Shooting Scene Investigation: Shooter Evidence and Trajectory Analysis - lect 9
Shooter Evidence
When a shooter is inexperienced or in a stressful situation, they may unintentionally acquire more evidence on themselves.
Incorrect Weapon Handling
Inexperienced (or other) shooters may hold weapons incorrectly or too tightly, leading to specific effects.
Slide Bite
Occurs when holding a self-loading pistol too high on the grip.
The slide moves backward during firing, potentially abrading the hand, specifically the webbing between the thumb and forefinger.
The bottom of the slide is often angled and sharp, causing scratches or bleeding.
Cuts between the forefinger and thumb are uncommon, making this a potential indicator of gun use, though not definitive proof of involvement in the investigated shooting.
Hammer Bite
Related to self-loading pistols with external hammers. The slide pushes the hammer back, potentially imprinting or abrading the skin between the forefinger and thumb if the grip is too high.
Hammers are often textured for grip, which can cause more abrasion and act as a source of DNA due to rubbing against the skin.
Textured surfaces on firearms (pistol grips, hammer grips) are good sources of DNA.
Firearms experts need to collaborate with other forensic services, such as DNA labs.
Incorrect Handling of Larger Weapon Systems
Inexperienced shooters may hold shotguns incorrectly, leading to injuries.
Holding the shotgun against the forearm rather than the shoulder, where there is more mass, can cause significant bruising due to recoil.
Bruising is internal bleeding and in an unusual place can be an indicator of firearms use.
Grip Markings
Holding weapons tightly can cause textured grips to imprint markings on the hand.
Scope Bite
Inexperienced rifle shooters may place their eye too close to the optical scope.
Recoil can cause the scope to hit the shooter between the eyes or on the nose, leading to injury.
Experienced shooters keep a few inches distance from the scope to avoid injury and for clearer image.
Scene Documentation and Trajectory Analysis
Document the scene in three dimensions to account for bullet trajectory, including the horizontal and vertical angles.
Create scale plans and sketches.
Digital models can be useful for visualizing the scene in court.
Two-dimensional views include side-on and top-down perspectives.
Determining Shooter Position
Consider multiple pieces of evidence to understand where the shooter was within the scene.
Eyewitness Accounts
Eyewitness accounts can be useful, but are often unreliable due to fight or flight response.
Adrenaline and the traumatic nature of a shooting can affect recall.
Multiple consistent eyewitness accounts, if available, can provide a starting point for investigation.
Corroborate with scientific evidence.
Bullet Trajectories and Velocities
For internal scenes, assume a straight bullet trajectory due to short distance and high velocity.
For external scenes, consider gravity and distance when determining bullet trajectories.
Gunshot Residue (GSR)
GSR can be found near the shooting location, originating mostly from the gun barrel, and some from the chamber and the breech area.
The presence of GSR can confirm a shooting took place in the area.
In long-range shootings, GSR at the target end is mainly from bullet wipe.
Gunshot residue can be a mixture if different types of ammunition have been fired/mixed up.
Cartridge Ejection Patterns
Semi-automatic weapons eject cartridges.
Ejection patterns depend on the weapon, ammunition, and how the shooter holds the firearm.
The direction of ejection will change based on sneaky vs traditional stance-fired shots.
Experimental firings with the suspect weapon can help determine typical ejection patterns.
Trajectory Analysis Techniques
Start analysis at the bullet hole and work backwards to determine the shooter's location.
Stringing Kits
Use physical strings to map trajectories.
Easy for a jury to understand.
Strings must be properly tensed and anchored.
Can be expensive.
Angle Gauges
Measure the angle of the bullet trajectory relative to the horizontal.
Digital versions are now most common.
Trajectory Rods
Can be pieced together and attached to lasers.
Protractors
Help measure angles.
Plywood
Used to analyze shotgun pellet patterns.
Firing shotguns at plywood from varying distances can help determine the distance of the shooter from the target.
String Trajectory Lines
Inexpensive if using suitable materials.
Anchoring is important by tethering the string to a bullet hole.
Various ways to understand bullet trajectory such as a centering cone.
Different colors of strings can be used for different shots/ weapons.
Laser Trajectories
Lasers are useful to replace strings.
Used as attachment to trajectory rods.
Useful for non-penetrating impacts where a centering cone can't be adhered to.
Centering Cones
Inserted into a bullet hole to indicate the trajectory path.
The trajectory rod can then be pushed into the cone to simulate the trajectory.
Angle Gauges and Protractors
Angle gauge placed on top of the rod indicates approximate angle.
Zero base protractors are used in conjunction with a flat line to measure angles.
Plumb Line
Used to create a vertical line in scenes without vertical or flat lines.
Bullet Hole Analysis for Angle Prediction
A crude calculation, but provides a reasonable approximation.
A circular bullet hole suggests a 90-degree impact.
Oval bullet holes indicate an angled impact.
Draw an oval of best fit inside the bullet hole.
Angle of impact
Creating a right-angle triangle to find measurements.
Trajectory in Vertical and Horizontal Planes
Vertical plane: up/down angle.
Horizontal plane: left/right angle.
Side-on view: used to determine the trajectory in the up-down direction.
Top-down view: used to determine trajectory in the left-right direction.
Ricochet and Deflection
Ricochet: bullet bouncing off a surface in the up/down plane.
Deflection: bullet changing direction in the left/right plane.
Angle of incidence: the angle at which the bullet strikes a surface.
Angle of ricochet: the angle at which the bullet bounces off a surface.
The angle of incidence and the angle of ricochet is not the same most of the time.
Experiments are often needed to recreate ricochet marks.
Angle of deflection: lateral movement in the left/right plane following the twist of the bullet.
Shooter Evidence
When a shooter is inexperienced or in a stressful situation, they may unintentionally acquire more evidence on themselves. This can include physical injuries from improper weapon handling, DNA evidence, or unintentional transfer of trace materials.
Incorrect Weapon Handling
Inexperienced (or other) shooters may hold weapons incorrectly or too tightly, leading to specific effects such as slide bite, hammer bite, scope bite, or bruising from larger weapon systems.
Slide Bite
Occurs when holding a self-loading pistol too high on the grip.
The slide moves backward during firing, potentially abrading the hand, specifically the webbing between the thumb and forefinger.
The bottom of the slide is often angled and sharp, causing scratches or bleeding. Medical attention may compound the evidence.
Cuts between the forefinger and thumb are uncommon, making this a potential indicator of gun use. However, it is not definitive proof of involvement in the investigated shooting. Other potential causes should be considered, documenting any pre-existing injuries.
Hammer Bite
Related to self-loading pistols with external hammers. The slide pushes the hammer back, potentially imprinting or abrading the skin between the forefinger and thumb if the grip is too high. Prolonged or repeated firing can exacerbate this.
Hammers are often textured for grip, which can cause more abrasion and act as a source of DNA due to rubbing against the skin. Sweat and skin cells can be deposited on the hammer.
Textured surfaces on firearms (pistol grips, hammer grips) are good sources of DNA. Proper collection methods must be employed by forensic teams to avoid contamination.
Firearms experts need to collaborate with other forensic services, such as DNA labs, to ensure comprehensive analysis.
Incorrect Handling of Larger Weapon Systems
Inexperienced shooters may hold shotguns incorrectly, leading to injuries. Lack of familiarity with recoil can lead to improper shouldering.
Holding the shotgun against the forearm rather than the shoulder, where there is more mass, can cause significant bruising due to recoil. The extent of bruising can depend on the gauge and load of the shotgun.
Bruising is internal bleeding and in an unusual place can be an indicator of firearms use. Documenting the size, shape, and color of the bruise can provide additional information about the potential source of the injury.
Grip Markings
Holding weapons tightly can cause textured grips to imprint markings on the hand. These markings can sometimes be photographed and compared to the grip surface for potential identification.
Scope Bite
Inexperienced rifle shooters may place their eye too close to the optical scope. Often a reflexive action to improve the sight picture.
Recoil can cause the scope to hit the shooter between the eyes or on the nose, leading to injury. The severity can range from minor bruising to lacerations.
Experienced shooters keep a few inches distance from the scope to avoid injury and for clearer image. Documenting the distance maintained by experienced shooters provides a baseline for comparison.
Scene Documentation and Trajectory Analysis
Document the scene in three dimensions to account for bullet trajectory, including the horizontal and vertical angles. Precise measurements are critical.
Create scale plans and sketches. Include reference points and landmarks.
Digital models can be useful for visualizing the scene in court. They provide an interactive representation.
Two-dimensional views include side-on and top-down perspectives. These views simplify the spatial relationships for presentation purposes.
Determining Shooter Position
Consider multiple pieces of evidence to understand where the shooter was within the scene. No single piece of evidence is definitive on its own.
Eyewitness Accounts
Eyewitness accounts can be useful, but are often unreliable due to fight or flight response. Stress and trauma alter perception.
Adrenaline and the traumatic nature of a shooting can affect recall. Memories can be distorted or incomplete.
Multiple consistent eyewitness accounts, if available, can provide a starting point for investigation. Consistency strengthens the reliability of the accounts.
Corroborate with scientific evidence to validate eyewitness accounts.
Bullet Trajectories and Velocities
For internal scenes, assume a straight bullet trajectory due to short distance and high velocity. Air resistance has negligible effect.
For external scenes, consider gravity and distance when determining bullet trajectories. Environmental factors such as wind can also play a role.
Gunshot Residue (GSR)
GSR can be found near the shooting location, originating mostly from the gun barrel, and some from the chamber and the breech area. The distribution pattern can indicate the shooter's proximity.
The presence of GSR can confirm a shooting took place in the area. However, it does not definitively identify the shooter.
In long-range shootings, GSR at the target end is mainly from bullet wipe. This can provide information about the bullet's path upon impact.
Gunshot residue can be a mixture if different types of ammunition have been fired/mixed up. Analyzing the composition of GSR can provide insights into the type of ammunition used.
Cartridge Ejection Patterns
Semi-automatic weapons eject cartridges. The ejection mechanism is designed for consistent expulsion.
Ejection patterns depend on the weapon, ammunition, and how the shooter holds the firearm. Variations can occur due to malfunctions or modifications.
The direction of ejection will change based on sneaky vs traditional stance-fired shots. This helps to understand the shooter's body position.
Experimental firings with the suspect weapon can help determine typical ejection patterns. Documenting these patterns provides a reference for comparison.
Trajectory Analysis Techniques
Start analysis at the bullet hole and work backwards to determine the shooter's location. This approach relies on reconstructing the path of the bullet.
Stringing Kits
Use physical strings to map trajectories. Strings represent the path of the bullet.
Easy for a jury to understand. The physical representation aids comprehensibility.
Strings must be properly tensed and anchored. Loose strings can distort the trajectory.
Can be expensive due to the cost of materials and equipment.
Angle Gauges
Measure the angle of the bullet trajectory relative to the horizontal. This measurement is critical for determining the vertical angle.
Digital versions are now most common. Digital gauges provide precise readings.
Trajectory Rods
Can be pieced together and attached to lasers. Rods provide a straight line for visualizing the trajectory.
Protractors
Help measure angles. Protractors are used to determine the precise angles of impact and trajectory.
Plywood
Used to analyze shotgun pellet patterns. The pattern on the plywood shows the spread of the pellets.
Firing shotguns at plywood from varying distances can help determine the distance of the shooter from the target. The density and spread of the pellets change with distance.
String Trajectory Lines
Inexpensive if using suitable materials. Cost-effective for initial assessments.
Anchoring is important by tethering the string to a bullet hole. Secure anchoring ensures accuracy.
Various ways to understand bullet trajectory such as a centering cone. Centering cones help align the string with the bullet path.
Different colors of strings can be used for different shots/ weapons. Color-coding helps differentiate between multiple trajectories.
Laser Trajectories
Lasers are useful to replace strings. Lasers provide a clear, straight line.
Used as attachment to trajectory rods. Lasers attached to rods enhance accuracy.
Useful for non-penetrating impacts where a centering cone can't be adhered to. Lasers allow for trajectory analysis without physical penetration.
Centering Cones
Inserted into a bullet hole to indicate the trajectory path. Cones align with the bullet's entry point.
The trajectory rod can then be pushed into the cone to simulate the trajectory. Rods extend the line of the bullet path.
Angle Gauges and Protractors
Angle gauge placed on top of the rod indicates approximate angle. Gauges measure the vertical angle of the trajectory.
Zero base protractors are used in conjunction with a flat line to measure angles. Protractors ensure accurate measurement.
Plumb Line
Used to create a vertical line in scenes without vertical or flat lines. Establishes a reference for vertical measurements.
Bullet Hole Analysis for Angle Prediction
A crude calculation, but provides a reasonable approximation. Preliminary assessment of impact angle.
A circular bullet hole suggests a 90-degree impact. Indicates a perpendicular hit.
Oval bullet holes indicate an angled impact. Suggests a non-perpendicular hit.
Draw an oval of best fit inside the bullet hole. Helps determine the major and minor axes.
Angle of impact
Creating a right-angle triangle to find measurements. This helps in trigonometric calculations.
Trajectory in Vertical and Horizontal Planes
Vertical plane: up/down angle. Determines the elevation of the shot.
Horizontal plane: left/right angle. Determines the direction of the shot.
Side-on view: used to determine the trajectory in the up-down direction. Illustrates vertical angle.
Top-down view: used to determine trajectory in the left-right direction. Illustrates horizontal angle.
Ricochet and Deflection
Ricochet: bullet bouncing off a surface in the up/down plane. Bounce is in the vertical plane.
Deflection: bullet changing direction in the left/right plane. Change in direction is in the horizontal plane.
Angle of incidence: the angle at which the bullet strikes a surface. Angle of arrival.
Angle of ricochet: the angle at which the bullet bounces off a surface. Angle of departure.
The angle of incidence and the angle of ricochet is not the same most of the time. The angles differ due to surface properties.
Experiments are often needed to recreate ricochet marks. Controlled experiments help understand the factors influencing ricochet.
Angle of deflection: lateral movement in the left/right plane following the twist of the bullet. Deflection is influenced by bullet spin.