Purpose of Fire and Explosion Investigations
Fires and explosions are energetic events with the potential to cause massive destruction to life and property. It is therefore not surprising that criminal justice, public concerns and industrial safety regulations require the forensic investigation of fires and explosions. These investigations typically revolve around two key objectives: (i) ascertain the origin and cause of the fire or explosion; and (ii) determine whether the incident was accidental or a crime has been committed. Hence, the forensic findings must provide sufficient information on the factors and circumstances that led to the fire or explosion, and assist the Court in assessing and apportioning the responsibility and liability of parties involved.
Key Concepts and Terminology
Combustion is a chemical process in which a substance reacts with oxygen and gives off heat. For a fire to occur and propagate, all four components of the fire tetrahedron must be present and interacting together: fuel, oxidisers, heat and uninhibited chemical reaction (Figure 1).
Fires can be prevented or suppressed by controlling or removing any of these components.
Fuel: Any substance – solid, liquid or gaseous – that can undergo combustion. Ignitable liquids refer to liquids that are capable of fuelling a fire.
Oxidiser: Oxidisers cause or contribute to the combustion of other materials by providing oxygen to support the combustion process. The most common oxidiser is atmospheric oxygen.
Heat: The ignition source must be capable of transferring sufficient thermal energy to the fuel to ignite it without a spark or flame.
Chain reaction: For a self-sustaining combustion to occur, the chemical reactions driving the rapid oxidation of the fuel, which produce heat and various chemical by-products need to be uninhibited and continuous.
Similar to fires, an explosion is the product of chemical reactions accompanied by the creation of gases and heat. However, the distinguishing characteristic of an explosion is the rapid rate at which the reaction occurs. The sudden build-up of expanding gas pressure at the origin of the explosion produces a violent physical disruption in the surrounding environment. Explosions are classified by the source or mechanism by which the explosive pressures are produced. They include mechanical, chemical, electrical or nuclear explosions. Some examples of commonly encountered explosions include the rupture of a gas storage cylinder (mechanical), and detonation of an improvised explosive device (chemical).
Common classes of energetic materials include high explosives, low explosives and materials that spontaneously ignite or explode when exposed to an oxidiser. High explosives are used in commercial and military explosives and are usually organic in nature, eg trinitrotoluene (“TNT”). Low explosives are intimate mixtures of oxidiser and fuel; the oxidiser is usually inorganic in nature, eg ammonium or potassium nitrate.
Homemade explosives have been increasingly used over the last two decades for terrorism and other types of crimes. They are clandestinely manufactured from easily sourced and legal precursor materials.
Common Types of Evidence Collected
One of the routine requests by fire investigators in cases involving suspicious fire scenes is the analysis of fire debris for the presence of ignitable liquid residues (“ILRs”). Ignitable liquids are used as accelerants by the arsonist to initiate a fire or increase its rate of growth or spread. Common accelerants include petroleum-based products such as petrol, diesel, kerosene, mineral turpentine and commonly available solvents such as thinners. The broken glass fragments and ILRs of Molotov cocktails (Figure 2) may at times be found at a suspicious fire scene.
For explosion investigations, in addition to the analysis of post-blast debris for the presence of explosive materials, precursors and combustion products, other requests may include types of packaging material and electrical components used, damage analysis to determine the cause of the explosion, and reconstruction of the improvised explosive device ("IED") and event.
Physical evidence commonly collected in fire and explosion scenes include, but are not limited to those listed in Table 1.
Table 1: Types of Evidence Collected From the Scene, Suspect, Vehicles and Premises
In addition to carrying out laboratory analysis, forensic scientists are sometimes activated to the scene to identify and collect relevant physical evidence, determine factors and postulate events leading to the incident. Refer to section – “Local Cases Involving Fire and Explosion” below for examples of how forensic scientists have added value to investigations in local cases.
Laboratory Examination of Evidence
The typical steps in fire debris analysis are depicted in Figure 3.
Preliminary examination: The nature of the debris (eg plastic, paper materials), and presence of other physical evidence (eg fingerprints and DNA on a Molotov cocktail) are identified in order to determine the proper sequence of examination (eg to examine for DNA or ILRs first).
Extraction: This process isolates ILRs from the fire debris. Common extraction techniques include adsorption-desorption and solvent extraction. Each technique has its own advantages and limitations, and the forensic scientist needs to select the most appropriate technique for the type of evidence submitted.
Analysis: Volatile chemical compounds in the extracts are
separated and identified by the analytical equipment.
Interpretation of results: The chemical fingerprints of the
compounds are analysed to determine whether or not an
ignitable liquid was present in the debris.
Report writing and Expert court testimony: The report should describe the evidence and examinations carried out, and state and explain the laboratory results and conclusions with the inclusion of appropriate qualifiers.1
Post-blast debris analysis generally follows procedures similar to fire debris analysis. It starts with a macroscopic and microscopic examination of the evidence for damage and explosive residues, followed by extraction and isolation of the residues. The extracts and isolated particles are then analysed and the data interpreted to identify explosive materials or combustion products present.
Guidelines on good laboratory practices for post-blast debris analysis require that multiple techniques be employed in forensic explosive residue identification.2 Screening techniques, eg colour spot test are used to provide a quick answer on the presence of explosives. These tests are relatively simple and useful for excluding negative samples and narrowing down the number of samples that require further examination. However, screening test results are only presumptive and should not be relied on as the sole method to identify explosives. Confirmatory techniques in the laboratory using analytical equipment are necessary for conclusive identifications.
Challenges in Fire Debris Analysis
The core essential skill of the fire debris scientist lies not so much in the technical ability to operate an equipment as in the scientific expertise to correctly interpret whether a sample’s chemical fingerprint is attributable to ignitable liquid residues.
When ignitable liquids are analysed “neat” (ie in the absence of fire debris), the interpretation of the resulting chemical fingerprint is generally straightforward. Unfortunately, most analyses in real-world cases are performed on “dirty” debris collected from fire scenes. Besides the complications resulting from fire suppression activities, other factors that affect interpretation include:
Weathering and other environmental factors: Weathering (ie the evaporation of ignitable liquids due to heat and turbulence in a fire) and degradation by soil bacteria result in the diminution and change in composition of ignitable liquids.
Interference from materials: Unburnt, burnt and materials which are exposed to heat contain compounds which either have similar chemical fingerprints to those of ignitable liquids or mask their presence, rendering the interpretation of ILRs more difficult.
Incidental liquids: These liquids have a legitimate presence at the scene and are not accelerants deliberately introduced by an arsonist. Their detection is encountered more often today due to the increased use of various petroleum-based household and commercial products. Incidental liquids significantly complicate the interpretation of the results, particularly as modern extraction and analytical techniques have increased in sensitivity.
Hence, an experienced forensic scientist must be able to sieve through the background “noise” and provide an accurate interpretation of the findings with appropriate qualifiers.
Challenges in Post-blast Debris Analysis
High explosives have a wide range of vapour pressures and physical properties. Appropriate techniques must be employed in order to collect, preserve and analyse the evidence. Most high explosives are thermally unstable and consumed during the explosion with little residue left for analysis. Hence, highly sensitive equipment are required to identify their traces. Applying high sensitivity analysis to trace levels requires stringent laboratory measures to prevent contamination, carry-over and false positives. As with fire debris analysis, improved detection limits increase the possibility of detecting background interferences, which can confound interpretation.
Low explosives contain oxidisers such as ammonium nitrate and common fuels such as charcoal, sugar, sulphur and fine metal powders. As ingredients of low explosives have widely differing physical and chemical properties, multiple analytical techniques are required to screen and identify them. Some low explosive ingredients and their combustion products may be found as background materials in household, outdoor and industrial environments, eg sugar, charcoal, nitrates in fertilisers, chlorates in weed-killers, finely divided metals and their oxides or chlorides in industrial materials. In interpreting the findings, the scientist must correctly differentiate compounds in low explosives from those legitimately present in the background.
A forensic scientist well-versed in post-blast debris analysis will take into account the limitations and advantages of various sample clean-up, extraction and analytical techniques, and employ the appropriate methods in the right sequence to identify the explosive materials in post-blast debris. Besides the technical challenges, post-blast debris analyses are usually performed under great time pressures and urgency.
Local Cases Involving Fire and Explosion
In Singapore, most of the fire and explosion investigations are associated with household or industrial accidents. Some of the local cases involving our experts are highlighted below to provide a snap-shot of the types of examinations conducted and the value forensic scientists bring to such cases.
MP set alight by resident3
On 11 January 2009, an elderly resident poured an unknown flammable liquid on former Yio Chu Kang MP Seng Hang Thong and set him alight while he was at a grassroots event at a community club. Remnants of Mr Seng’s burnt clothing were submitted for forensic analysis. The flammable liquid used was identified to be thinner.
Chevron Oronite chemical explosion4
One worker was killed and another injured during an explosion at Chevron Oronite’s lubricant additive factory on Jurong Island in 2005. The incident occurred in a manufacturing unit of the factory as employees uploaded a highly flammable chemical, phosphorus pentasulfide, which was used as an intermediate in the production of lube additives. Forensic scientists were required to determine the nature of the explosion, type of chemicals and their reactions, and how the fire and explosion could have occurred.
Kreuz Shipyard fire and explosion5
On 8 June 2008, a flash fire broke out on the “Rainbow Star” vessel which was in its final stages of repair. A massive explosion subsequently occurred, injuring 15 and killing three shipyard workers. Flammable vapours were found in the gas samples recovered from the vessel. These flammable vapours which likely originated from the spray-painting works conducted in the water ballast tanks, fuelled the fire and subsequent explosion.
Compressed natural gas (“CNG”)-fuelled bus explosion6
A CNG-fuelled bus exploded 15 minutes after 19 full-time national servicemen (“NSF”s) got off the bus on 13 August 2010. The NSFs had alerted the driver to a smell resembling petrol or fuel when they boarded the bus earlier, but the driver dismissed it. Forensic scientists were activated to the scene and the bus was examined to determine the cause of the explosion. The absence of any seat(s) of explosion indicated that an IED was unlikely to have been used. We conducted damage analysis and established that the cause of the fire and explosion was likely due to the leakage of flammable CNG from a joint with a missing ferrule located in the rear left compartment of the bus. The findings provided valuable inputs to the Coroner’s inquiry.
Superbowl underground gas mains explosion7
In 2002, two workmen were hacking tiles in a water valve pit when a series of explosions was triggered in the ground below a restaurant erected on reclaimed land. The area around an underground town gas pipe sustained the most severe damage. Investigations by other agencies concluded that separation of the elbow joint from the spigot of this pipe was the effect and not the cause of the explosion; the cause was attributed to the accumulation and ignition of methane gas from the soil or sewage system. However, our forensic reconstruction revealed that the horizontal section of the pipe (with the elbow) was supported by reinforcement bars and embedded in a concrete slab, whereas the spigot end of the pipe was not adequately supported in the soil. Soil settlement on the reclaimed land for a period of over 25 years had caused a void of 30 cm under the concrete slab, and created stress on the joint between the spigot and elbow of the pipe, resulting in joint misalignment and town gas leakage. The leaked town gas accumulated in the void spaces below the concrete slab of the restaurant and were likely ignited by the sparks made by the electric drills used by the workmen, triggering the series of explosions.
Coroner’s inquiry: Bukit Merah LPG gas explosion8
In 2007, a blast and an ensuing blaze occurred in a one-room rental flat in Jalan Bukit Merah. The force of the explosion blew out the front wall of the unit, and sent the windows flying about 15 metres from the block. The fractured liquefied petroleum gas (“LPG”) tubings were recovered from the scene. Experiments were conducted to study the effect of stress, heat and cuts by sharp objects on the surface characteristics of the tubing.9 No evidence of tampering was observed on the recovered rubber tubing, and the fractures were determined to be due to breakage under stress. As a result of these findings, the regulatory body in Singapore recommended that regular checks and changing of worn tubings be performed by suppliers of LPG cylinders.
1998 Jurong blast10
A rag-and-bone man was killed in an explosion while rummaging through a rubbish bin at a bus-stop in Jurong. Examination of fragments of the rubbish bin indicated that the blast occurred outside the bin. Bits of blackened blue plastic, believed to have held the explosive, were found on his body and at the scene. These plastic pieces were stained with nitroglycerin (“NG”) and ethylene glycol dinitrate (“EGDN”), two high explosives which are common ingredients of dynamite.
Unlike most scenes, the inherent destructiveness of a fire and explosion often compromises much of the evidence left behind. Ignitable liquid and explosive residues are hence found in trace amounts. While the analytical procedure for ignitable liquid residues is generally straightforward and follows standard guidelines, the interpretation of results is made complicated with ironically, the improved sensitivities of analytical equipment, and increased use of petroleum products and other chemicals in the environment. The forensic report should explain clearly the significance of the results obtained, and conclusions reached (with appropriate qualifiers) so that readers of the report are not misled by a positive or negative finding.
Explosive ingredients, their precursors and by-products are either consumed in the blast or present in the environment. The analysis of post-blast debris requires the scientist to take into account the limitations and advantages of various sample clean-up, extraction and analytical techniques, and employ the appropriate methods in the right sequence to identify the explosive materials in post-blast debris. The scientist must be able to correctly differentiate compounds in low explosives from those legitimately present in the background. Incorporating findings from damage analysis allows the reconstruction of events that led to the explosion.
Flammables and explosives form a small subset of the forensic discipline related to unknown chemicals and materials. Unknown chemicals and materials are commonly encountered in criminal and civil cases involving robbery, adulteration, counterfeit drugs, controlled drugs, poisons and homemade explosives. They are also encountered in day-to-day quality control operations to ensure product quality. Identifying the unknown substances may provide important investigative leads such as “What is it used for?”, “Who may have access to such a chemical?”, “What did it originate from?”, etc.
Look out for our next article on “Analysis of Unknown Chemicals and Materials”, which aims to address the science behind the discipline as well as the methodology commonly adopted for examining and reporting on cases involving unknown substances.
► The Forensic Experts Group*
* 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 and innovative forensic scientists, who are combining 75 years of specialised knowledge, unique experience and skillsets to deliver high quality forensic services both locally and overseas. While leading the Criminalistics Laboratory and the Forensic Chemistry & Physics Laboratory at the Health Sciences Authority from the mid-1990s, TFEG’s forensic scientists developed and conducted fire debris and explosives analyses for numerous cases in Singapore.
1 ASTM E1618 Standard Test Method for Ignitable Liquid Residues in Extracts from Fire Debris Samples by Gas Chromatography-Mass Spectrometry.
2 TWGFEX Laboratory Explosion Group, Standards & Protocols Committee, Recommended Guidelines for Forensic Identification of Post-Blast Explosive Residues.
3 “MP set alight by resident, sent to SGH”, Asia One (11 Jan 2009).
4 “Accident Kills Oronite Worker”, Lube Report Vol 5 (10) (9 Mar 2005).
5 “Flash fire in ship docked in Tuas”, Asia One (9 June 2008).
6 “NSFs escape death by 15 minutes”, The New Paper (26 Nov 2012).
7 “Forensic investigation of an underground gas main explosion” (2004), Lim Chin Chin, Michael Tay Ming Kiong, Chia Poh Ling, 56th Annual Meeting of the American Academy of Forensic Sciences, USA, Proceedings Vol 10, Abstract No. C44
8 “Gas explosion victim dies”, Today (6 Aug 2007).
9 “Comparing the effects of stress, cuts by sharp objects and heat on the surface characteristics of the rubber tubing of liquefied petrol gas (“LPG”) cylinder” (2009), Chia Poh Ling. Kuah Kim Lian. Lim Chin Chin and Michael Tay Ming Kiong. 6th Forensic International Network of Explosives Investigation (FINEX) conference.
10 “1998 Jurong blast still a mystery”, The Straits Times (28 Aug 2003).
* 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 75 years of specialised knowledge, unique experience and skillsets to deliver high quality forensic services both locally and overseas. While heading the Forensic Chemistry & Physics Laboratory at the Health Sciences Authority, TFEG’s senior consultants developed and instituted bloodstain pattern analysis as a distinctive forensic discipline in Singapore, successfully applying the expertise to many high profile local cases since 2005.