class evidence in exact mathematical terms is difficult if not impossible

Class evidence by definition is not unique

Only objects that exhibit a significant amount of diversity are appropriate for physical evidence

colored fibers v. white cotton

CLASS CHARACTERISTICS

• Must provide evidence that would convince a reasonable man that there is no other explanation

• Dealing with more than one type of physical evidence, their collective presence leads to a high degree of certainty

CLASS CHARACTERISTICS

• Must consider reality in courtroom

• Weight or significance left entirely to jury

• Lay people usually give scientist high degree of significance

• Scientific testimony takes on more reliability in courtroom

• Given great weight in deliberations

PHYSICAL EVIDENCE

• When does class characteristic become individual characteristic

• Source of debate and disagreement

• EXAMPLES– How many striations to match bullet– How many paint layers to individualize– How many ridges to match fingerprint

PHYSICAL EVIDENCE

• Find as many characteristics as possible to compare substances

• Significance decided by– Quality of evidence– Composition of evidence– Case history – Examiners experience

• Conclusion can reach from speculation to near certainty

PHYSICAL EVIDENCE

• Properties and characteristics– Practical limits– Extreme – No two things are alike in every

detail– Modern analytical techniques have become too

sensitive in some cases– Learning how to use instruments part of work– Proficiency to interpret findings critical

PHYSICAL EVIDENCE

CRIME SCENE RECONSTRUCTION

CRIME SCENE RECONSTRUCTION

• Team event• Reconstruct events that occurred

– Prior to crime– During crime – Subsequent to crime

• Collaborative effort including– Law enforcement, criminalist, Medical

examiners

RECONSTRUCTION

• All professionals bring unique perspective

• Try to answer questions about crime– How many people involved– Cause of death– Was there an attempt to cover up

• Physical evidence plays critical role in reconstructing events

RECONSTRUCTION

• Physical evidence can – Support accounts given by witnesses/suspects– Contradict accounts given by

witnesses/suspects– Generate leads– Confirm reconstruction to jury– Physical evidence is the foundation of

reconstruction

RECONSTRUCTION

• Actions law enforcement must take to optimize reconstruction– 1. Crime scene protection and security

• A continuous process (beginning to end)

• Evidence can be destroyed by walking through

• There is the possibility of contamination from persons allowed in the scene

RECONSTRUCTION

• 2. Preliminary examination– See scene as left by perpetrator– Experience and physical evidence critical– Captures nature of scene– Hypothesize what happened– Document observations– Determine how scene should be worked

RECONSTRUCTION

• 3. Involve other professionals– Medical examiners

• Position of body

• Injuries

• Has body been moved

• Clothed before or after death

• Cause of death

• Estimate of time of death (not very accurate)

RECONSTRUCTION

• 3. Other professionals– Criminalists

• Approximate bullet path (lasers)

• Blood spatter analysis (chapter 12)

• Glass penetration (Chapter 4)

• Gunshot residue for distance (Chapter 15)

RECONSTRUCTION

• From text

• “Reconstruction supports a likely series of events by the observation and evaluation of physical evidence, as well as statements made by witnesses and those involved with the incident”

RECONSTRUCTION

• Team effort

• Put together different pieces of the puzzle

• Need right connections to show relationship between victim, suspect, and crime scene

• Can play a vital role in helping jury arrive at proper verdict