Chapter 3 PHYSICAL EVIDENCE

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.

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.

Examples 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 Soil and minerals Tool marks Vehicle lights Wood and other vegetative matter PHYSICAL EVIDENCE

Examples of Physical Evidence Impressions – Tire markings, shoe prints, depressions in soft soils, and all other forms of tracks. Bitemarks in skin or food, are also included.

Examining Physical Evidence The examination of physical evidence is done for two reasons: identification or comparison. Identification determines the physical or chemical identity Comparison subjects a suspect specimen and a standard/reference specimen to the same tests to show origin PHYSICAL EVIDENCE

Identification PHYSICAL EVIDENCE Identification determines the physical or chemical identity as best as existing analytical technology will allow. Identification requires that the tests establish the identify a substance to exclude all other substances. Examples: Identifying the composition of a drug Identifying blood or hair from species of origin (human, dog, cat?) PHYSICAL EVIDENCE

Evidence Identification The crime laboratory is frequently asked to identify: the chemical composition of an illicit drug. gasoline in residues recovered from arson debris explosive residues from bombs blood, hair, or wood and include the species origin. It may be asked to identify gasoline in residues recovered from the debris of a fire, or it may have to identify the nature of explosive residues—for example, dynamite or TNT.

Comparison PHYSICAL EVIDENCE A comparative analysis determines whether a suspect specimen and a standard/reference specimen are the same Both the standard/reference and the suspect specimen are subject to the same tests. The forensic comparison is actually a two-step procedure: Step One: Compare the standard/reference to the unknown specimen. Step Two: Give a conclusion about where the specimen came from. PHYSICAL EVIDENCE

Role of Probability PHYSICAL EVIDENCE Value of comparison depends on the probability of matching unknown specimens with known examples (controls/standards) Probability is the frequency of occurrence of an event. Ex: When flipping a coin, probability is easy to establish PHYSICAL EVIDENCE

Classifying Characteristics Individual Characteristics Evidence that can be linked to a single source with an extremely high degree of probability. Class Characteristics Evidence linked to only a group of like objects. PHYSICAL EVIDENCE

Individual Characteristics Examples of Individual Characteristics: the matching ridge characteristics of two fingerprints the comparison of random striation markings on bullets or tool marks the comparison of handwriting characteristics the comparison of irregular and random wear patterns in tire or footwear impressions 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 Characteristics Class characteristics are associated with only a group (such as a manufacturing lot) and not a single source (such as one individual). Due to mass manufacturing matching to a specific company or lot is difficult.

Determine whether the evidence is more likely to possess individual or class characteristics Individual characteristic Impression from an automobile tire A fingerprint A spent bullet cartridge Pieces of a shredded document Commercial potting soil DNA Fragments of silver paint recovered from a Honda Accord

Crossing Over PHYSICAL EVIDENCE Evidence that changes from class to individual Examples: Number of striations are necessary to individualize a mark to a single tool. Performing DNA profiling on a hair sample These are all questions that are addressed in court . PHYSICAL EVIDENCE

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/crime scene. Someone can also be exonerated (or excluded from suspicion) if the physical evidence collected at a crime scene is found to be different from the reference samples collected from that suspect. PHYSICAL EVIDENCE

Forensic Databases PHYSICAL EVIDENCE 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. PHYSICAL EVIDENCE