Determining the type or types of weapons used in a murder is done by looking for the taletell evidence left behind by its use.
The Gun
The evidence collected at the scene of a shooting is numerous and varied:
- Gunshot residue
- Cartridges
- Bullets
- The body (entrance and exit wounds)
Gunshot residue is soot left behind by the burning of the accelerant used to propel the projectile from the bullet. This residue will be present on the hands and clothes of the shooter or the victim if the shot was made at point blank range.
The factors that can affect the amount and distribution of gunshot residue (GSR) on skin and clothing include:
- firing distance
- length and diameter of the firearm barrel
- characteristics of the gunpowder
- angle between the firearm barrel and target
- characteristics of the cartridge
- the environment (moisture, wind, heat)
- type of clothing
- intermediate targets
- characteristics of the target (tissue type, putrefaction, blood marks)
Shelling Out Evidence: NIST Ballistic Standard Helps Tie Guns to Criminals
National Institute of Standards and Technology (YouTube)
The cartridge is the cylindrical portion of a round that remains after a gun is fired while the bullet is the projectile.
Cartridges and bullets are regularly analyzed for both firing pin impressions as well as rifling marks that will help identify the gun that was used. Using a specialized microscope called an Integrated Ballistic Identification System (IBIS), lab technicians acquire digital images of three markings, or "signatures," impressed on the cartridge case by the gun that fired it. These signatures—the firing pin impression, the breech face impression and the ejector mark—are unique when fired from a specific firearm and can serve as "fingerprints" for that gun once the digital images are entered into a national database known as the National Integrated Ballistic Information Network (NIBIN).
The body of the victim provides a great deal of ballistic evidence. One of the most important determinations of the forensic pathologist is the range of fire. Gunshot wounds are typically classified as:
- Contact
- Intermediate range
- Distant range
Contact wounds characteristically have soot on the outside of the skin, and muzzle imprint, or laceration of the skin from effects of gases.
Intermediate, or close-range, wounds may show a wide zone of powder stippling, but lack a muzzle imprint and laceration. The area of powder stippling will depend upon the distance from the muzzle.
Distant range wounds are lacking powder stippling and usually exhibit a hole roughly the caliber of the projectile fired.
The Knife
Knife Wounds
Joseph Giacalone (YouTube)
Knife wounds have 3 catergories:
Slashing wounds are identified as those that are horizontal across the victim. Incisions are precision up and down cuts and punctures are inserted thrusts of the knife. Each type of knifing motion creates a distinctive wound that can tell a story about the manner of the attack that took place.
Investigators and more specifically pathologists identify knives by the track marks the knife leaves when it is thrust into the body. Knives also leave trace metals in the wounds that can often be matched metallurgically (by metal chemistry) to the weapon used. Even if an exact match cannot be made, it is always possible to determine whether a particular knife is compatible with a distinct stab-wound.
Wound analysis can determine the location, size, borders and pattern of the injury; the orientation on the skin's surface; the strength and height of the perpetrator; and whether the wounds are consistent with defensive motions.
The Bat
Wounds caused by blunt force trauma may not leave the same degree of evidence as the gunshot or knife wound, but a skilled investigator can still determine many aspects of the trauma using the bruising, abrasions and fractures left behind.
Signs of Blunt Force Trauma
Bruising: Bruising occurs when blood vessels at or near the surface of the skin rupture or when blood seeps from deep tissue to low lying spots in the body due to gravity. Trauma in the form of extreme pressure from the impact of a bat or similar weapon causes blood cells to rupture forming bruises.
Abrasions: Abrasions refer to any number of cuts, scrapes, friction burns or grazing on the victim that indicates that the victim was beaten. Specific patterns within these abrasions can be used to determine the type of weapon used and the environment where the beating took place.
Lacerations: This is the tearing of tissue underneath the skin. An individual may be beaten severely or have sustained a severe bump against a stationary object and underneath the skin (subcutaneous) there may be severe damage caused to tissue and organs. Visual examinations do not always show this to be case and if the victim has died an autopsy will most certainly be carried out.
The Ligature
When ligature strangulation is suspected the investigation starts with a visual inspection of the victim's neck. A ligature pattern is generally going to be located about the level of the victim's larynx or lower. In homicides as opposed to hangings, the line of the ligature pattern tends to be more horizontal across the front of the neck as the ligature was pulled from behind. Ligature marks from hanging tend to have a higher point within the mark indicating the location of suspension.
Another key indicator of strangulation is the presence of petechiae on the skin and eyes. Petechial hemorrhages occur when blood flow is cut off in the veins and blood pressure increases rapidly due to the pooling blood. This damage produces minute points of bleeding which can be visible as pinpoint hemorrhages in the softer tissues like the eyes. These tiny points of bleeding are called petechiae (pi-tee-kee-uh).
In addition to ligature marks and petechiae, the victim may also show bleeding from the mouth, nose or ears. While this is possibly present it should not be the only indicator for asphyxia as many other types of trauma can cause similar symptoms.
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