Physical Evidence

Physical Evidence

Class characteristics:Evidence can only be associated with a group and not a unique source

Individual characteristics:Evidence can be attributed to a unique source with a high degree of certainty (or even to the exclusion of all others)

Examples of Class Characteristics

Shoes (Nike, Air Jordan, size 9)Paint (Cherry red 1967 Chevy Mustang)Fibers (Red polyester fiber, braided weave)Fingerprints (loops, whorls, arches)Toolmarks (1/4” flat blade B&D screwdriver)Firearms (.40 caliber S&W magnum pistol)Arson (accelerants)DNA (male)

Examples of Unique Characteristics

Shoes (natural wear, bubble gum)Paint (physical match)Fibers (physical match, unique “trait”)Fingerprints (ridge detail)Toolmarks (stria markings)Firearms (stria, firing pin markings)DNA (profile)

Scenarios

Pathology wound matches murder weapon…Glass fragments found in burglar’s coat pocket…Burglar’s tools…Poison found in suspect’s home with rare component…

Analysis of Evidence

KnownUnknownCommon Source

e.g. the unknown XXX collected at the crime scene was compared to known XXX collected from Ms. Y and could have originated from a common source.

Edmond Locard (1877-1966)

French investigatorBackground in medicine and law1910-started a police laboratoryFounded Institute of Criminalistics at the University of LyonsContributed to fingerprints (poroscopy)

Locard’s Exchange Principle

Principle of cross-transferWhenever two objects come into contact there is evidence of that contact through cross-transfer…MicroscopicMacroscopic

Every contact leaves a trace!

Physical Evidence Can…

1. Prove that a crime has been committed or establish key elements of a crime

Example Rape = victim has torn clothing, bruises, broken arm =

non-consensual

Physical Evidence Can…

2. Link a suspect with the victim or crime scene

Example Burglary = suspect has broken glass shards in his

pants cuff that are consistent with glass shards at scene

Physical Evidence Can…

3. Establish the identity of persons associated with a crime

Example Latent fingerprints identified through AFIS. DNA

identified through CODIS.

Physical Evidence Can…

4. Exonerate the innocent

Example Rape = DNA preserved was compared against man

convicted of the crime = no match.

Physical Evidence Can…

5. Corroborate the victim’s story

Example Officer involved shootings

Physical Evidence Can…

6. Contradict information provided to investigators

Example Discredit a witness; elicit a confession

Physical Evidence Can…

7. Be more reliable than eyewitness testimony

“[Physical Evidence] does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. It cannot be wrong; it cannot perjure itself; it cannot be wholly absent. Only in it’s interpretation can there be error. Only human failure to find, study, and understand it can diminish its value.”

Paul Kirk, Crime Investigation, 1953

Physical Evidence Can…

8. Be the cornerstone of a case

Police/prosecutors are expected to obtain physical evidence and rarely rely on a confession alone

Physical Evidence Can…

9. And IS expected by a jury/the public

TV, books, media focus on physical evidence and the miracles performed (reality vs. fantasy)

The “CSI Effect”

Lack of Physical Evidence Can…

10. Support or refute a theory

The absence of forced entry in a burglary case…