Trace Evidence Flashcards
(60 cards)
Definition
Not a proper evidence type - more of an umbrella term which covers a wide varitey of different materials. Any physical evidence that is found in small measurable amounts and can’t be calssified into one of the main traditional fields.
Definition by Brenner
‘Physical evidecne that results from the transfer of small quantities of materials (hair, textile fibres, paint chips, glass fragments, gunshot residue particles)
All sub-categories of trace evidence are more specifically characterised by what common characteristics?
- particulate nature - they come in discrete units, such as particles, fibres, fragments, etc
- small size - usually, these units are in the mm or um range (micro-traces). Examples
-Human hair - OD:0.1MM, Length: in the order of cm
-Textile fibres - OD: 0.03mm, Length in the order of mm
-Glass fragments - Size 0.1-1mm - difficult to see with naked eye - they need a systematic approach to be observed and/or recovered
-Low and high power microscopy
Classification according to their source and nature - examples: textile fibres, glass fragments, paint fragments, etc
Common types
- textile fibres
- glass fragments
- paint chips
- soils
- gunshot residue
- pollen particles
- hair
- othersI
Importance in forensic science
Trace evidence is a critical evidence type in crime investigation. This is because:
1. It’s easily lost from their source after application of adequate force/energy
2. Usually, unintentionally and without realising
3. it’s easily transferred to surrounding people (offender, victim, by-standers) and/or objects
As a consequence, it can be:
* be produced following different types of activities and found in a very large number of different cases/crimes
* Establish different kinds of links (i.e. associations). Examples (but not only): suspect -> crime scene, victim -> crime scene, suspect -> victim
Trace evidence that can be found at volume crimes
- Burglaries - (1) paint from door left on lever, (2) glass from broken window on offender, (3) fibres left at a point of entry (window or door)
- Car accidents - (1) glass from broken windshields/lights on victim or crime scene, (2) fibres from victim left on car
- Car thefts (TWOCs) - (1) glass from broken windshields on offender, (2) fibres from offender onto seat and vice versa
- Assaults/fights - fibres transfer between people
Trace evidence that can be found at Serious/violent crimes
- Sexual assaults - fibres transfer between people
- Murders - fibres transfer between people
- Shooting - gunshot residue from discharge firearm on offender, target/victim and surrounding objects
Where can trace evidence potentially be transferred?
Everywhere and in every case.
Trace evidence can potentially be transferred ….. THROUGH ANY ACTIVITY!
Thus, it can potentially be found everywhere, in every case and after any type of crime.
Case example - hit and run
In February 1987, at 9.30pm, 19 year old Craig Elliot Kalani went for a walk with his dog in his neighbourhood but never returned. He was killed by a hit-and-run driver.
- Pieces of glass were found on Craig’s jacket and around his body at the crime scene
- These pieces of glass helped crime scene investigators figure out what happened
Police searched for a vehicle that had the same damage as the hit-and-run car. Then they found a car with those damages that belonged to Susan Nutt
- In order to connect Susan’s car, glass analysis has been done. The scientists found that the windshield glass contained the same 22 chemicals as the ones in Susan’s car, and that it also had the same refractive index (RI). Both samples of glass were a ‘definite match’.
- The glass evidence helped convict Susan Nutt, who was sentenced to 5 years in jail.
Transfer mechanisms
- a particularly important concept in trace evidence
- Is the concept of : Transfer (or exchange)
- Trace evidence is sometimes also reffered to as ‘transfer evidence’(ider group) and has a really strong relationship with Locard’s exchange principle
Locards exchnge principle
‘It is impossinle for a criminal to act without leaving traces of his/her presence, especially considering the intensity of a crime. Additionally, through inverse action, he/she will collect on his body or clothes tracesof his/her presence and actions.’
What 2 entities does transfer occur?
- donor surface/source
- recipient surface
Extent of transfer
- One-way - trace material exclusively transfer from the source to the recipient surface
- Two-way (cross transfer) - reciprocal transfer of trace material between the 2 objects
Mode of transfer
- Direct (primary) - transfer of trace material from the source to a recipient surface (A to B)
- Indirect (secondary, tertiary, etc) - transfer of trace material from the source to a recipient material, through a further intermediary surface (A to B to C)
Source
Source - to determine what is the source of analysed (questioned) trace material
- Usually, these questions are addressed using the intrinsic characteristics (IC), i.e. those characteristics that are inherent to each single element of the trace and solely depend on those of the original material
Activity
Activity - to determine how has the analysed (questioned) trace material been transferred on the recipient surface
- Usually, these questions are addressed using the extrinsic characteristics (EC), characteristics that are inherent to the trace when taken as a group and depend on the type of activity that left the trace
Source problem
Estalishing the source of the questioned material is the first aim in any criminal case. At this level, the typical question that forensic scientists help to reply is:
Does the analysed trace material come from that specific source?
How is the source problem answered?
1.Comparison of the ICs between the questioned material and reference source
- Inference of common source, by integration with additional information on the feature variability and rarity
Activity problem
Even if the source can be confirmed, this does NOT mean that questioned material has been transferred during the alleged activity! At activity level, the typical question that forensic sceientists help to reply is: Was the suspect involevd in that specific activity?
How is the activity problem answered?
- Study of the circumstances surrounding the case: what is the alleged activity?
- Comparison of the ECs observed on the questioned material with a series of expected findings
- Inference of activity, by integration with additional information on the transfer and persistence mechanisms
At activity level, tere are many additional difficulties in the interpretation of findings!
The most important issuesare that the ‘expected findings’ (EC) …
* They are NOT directly observable by sampling from reference material on the crime scene (IC)
* They strongly depend on the circumstances of the case!
What additional information does interpretation strongly rely on?
- Transfer
- Persistence
Relevant mechanism 1 - transfer
The amount of trace material transferred durign an
activity depends on a series of factors.
Main ones:
* Donor material (shedability)
* Recipient surface (retention)
* Area of contact
* Number of contacts
* Duration of contact
* Pressure of contact
Relevant mechanism 2 - peristence
After transfer, trace material is lost!
The loss rate (and, so the amount of trace material expected to have persisted between the alleged activity/seizure of the suspect-related material) also
depends on a series of factors.
Main ones:
* Donor material (adhesion)
* Recipient surface (retention)
* Intermediary activities
* Time since deposition
Loss rate essentially depends on type of activity:
*80% of transferred fragments are
lost 3-4h after contact through normal
daytime activities
* This loss rate strongly accelerates
with direct aggressive activities, such as
washing
Expected findings
- transfer
- persistence
- recovery
