Chapter 3: Physical Evidence

Any tangible objects that can be collected from a crime scene

Common Types of Physical Evidence

• Body fluids– Blood, semen, saliva– Liquid/dry– Animal/human

• Documents– Hand-/typewriting– Paper, ink, etc.

• Drugs– Any illegal substance– Sale, manufacture,

distribution

• Explosives– Devices - explosive charge– Explosive objects/residues

• Fibers– Natural/synthetic– Establishes connections

between objects/people

• Fingerprints– Individual evidence– Can be visible/latent

(hidden)

• Firearms/ammunition– Guns/casings/shells, etc.

Types of Physical Evidence (continued…)

• Glass– Glass particles/ fragments

in a crime– Bullet holes

• Hair– Animal/human– Link to a crime/scene

• Impressions– Tires/shoe prints/tracks– Bite marks

• Organs/Physiological Fluids– Detection of drugs/poisons

or alcohol

• Paint– Liquid/dried– Transferred during a crime

• Petroleum Products– Suspect/crime scene– Gasoline, grease, etc– Residues left at a crime

• Plastic bags– Polyethylene– Homicide/drugs

• Plastic/Rubber/Polymers– Remains recovered– Link to crime/suspect

Types of Physical Evidence (continued…)

• Powder Residues– From the discharge of a firearm

• Serial Numbers– Stolen property– Restoration of erased ID numbers

• Soil and Minerals– Link person/object to a particular

location– Soil embedded in shoes, etc.

• Tool Marks– Causes an impression– Tool in a crime leaves

marks

• Vehicle Lights– Headlights/taillights– On/off at time of impact

• Wood/Vegetative Matter– Fragments/shavings/twigs,

etc. on clothing, shoes, etc.

– Link a person/object to a location

Key Points

• Biological crime scene evidence includes blood, saliva, semen, DNA, hair, organs, and physiological fluids;

• Impression crime-scene evidence includes tire markings, shoe prints, depressions in soft soils, all other forms of tracks, glove and other fabric impressions, tool marks and bite marks;

• Manufactured items considered common items of crime-scene evidence include firearms, ammunition, fibers, paint, glass, petroleum products, plastic bags, rubber, polymers, and vehicle headlights/taillights.

Examination of Physical Evidence

• Identification– Determine

physical/chemical identity

– Body fluids can determine the species of origin

– Determine the chemical composition of a drug, residue, etc.

Identification

• Identification process– Uniform testing

procedures– Scientific process– Sufficient number/type of

tests conducted (multiple trials) depending upon type of evidence

– Comprehensive enough to exclude any uncertainty

– Education/experience of scientist also plays a role

Comparison Analysis

• Specimen in question compared to a known standard

• Usually a 2-step procedure– Properties are selected to

compare the evidence with the standard (reference)

– Conclusion(s) drawn about the origins of the sample/specimen

Probability

• If one or more of the selected properties do not agree (between evidence and standard), the scientist will conclude that the specimens are not from the same source

• Probability plays a role in establishing evidence of value– Frequency of occurrence of an event– Defines the odds of an event occurring and that

the evidence is connected to that particular event

Evidence: Class Characteristics

• Associated with a group and not a single source

• Used for elimination rather than identification

• Blood, paint, hair and fibers are some examples

Evidence: Individual Characteristics

• Evidence from a single source with a high degree of probability

• Can be linked to a specific suspect and/or location

• Examples are DNA and fingerprints

Key Points• Two methods used by forensic scientists when examining

physical evidence are identification and comparison.• Identification is the process of determining a substance’s

chemical or physical identity to the exclusion of all other substances.

• A comparison analysis determines whether a suspect specimen and a standard/reference specimen have a common origin.

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

• The overall frequency of occurrence of an event can be obtained by multiplying the frequencies of all independently occurring instances related to the event. This is known as the product rule.

Significance of Physical Evidence

• Hard to assign exact or even approximate probability values to most class evidence

• Very few statistical data are available (mass-produced materials, etc.)

• Statistical databases must be created/updated to help evaluate the significance of physical evidence

• Personal experience often plays a role in the evaluation – subjective – can detract from credibility rather than support

Assessing the Value of Evidence

• Lies in its ability to corroborate events with data that is as free from human error as possible

• Methods/instruments are continually being revised/improved

• Objects that exhibit significant diversity are appropriate for classification as physical evidence

• As the number/type of evidence increases to link a specific person to a crime scene, the probability of involvement increases dramatically

Cautions and Limitations

• The weight/significance of evidence in a courtroom is usually decided by the jury

• Reliability and trustworthiness• Given great weight in jury deliberations• Proper safeguards must be taken to protect the

evidence to prevent bias against the accused– Thorough collection and scientific evaluation– Find as many characteristics as possible to compare one

substance/piece of evidence to another– Decided by the quality and composition of the evidence,

case history, examiner’s experience– There are practical limits to the properties/characteristics

selected for comparison (sophisticated instruments help)• Also used to exclude/exonerate someone

Key Points

• The value or class physical evidence lies in its ability to corroborate events with data in a manner that is, as nearly as possible, free of human error or bias.

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

• 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

• Fingerprint Databases– IAFIS (Integrated Automated Fingerprint ID System)

• Maintained by the FBI• 50 million suspects (500 million images!)• Submitted by state, local and federal law enforcement• Many other countries maintain a fingerprint database like IAFIS

and these can be available through Interpol

• DNA Databases– CODIS (Combined DNA Index System)

• Also maintained by the FBI• Enables federal, state and local crime labs to electronically

exchange and compare DNA profiles• Forensic Index and Offender Index to match• 3 million files with lots backlogged!• Info from other countries available through Interpol

Other Databases

• NIBIN (National Integrated Ballistics Info Network)

– Maintained by the ATF– Acquires, digitizes and compares markings– Heart of this is IBIN – Integrated Ballistics ID

System• Made up of a microscope and computer unit that can

capture the image of a bullet/casing• 90,000 pieces of crime scene evidence

• PDQ (International Forensic Automotive Paint Data Query)

• SICAR (Shoeprint Image Capture and Retrieval)– Privately run, comprehensive shoe sole database

Key Points

• The creation of computerized databases for fingerprints, criminal histories, DNA profiles, markings on bullets and cartridges, automotive paints and shoe prints has dramatically enhanced the role of forensic science in criminal investigation.

• AIFIS is a national fingerprint and criminal history database maintained by the FBI. AIFIS allows criminal investigators to compare fingerprints at a crime scene to an index of 500 million known prints. CODIS is the FBI’s DNA database. It enables federal, state and local crime labs to electronically exchange and compare DNA profiles, linking crimes to each other and to convicted offenders.

Crime Scene Reconstruction

• Principles of Crime Scene Reconstruction– Physical evidence left behind is crucial– Supports or contradicts the hypothesis

and/or witness testimony– Generate leads/assist the jury– Collection/documentation of physical

evidence is the foundation of reconstruction

– Supports likely sequence of events

Steps in Reconstruction

• Secure/protect the crime scene– If not protected/secured, evidence could be

rendered useless– Prosecution is difficult

• Processing the crime scene– Crime scene walk through/drawing/document all

observations– Contemplate events that might have occurred– Bag/tag physical evidence– Photograph the scene

Steps in Reconstruction (continued…)

• Personnel Involved in Reconstruction– Medical examiner

• Make observations about livor (settling of blood closest to the ground) as to whether the body was moved, etc.

• Clothed/unclothed (livor will not develop if skin constricted by clothing)

– Criminalist• Plot the trajectory of a bullet/position of shooter• Analyze blood spatter, residues and other physical

evidence

– Establish a relationship between suspect, victim and crime

Key Points

• Crime-scene reconstruction relies on the combined efforts of medical examiners, criminalists, and law enforcement personnel to recover physical evidence and sort out the events surrounding the occurrence of a crime.

• Examples of crime-scene reconstruction include determining whether a body was moved after death, determining whether a victim was clothed after death, analyzing bullet trajectory, analyzing blood spatter, determining the direction from which penetrated glass objects, estimating the distance of a shooter from a target, and locating primer residue on suspects.