SFS4. Students will evaluate the role of ballistics, tool marks and evidence of arson - PowerPoint Presentation

SFS4. Students will evaluate the role of ballistics, tool marks and evidence of arson in forensic investigation. D. Evaluate possible indicators of arson and criminal bombing. FORENSIC ASPECTS of Arson and Explosives

Introduction Arson and explosions often present complex and difficult circumstances to investigate becausethe perpetrator has, typically, thoroughly planned the actthe perpetrator is not present during the actthe destruction is so extensive

INTRODUCTIONThe criminalist’s function is rather limited to…detecting and identifying relevant chemical materials collected at the scenereconstructing and identifying igniters or detonating mechanisms

ARSON

DEFINITIONSArson is the conflagration, or uncontrolled burning, of any real property without consent or with unlawful intentArson is prosecuted with attention to degree of severity in the alleged offenseThe analysis of fire-related incidents is called fire investigation

The Chemistry of FireChemically, fire is a type of oxidation, which is the combination of oxygen with other substances to produce new substances.CH4 + 2O2  CO 2 + 2H2OOxidation reactions typically result in large energy release, mainly as heat and/or light.

The FIRE TRIANGLE (or TETRAHEDRON)Fuel Oxygen in sufficient quantity to combine with the fuel.Sufficient heat to initiate the combustion and sustain the reaction.

The Chemistry of FireThe autoignition temperature of a fuel is the minimum temperature needed to spontaneously ignite it without a spark or flame.White Phosphorus – 34°CGasoline – 280°CHydrogen – 530°C

The Chemistry of FireThe heat evolved when a substance burns is known as heat of combustion.Wood – 21.7 kJ/kgGasoline – 47.3 kJ/kgHydrogen – 141.8 kJ/kg

The Chemistry of FireAn additional factor, besides the liberation of energy, needed to explain fire isthe rate or speed at which the oxidation reaction takes place.A fuel will achieve a reaction rate with oxygen sufficient to produce a flame only when it is in the gaseous state .

The Chemistry of FireA liquid burns when the temperature is high enough to vaporize it – at the flash point.A solid must be hot enough to decompose into gaseous products – pyrolysis.

The Chemistry of FireGlowing combustion or smoldering is burning at the fuel-air interfaceCigarette embers, charcoalSpontaneous combustion , which is rare, is the result of a natural heat-producing process in poorly ventilated containers or areas.

The Fire SceneThe arson investigator needs to begin examining a fire scene for signs of arson as soon as the fire has been extinguished.The necessity to begin an immediate investigation even takes precedence over the requirement to obtain a search warrant.

The Fire SceneExperience shows that most arsons are started with petroleum-based accelerants. The search of the fire scene must focus on finding the fire’s origin, which may be most productive in any search for an accelerant or ignition device.

The Fire Scene Some telltale signs of arson include…evidence of separate and unconnected firesuse of “streamers” to spread the fire from one area to anotherevidence of severe burning found on the floor as opposed to the ceiling of a structure, due to a flammable liquid .

The Fire Scene Normally, a fire has a tendency to move in an upward direction, so the probable origin will most likely be the lowest point showing the most intense characteristics of burning.Fortunately, combustible liquids are rarely entirely consumed during a fire.

EVIDENCE CollectionAt the suspected point of origin of a fire, ash and soot, along with porous materials which may contain excess accelerant, should be collected and stored in airtight containers, leaving an airspace to remove samples.It is important that a sampling of similar but uncontaminated control specimens be collected.

EVIDENCE CollectionTraces of flammable liquid residues may be located with a vapor detector (sniffer).Trained dogs can detect 1:1000000000 (1 part per billion)

EVIDENCE CollectionA search for igniters must also be conducted.MatchesAn electrical sparking deviceParts of a “Molotov cocktail”

EVIDENCE ANALYSISIn the laboratory, the gas chromatograph is the most sensitive and reliable instrument for detecting and characterizing flammable residues. The GC separates the chemical components and produces a chromatographic pattern characteristic of a particular fuel.

EVIDENCE ANALYSISBy comparing select gas chromatographic peaks recovered from fire-scene debris to known flammable liquids, a forensic analyst may be able to identify the accelerant used to initiate the fire.

GC/ms ANALYSIS

GC Analysis Gas chromatograph of vapor from a genuine gasoline sample. Gas chromatograph of vapor from debris recovered at a fire site.

EXPLOSIVES

DEFINITIONSAn explosive is a reactive substance that can produce an explosion.An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases.

ExplosionsA sudden buildup of gas pressure produces a shock wave.

Types of ExplosivesIf the shock wave is subsonic, the oxidation reaction is called a deflagrationThe source explosive material is said to be a “low explosive”If the shock wave is supersonic, the oxidation reaction is called a detonation The source explosive material is said to be a “high explosive”

“low explosives”The most widely used are black powder and smokeless powder. Black powder is a mixture of potassium or sodium nitrate, charcoal, and sulfur.Because it burns slowly, it is often used as a fuse to a high explosiveSmokeless powder consists of nitrocellulose (plant fiber + H 2SO4/HNO3)More stable, yet powerful, than BP

“high explosives”They can be characterized by their sensitivity toImpactHeatFrictionPrimary explosives are highly sensitiveSecondary explosives are fairly insensitiveTertiary explosives are least sensitive

“high explosives”Primary explosives provide the major ingredients found in blasting caps or primers used to detonate other explosives.Nitroglycerin (in its pure form) 30

“high explosives”Secondary explosives will normally burn rather than detonate if ignited in small quantities in the open air.Dynamite – composed of nitroglycerin mixed with diatomaceous earthTrinitrotoluene (TNT)

“high explosives”Research Department Explosive (RDX)Composition C-4 – plasticized RDX

“high explosives”Tertiary explosives, also called blasting agents, are so insensitive to shock that ignition by both primary and secondary explosives is required.Primarily used in large-scale mining and construction and terrorismAmmonium Nitrate/Fuel Oil (AMFO)

EVIDENCE CollectionThe entire explosion site must be systematically searched with great care given to recovering any trace of a detonating mechanism or any other item foreign to the explosion site.

EVIDENCE CollectionOften a crater is located at the origin and loose soil and other debris must be preserved from its interior for laboratory analysis.

EVIDENCE CollectionDebris and articles collected from different areas are to be packaged in separate air-tight containers.It has been demonstrated that some explosives can diffuse through plastic and contaminate nearby containers.

Evidence analysisTypically, in the laboratory, debris collected at explosion scenes will be examined microscopically for unconsumed explosive particles. Recovered debris may also be thoroughly rinsed with organic solvents and analyzed. 37

EVIDENCE AnalysisIon Mobility Spectrometry can be used to preliminarily identify an explosive residueUsed in airports and by the US Army in Improvised Explosive Device (IED) examination. 38

Evidence analysisConfirmatory identification tests may be performed on unexploded materials by either infrared spectrophotometry or X-ray diffraction. 39