There is a lot of evidence present in varied quantities at a crime scene. They can be in abundant amounts like a pool of blood in case of murders, while in some cases the evidence may present in small quantities like soil samples, a semen sample, etc. The evidence present at the crime scene in small amounts or in trace amounts is called trace evidence. Though they are present in trace amounts still they play a vital role in the investigation of any case.
The principle behind the appearance of trace evidence is Locard’s Principle of Exchange which states that whenever two entities come in contact with each other, they leave their traces on each other.
Since Trace evidence is always in minute quantities, therefore they usually remain unnoticed and if neglected can be a big drawback in the investigation. Trace evidence is also known as “Silent Witness” because it has the potential to tell what actually happened and who has been involved. Hence, it is important to search the crime scene carefully for such pieces of evidence.
Evidence Termed as Trace Evidence
The evidence that are considered trace evidence are:
- Hair:- Usually found in crimes such as homicide, sexual assaults, etc.
- Glass:- It is a piece of common evidence in theft, robbery, burglary, etc.
- Plant materials(leaves, pollens, wood, seeds, etc.):- Plant species are particular to specific regions, so they can be of forensic significance as they can be found stuck to the clothes, footwear, etc. of the victim or suspect. Usually in cases where primary and secondary crime scenes can be established.
- Animal materials(animal hair, feathers, etc.):- This evidence can be encountered in cases where endangered animals are directed to smuggling or animals are used as a weapon to commit a crime.
- Dust:- Dust is present everywhere and varies from place to place.
- Soil:- Usually found in outdoor crime scenes.
- Fibers:- As each human being wears clothes to cover their body, the fibers of clothes are obvious to be encountered at a crime scene such as in hit-and-run cases where the fibers get stuck to the hitting vehicle, which may give a lot of important information from the perspective of a criminal investigation.
- Paint– Usually paint is encountered in car accidents or hit-and-run cases where the paint chips are found at the scene of occurrence.
- Gunshot Residue– Encountered in cases where a firearm is used as the weapon of offense.
- Minerals– Minerals are constituents of soil and are particular to a geographical area, whose analysis can be helpful in a criminal investigation.
- Polymers:- These include plastics, rubbers, etc.
Forensic Significance of Trace Evidence
Although trace evidence is small in size and amount, they still have great significance in forensic science as they are exchanged in all types of crime scenes.
- Due to their minute size, they often remain unnoticed which indicates that the chances of the destruction of this evidence are quite less which is of great use in forensic examination.
- The trace evidence may link the crime scene, suspect, and victim.
- It helps to reconstruct a sequence of events, to determine the modus operandi, and also the place of occurrence of crime.
- They can be identified and matched with the possible source of their origin with much accuracy like any other clue material.
Location of Trace Evidence
The location of trace evidence can be the victim, suspect, scene of occurrence, or weapon used to commit any offense. For example, hair, skin, fiber, soil, etc. can be found on the body, clothes, or belongings of the victim as well as the suspect. Also on the weapons such as knives or any such sharp object, traces of hair, dust, soil, fiber, etc. can be found.
In case of hit and run or any vehicular accident, the fiber of the clothes may be found on the vehicle and the paint chips from the vehicle may stick to the clothes of the victim or are found shed on the scene while the glass pieces of the headlights or any other vehicular part may be found on the scene.
The mud or dust picked up by the vehicle from the scene of occurrence or deposited at the scene provides a useful link between the crime vehicle and the crime scene. The exchange of particles may also indicate the reason or the responsible person behind the accident.
Collection of Trace Evidence
The trace pieces of evidence are usually tiny therefore they require specific instruments to locate them. The use of magnifying glasses, strong light, UV lamps, chemical agents, large shadow-less lamps, etc. can make the locating process quite easy for the crime scene investigator.
After locating the trace evidence on the crime scene, they are next directed to their collection. The collection method used can be of the following types:
- Hand Picking: Trace evidence like hair, and fiber can be picked up by using clean forceps.
- Lifting: In this method, an adhesive-bearing substrate such as tape is used for the collection of micro traces like dust, pollens, etc.
- Scrapping: This method is used to collect traces like bloodstains, paint chips, etc. A flat object such as a clean spatula is used to dislodge traces from an object and then transfer them to a collecting surface.
- Vacuum Sweeping: In some cases, a vacuum cleaner along with the filter trap is used to recover trace evidence from an item or area.
- Clipping: Trace evidence present in fingernails can be recovered from clipping. Clean clippers or scissors are used to clip fingernails.
- Combing: Sometimes a clean comb or brush is used to recover trace evidence from the hair of an individual.
Packaging of Trace Evidence
The trace evidences are usually packed in:
- Paper Fold– Hair, fiber, pollen grains, dust, etc.
- Glass Vials- Minerals, dust, pollens, polymers, food grains, spices, wood, etc.
- Plastic Envelopes- Fibre, hair, food product grains, diatoms collected on glass slides, etc.
- Herbarium Sheets – Plant evidence is usually collected on such sheets.
Examination of Trace Evidence
The analysis of the trace evidence is specific to each piece of evidence but the most common tools involved in the examination are:
- Microscopy– Since the trace evidence is small in size so microscopes are mandatory for their analysis. It reveals the morphology, size, crystal structure, refractive index, density, etc. The commonly used microscopes are- a stereomicroscope, petrographic microscope, and scanning electron microscope.
- Spectroscopy– Techniques of X-Ray diffraction and micro-FTIR are very useful for the analysis of trace evidence. Both these techniques elucidate information like compositions, crystalline nature, and other molecular properties of the trace evidence.
- Chemical Tests– It is a destructive method, therefore it is not frequently used. But it is useful to detect the chemical composition of traces of gunshot residue, metallic components in trace evidence, etc.
Trace evidence is the material found in trace amounts that act as corroborative evidence. They can be used for investigation as they are found in every criminal case.
The location, nature, and condition of the trace evidence can give a lot of information helpful for the purpose of a criminal investigation. Though they are small in size, still they play a crucial role in linking the crime scene, victim, and suspect, therefore they should not be left unnoticed.
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