Trace Evidence – A Primer for Lawyers
What is Trace (Transfer) Evidence?
Gross evidence is physical evidence which looks intuitively “out of place” in the scene and is large enough to be readily visible and “obvious” to a scene investigator. Its association and relevance to a crime or incident is easily recognised. Examples include a bloodstained knife, a damaged lock, and a partially burnt document. Trace evidence refers to small, often microscopic amounts of material scarcely visible to the naked eye, which are transferred through physical contact between persons, objects or places. Examples include textile fibres, paint fragments, glass particles, soil and dusts, explosives, gunshot residues, adhesives, ignitable liquids and vapours, wood and paper products, plant material, household and industrial chemicals, corrosive and noxious substances, suspicious white powders and unknown substances.
Locard’s Exchange Principle
The foundational principle of forensic science was first enunciated in 1923 by French scientist, Edmond Locard.1 The Exchange Principle is commonly paraphrased as “Every contact leaves a trace”. When a perpetrator comes in contact with a scene or victim, he adds or removes something from the scene or victim, unintentionally and usually unknowingly. Hence, trace evidence that is found at a scene and examined by a forensic scientist can establish that contact had occurred, and reveal details of the presence and physical activities of a perpetrator in a crime.
How is Trace Evidence Formed?
Trace evidence forms when tiny bits of material break or detach from the source surface and transfer to a recipient surface during physical contact. The amount of material that transfers from the source and is retained on the recipient surface depends on the following factors:
1. Properties of the source and its sheddability;
2. The duration, forcefulness and surface area of the physical contact;
3. The size and properties of the transferred materials;
4. The properties of the recipient surface;
5. The time elapsed, post-transfer activities and environmental conditions affecting the recipient; and
6. Transferred materials after the transfer.
Where Can Trace Evidence Be Found?
Investigators who process a scene need to look beyond obvious gross evidence. An inexperienced or negligent investigator is liable to overlook trace evidence at the scene. It is hence not surprising that trace evidence is often under-utilised, and its value understated.2
Trace evidence at the scene must be quickly recognised, preserved and collected as it is transient and easily lost. It is often masked on gross evidence such as furniture, walls, clothing, footwear, vehicles, weapons, tools, and even in food. A forensic scientist with expertise and experience in trace evidence examination has a keen eye for recognising and preserving trace evidence at the scene, as well as analysing and interpreting it in relation to the physical context in which it was discovered. Table 1 provides examples of trace evidence which are commonly encountered for different types of cases.
Table 1: Common types of trace evidence found in Singapore cases
Types of Cases
Transfer Material on Gross Evidence
Theft and robbery cases
Paint fragments on the tip of a screw-driver; metal filings and smears on a pair of pliers or saw
Glass fragments and soil on the soles of footwear
Cases causing grievous hurt
Corrosive residues in a container or on clothes and surfaces
Lachrymatory (tear-inducing) traces on the clothing of a person who flung chilli powder or discharged pepper spray or a riot agent
Toxic or poisonous substances spiked into food and beverages
Presence of flammable vapours where hot works are carried out on board a ship
Presence of toxic gases in scuba diving tanks
Drug consumption or trafficking
White powder residues in a plastic bag or on utensils for drug consumption
Hairs on tapes used for wrapping around drug packages
Paint smears on collided vehicles and boats
Windscreen glass fragments and lamp glass/plastic fragments on the roadway, vehicle paint fragments and glass fragments on the clothing of a victim
Fibres and body tissue adhering to the undercarriage of a vehicle that has run over a road-user
Cases involving firearms discharge, explosives and arson
Wall plaster, paint, glass or bone fragments on a bullet
Gunshot residues on a suspect’s hands, clothing and on surfaces near the location of discharge of a firearm
Explosives and combustion residues in post-blast debris
Ignitable liquids (petrol, kerosene, thinners, etc) in fire debris
Evidential Value of Trace Evidence
Trace evidence is the most diverse type of physical evidence.3 Virtually any material present in the indoor or outdoor environment can become trace evidence. Practically every incident involves trace evidence in one form or other. This cloud of silent witnesses can explain what transpired at the scene, and the interactions between objects and persons. Unlike blood, trace evidence is not obvious to the perpetrator, and hence unlikely to be noticed and cleaned up by the perpetrator.
Trace evidence is most commonly used as associative evidence as opposed to evidence that yields an identification of a substance or a suspect. Even though it is usually circumstantial, it provides vital associations, linking the source and the recipient item. In the absence of a suspect, finding unusual trace materials can help narrow the search and point investigators to a more specific and fruitful line of inquiry. Trace evidence is also useful in linking serial cases to a common perpetrator, modus operandi and source. For example, similar red paint smears found on the damaged doors and windows of several break-in cases were traced to a crowbar with a coat of red paint.
DNA profiling is usually the method of choice for identifying the perpetrator (“Who”) in an incident. However, it is common these days for informed perpetrators to take precautions against leaving their fingerprints or DNA at the scene. Trace evidence is complementary and indispensable for establishing non-DNA contacts, ie contacts that do not transfer human genetic materials. Non-DNA contacts include paint smears and broken glass in hit-and-run accidents, incendiary materials and explosive residues in fires and explosions, and the use of noxious or toxic chemicals to cause grievous hurt.
While trace evidence may not be able to positively identify a perpetrator, it can narrow the pool of suspects to a more manageable group. More importantly, trace evidence goes beyond identifying the perpetrator to unveiling how events transpired by revealing critical information on the “What”, “When”, “Where” and “How” of an incident. Answers to these interrogatives are necessary for scene reconstruction4 to ascertain the actions and reactions of persons, and the sequence of events at the scene. This is particularly useful in cases where a suspect can be linked to the incident but information on actions and individual responsibilities, and the role of other persons present at the scene are unclear or disputed.
Factors Affecting the Value of Trace Evidence
Most trace evidence originate from mass-produced materials and products, and hence are not unique in the same manner that DNA profiles are associated with a single individual. The value of trace evidence5 depends on several factors such as:
1. domain knowledge, experience and level of expertise of the forensic scientist;
2. knowledge of the forensic context in which the transfer occurred;
3. commonness of the trace material in the setting of the crime;
4. nature and extent of the transfer: number of types, persistence, one-or two-way transfer; and
5. discriminating power of the physical and analytical techniques employed.
Domain Knowledge, Experience and Level of Expertise of the Forensic Scientist
The value of trace evidence increases when the examiner has sufficient domain knowledge of the forensic discipline to accomplish the following:
• Recognise, identify and classify different types of materials;
• Recognise transfers that are relevant; and
• Narrow down to a few manageable materials by mining information from the database
Knowledge of the Forensic Context in Which the Transfer Occurred
Conducting experimental studies which mimic the forensic reality can increase the evidential value of trace evidence. Such studies enable a more complete understanding of the specific forensic context within which the trace material is generated and subsequently collected.6 It also provides insight and knowledge into the behavior of certain forms of trace materials under specific conditions, adding depth to the analysis, understanding and interpretation of the evidence.
The value of trace evidence depends on its commonness in the setting of the crime. For instance, white cotton fibres are so common as to have practically no evidential value, but red delustered nylon fibres of a particular chemical composition, melting point, crystallinity, colour, cross sectional shape and size will be rarer and hence have greater probative value than white cotton fibres.
Evidential value increases when the transfer occurs from an object which has been altered from its original condition because alteration introduces variation. Thus, the transfer of repaints from a vehicle has higher evidential value than the original manufacturer’s finish.
Nature of Transfer
A primary transfer involves the direct transfer from one surface (eg blue fabric of socks) to another (black fabric of shoe). Secondary transfers occur when already transferred trace evidence (eg red carpet fibres on the blue socks) transfers to another object. Transfer of materials may occur in one or both directions between the two contacting objects or persons. Two-way transfers, the concomitant transfer of multiple types of trace evidence, and the transfer of unusual materials normally absent in the environment greatly strengthen the probative value of the evidence.
For example, in a case where a kidnapped victim was bundled onto a fabric seat of a car, having several different types of fibres from the victim’s clothing transferred to the car seat and different types of fibres (both primary and secondary transfers) from the car seat fabric on the victim will further increase the associative value. On the other hand, textile fibre matches between two individuals who share the same environment (eg living in the same home) are essentially meaningless.7
The evidential value also increases when:
1. the transfer is not obvious;
2. the transfer process results in a change in the state of the recipient item; and
3. the transferred material is secured and less transient in form.
For example, in a traffic accident case where vehicle occupants forcefully contact with the vehicle interior when it is suddenly impacted or stopped quickly under heavy braking, the fibres from their clothing may be fused into the plastic material of the interior components of the vehicle. These plastic-fused fibres will remain intact for a long time within the vehicle. Identifying such fibres will provide unequivocal evidence of the sitting positions of the occupants. This evidence is especially useful in cases where the identity of the driver is disputed and several DNA profiles are obtained from the steering wheel.
Discriminating Power of the Physical and Analytical Techniques Employed
The discriminating power of analytical methods depends on the sensitivity, specificity and reproducibility of each instrumental analysis, variations of the measured property in the population of the material, heterogeneity of the sample, and number of independent techniques that can be applied (which may be dependent on the quantity of trace evidence available).
Like all other physical evidence, trace evidence should never be considered in isolation, but should be interpreted with other evidence types. Trace evidence is often synergistic in probative value when combined with marks, prints and damage, DNA, bloodstain patterns and other physical evidence. The whole can be greater than the sum of the individual parts.
Local Cases Involving Trace Evidence
In Singapore, the value of trace evidence became more evident following the increased use of forensic reconstruction8 in our cases. The unique ability of this form of physical evidence to provide “the missing link” is highlighted through a handful of our local cases.
Murder of Inspector T Maniam (vehicle paint)9
In the ambush and murder of a former police officer T Maniam by several hired men, small blue paint fragments were found on the rear bumper of Maniam’s car. The two-layered paint chips were found to match the paint on the perpetrators’ pick-up truck, indicating that the pick-up was present at the scene and had collided into the rear of Maniam’s car.
Vandalism cases involving loan sharks and foreigners (architectural paint and spray paint)10
In a vandalism case involving loan sharks, black paint stains on the suspect’s haversack were matched to the black paint found at the scene and to a can of black paint found in his home. In another case involving foreigners, an MRT train was spray-painted with graffiti. Graffiti paint matched the special type of spray paints the two perpetrators had purchased from a specific source.
Sexual assault and murder of six-month old Anjeli Elisaputri (adhesive, fibres, metallic particles)11
A six-month-old deceased infant was found dumped in a HDB rubbish chute wearing only diapers, with her hands and feet tied behind her back, and her mouth sealed with black duct tape. Her red dress was found nearby. A pair of scissors, fabric gloves, some string, a roll of black duct tape, and a crumpled strip of black duct tape were found in the accused, Soosainathan’s bedroom. Trace evidence examinations revealed that the accused’s gloves came in contact with the baby’s red dress and the crumpled tape. The pair of scissors had been used to cut the black duct tape and had come in contact with the baby’s red dress.
1. Fibres on the gloves and the scissors matched those from the baby’s red dress.
2. A small piece of black polypropylene matching the backing, and fibres matching the reinforcement fibres of the roll of tape were found on the scissor blades, suggesting that the scissors had been used to cut the tape.
3. Tiny dark-coloured metallic particles on the crumpled tape and on the gloves linked these two items.
Murder of eight-year-old Huang Na (fibres, adhesives, fruit stains and residues)12
In 2004, there was an island-wide manhunt for a missing 8-year-old girl. Initially, Police had no leads to Huang Na’s whereabouts. Eventually, Police suspected Took Leng How, a vegetable packer at Pasir Panjang Wholesale Centre. Forensic scientists looked for clues in the storeroom where Took worked and found many blue denim fibres on several items. They matched the blue denim fibres of the jacket Huang Na was wearing when last seen. Confronted with fibre and DNA evidence, Took led Police to the deceased’s decomposed body which he had disposed of three weeks earlier. Besides fibre evidence, several other types of trace evidence associated Took to the crime: adhesives, fruit stains and Huang Na’s stomach contents. Detecting mango residues in her stomach contents was significant as Huang Na loved mangoes, and Took had bought mangoes on the very day she disappeared. Its detection also provided an estimation of her time of death.
“Gas Scare” at Golden Landmark Shopping Complex (noxious chemical)13
More than 100 tenants were evacuated and 20 casualties treated after the release (spraying) of an unknown noxious substance from an aerosol can by two assailants. Forensic analysis of the hand swab, clothes and bag of a victim detected nonivamide, a synthetically manufactured capsaicinoid which is also naturally-occurring in chilli, and found in pepper sprays. As a result, the two offenders were jailed for using an illegal spray.
Recall of hot/cold gel packs in Singapore (prohibited substance)14
The accidental poisoning of a child in Australia through the ingestion of the ethylene glycol contents of a commercial hot/cold pack prompted the precautionary investigation of hot/cold packs and baby teethers available in Singapore. Instead of limiting to target analysis of ethylene glycol, we opted for a comprehensive, multi-disciplinary analysis of unknown chemicals to determine the chemical contents of these household products. Forensic findings led to the voluntary recall from the Singapore market of several hot/cold packs containing ethylene glycol.
Adulteration of milk powder (detergent)15
Cases where foreign domestic helpers suspected of adulterating the beverages of employers are typically processed with urgency due to their potential political implications. In 2009, a baby fell ill and was hospitalised after feeding from a milk bottle. The maid was suspected to have mixed small amounts of detergent powder into the baby’s milk powder. Forensic examinations identified the detergent in the milk powder, and associated it to one of the detergents found in the house. In another case, a maid fed a baby with a corrosive liquid, severely injuring his mouth, throat and windpipe. Examinations of the stains on the baby’s belongings and the cleaning agents found in the house enabled forensic scientists to pinpoint the cause of the chemical burns and link them to a drain de-clogging chemical.
Trace evidence occurs in most crimes because every contact leaves a trace. It is diverse and unparalleled in breadth of applicability in criminal investigations. Trace evidence complements and corroborates other types of physical evidence. To obtain a more complete and clearer picture of a crime, investigators must look beyond obvious gross evidence to recognise the occurrence of trace evidence, and collect potentially relevant transfer materials at the scene. Careful forensic analysis and holistic interpretation in the context of the case can lend a voice to these silent witnesses, revealing useful details of the crime.
Trace evidence provides the trier of fact with invaluable associative evidence. It provides answers to the “What”, “When”, “Where” and “How” of a crime, and is essential for scene reconstruction. It establishes linkages and modus operandi in an incident, uncovers details of individual responsibilities, and can furnish insightful investigative leads, especially for serial crimes.
The formation of trace evidence such as paint fragments, wood chips, metal particles and textile fibres is closely associated with hand tools due to the significant forces transmitted while using a tool. Dust, soil, glass particles and other materials transfer to or from footwear owing to intimate physical contact of the patterned outsole with the ground or other recipient surface. The same forces that create trace evidence often concomitantly create “Damages, Marks and Prints” – another important forensic discipline. The duality of trace evidence, and marks and prints are a potent synergistic multiplier of the value of physical evidence.
Look out for our next article “Damages, Marks and Prints” which will address the science behind the discipline, the methodology and conclusion scale commonly adopted for examining and reporting on cases involving marks and prints.
► The Forensic Experts Group*
E-mail: [email protected]
* The Forensic Experts Group (“TFEG”) is Singapore’s first one-stop private and independent provider of forensic consultancy, analysis, research, training and education for legal and law enforcement agencies, forensic and tertiary institutions, and private organisations. It comprises a team of accomplished forensic scientists, who are combining more than 70 years of specialised knowledge, unique experience and skillsets to deliver high quality forensic services both locally and overseas. While heading the Criminalistics Laboratory (Forensic Chemistry & Physics Laboratory) at the Health Sciences Authority from the mid-1990s till 2013, TFEG’s senior consultants developed trace evidence and its many sub-specialities as distinctive forensic disciplines in terms of analytical instrumentation, domain knowledge, methodology, interpretation, reporting guidelines and training programs. They successfully applied the expertise to hundreds of local cases.
1 Keith Inman, Norah Rudin, Principles and Practice of Criminalistics: The Profession of Forensic Science (CRC Press,2000), p 44.
2 “The understated value of trace evidence” (2009) Michael Tay, Singapore Academy of Law (“SAL”) Forensics Conference, 8-9 Oct (Oral presentation and paper).
3 The Forensic Experts Group, “The Essence of Forensic Science”, Singapore Law Gazette (2015, Jan issue).
4 The Forensic Experts Group, “Forensic Reconstruction”, Singapore Law Gazette (2015, May issue).
5 “Maximising the Value of Trace Evidence” (2012) Lim Chin Chin, Asian Forensic Sciences Network, Annual Meeting & Symposium (Oral presentation).
6 R. M. Morgan et al, “The relevance of the evolution of experimental studies for the interpretation and evaluation of some trace physical evidence” (2009) Science and Justice 49(4): 277-285.
7 “Trace evidence overview” (2011) Tammy Jergovich, Dept of Forensic Science, Georgia Bureau of Investigation.
8 Supra (note 4 above).
9 Supra (note 2 above).
10 PP v Fricker Oliver  SGDC 289; Fricker Oliver v PP  SGHC 239.
11 Supra (note 2 above); “Man who murdered baby hanged”, The Straits Times (22 May 2004). Also PP v Soosainathan s/o Dass Saminathan  SGHC 153 and Soosainathan s/o Dass Saminathan v PP  SGCA 45.
12 “A mango bait, a missing girl and a murder” (2007) Lim Chin Chin et al, American Academy of Forensic Sciences (“AAFS”) 59th Annual Meeting, USA, abstract no. B81 (poster presentation). Also Took Leng How v PP  2 SLR 70;  SGCA 3; “It is time to activate the forensic scientist” (2009) Lim Chin Chin, SAL Forensics Conference, 8-9 Oct (oral presentation and paper).
13 Supra (note 2 above); “Duo attacked customer with pepper spray, sent 20 to hospital” (2009); available at: <http://thecourtroom.stomp.com.sg/courtroom/cases/duo-attacked-customer-with-pepper-spray-sent-20-to-hospital>.
14 “Ethylene Glycol in Medical Devices and Related Products” (2010) Chia Poh Ling et al, AAFS, 62nd Annual Meeting, USA, (poster presentation).
15 “The Use of Corrosive Substances in Crime” (2002), Chia Poh Ling et al, HSA-NUS Joint Scientific Seminar Proceedings (oral presentation). Also “’Acid attack’ baby an active boy now”; available at:< http://yourhealth.asiaone.com/content/acid-attack-baby-active-boy-now>; “Maid charged over baby’s food”, The Straits Times (13 August 2009). Also: <http://news.asiaone.com/News /AsiaOne+News/Crime/Story/A1Story20090813-160971.html>.