Arson, Ballistics, & Tools— Forensics

Arson evidence can be difficult to investigate because evidence may be…

• burned

• charred

• wet (water damage)

fire investigators detect & identify:

• chemical materials

• igniters

Fire & Investigation

• fire is a product of oxygen combining heat & light (a flame)

• Investigators look for a V-pattern of char/soot that leads them to the fire’s origin

Accelerants are detected by…

• portable hydrocarbon detectors

• accelerants sniffing dogs

• identifiable pour patterns

Fire evidence should be…

packaged immediately in airtight containers to avoid evaporation

Reasons for arson crime:

• insurance fraud (#1 reason)

• crime concealment

• pyromania

• revenge

classification of arson

• 1st degree: burning an occupied building

• 2nd degree: burning an unoccupied building

• 3rd degree: burning an abandoned building/ area

firearm

weapon capable of firing a projectile using a confined explosive

ballistics can tell investigators…

• firearm type

• caliber of bullet

• # of bullets fired

• angle of impact from shooter to victim

• if firearm was used in previous crime

Cartridge

• Bullet

• Casing

• Gunpowder

• Rim

• Primer

Handgun (firearm classification)

• pistols & revolvers

• fire with 1 hand

• can be semi-automatic (1 bullet per trigger) or fully automatic

Long gun (firearm classification)

• rifles & shotguns

• require 2 hands

• rifles fire bullets

• shotguns fire pellets (multiple projectiles) or a slug (single projectile)

How a firearm works

1. trigger pulled

2. firing pin hits base of cartridge

3. ignites powder inside bullet

4. pressure pushes bullet from casing into barrel of firearm

5. bullet follows land & groove patterns of barrel & spirals out of barrel

Identification

Every weapon leaves its own unique, reproducible marking on a bullet & cartridge (regardless of gun type, style, manufacturer, etc.)

• useful in convicting criminals to crimes involving guns

Striations

stratch marks on bullet from rifiling lines inside the barrel

lands & grooves

lands & grooves in barrel produce bullets with specific patterns that make the bullet spin upon firing

Breech markings

marks left on spent cartridge casings

• explosive force pushes bullet forward

• Newton’s 3rd law sends casing backward against breechblock

Fire pin markings

leave unique stamping on cartridge primer/rim

Gunshot Residue (GSR)

• traces of smoke & unburned powder are released as the bullet is fired

• GSR contains nitrates that can stick to the person holding the firearm or anyone nearby

• that amount of GSR decreases as the distance between firearm & victim increases

• can be removed by washing, but chemical tests can detect residual residue

Bullet Trajectory

can be calculated to find the location of a shooter

• you need 2 reference points along the flight path of the projectile

  • ex) bullet holes, wounds, spent cartridge casings