Notes #2 Blood and Blood Spatter Analysis Learning Goal: How does blood spatter evidence help crime scene investigators reconstruct a crime scene?
Blood in Crime Scene Investigations Blood as Class Evidence - Blood typing can be used as class evidence because more than one person has the same blood type. - This can narrow the suspect list by excluding people with different blood types Blood as Individual Evidence - White blood cells contain DNA, so it is possible to determine the origin of the blood by doing DNA profiling - This can link a blood sample to a specific individual
Blood in Crime Scene Investigations Blood to Reconstruct a Crime Scene - Blood Spatter Evidence can be used to determine where the blood originated from and the angle it fell from - This can corroborate witness stories and help develop a clearer picture of the crime
Bloodstain Pattern Analysis The examination of the shapes, locations, and distribution patterns of bloodstains, in order to provide an interpretation of the physical events which gave rise to the origin of the bloodstains. - The determinations made from bloodstain patterns at the scene or from the clothing of principals (suspects) in a case can be used to: 1. Confirm or refute assumptions concerning events and their sequence: - Position of victim. (standing, sitting, lying) - Evidence of a struggle. (blood smears, blood trails)
Bloodstain Pattern Analysis The examination of the shapes, locations, and distribution patterns of bloodstains, in order to provide an interpretation of the physical events which gave rise to the origin of the bloodstains. - The determinations made from bloodstain patterns at the scene or from the clothing of principals (suspects) in a case can be used to: 2. Confirm or refute statements made by principals in the case: - Are stain patterns on a suspect's clothing consistent with his reported actions? - Are stain patterns on a victim or at a scene consistent with accounts given by witnesses or the suspect?
Properties of Blood Surface Tension - The elastic like property of the surface of the liquid that makes it tend to contract, caused by the forces of attraction between the molecules of the liquid. The cohesive forces tend to resist penetration and separation - This is an important property to understand when look at and analyzing blood spatter
Categories of Bloodstains Passive Bloodstain Transfer Bloodstain Projectile Bloodstain
Categories of Bloodstains Passive Bloodstains - Drops are created or formed by the force of gravity acting alone - Sub categories of study include: - Drops - Drip patterns - Pools - Clots
Categories of Bloodstains Passive Bloodstains - Target Surface Texture - There a variety of different surfaces that blood lands on, such as carpet, tile, wood, wallpaper, clothing - The type of surface the blood lands on affects the splatter, size and appearance of blood drops - Blood drops that strike a hard surface like glass will have little or no distortion around the edge. - Blood that lands on linoleum will have distortion because of the uneven surface of the floor tile - Wood and concrete have even more distortion and satellite spatter
Categories of Bloodstains Transfer Bloodstains - Created when a wet, bloody surface comes in contact with a secondary surface - An image that is recognizable as all or a portion of the original surface may be observed in the pattern (shoe print, fingerprint, bloody hand) - Subcategories of transfer bloodstains are: contact bleeding, swipe or smear, wipe, smudge
Categories of Bloodstains Projectile Bloodstains - Created when an exposed blood source is subjected to an action or force greater than the force of gravity (internally or externally produced) - The size shape and number of resulting stains will depend on the amount of force used to strike the blood source
Categories of Bloodstains Projectile Bloodstains - Projected Bloodstain Subcategories - Arterial Spurt/Gush – bloodstain pattern resulting from blood exiting the body under pressure from a broken artery
Categories of Bloodstains Projectile Bloodstains - Projected Bloodstain Subcategories - Cast-off stains – Blood released or thrown from a blood bearing object in motion
Categories of Bloodstains Projectile Bloodstains - Projected Bloodstain Subcategories - Impact Spatter – bloodstain patterns created when a blood source receives a blow or force resulting in the random dispersion of smaller drops of blood. - This category is further subdivided into: - Low velocity: gravitational pull up to 5 feet/sec relatively large stains 4 mm in size and greater - Medium velocity: force of 5 to 25 feet/sec, stain size 1 -4 mm - High Velocity: force of 100 feet/sec and greater, stain size 1 mm and smaller, mist like appearance
Direction of Bloodstains - A drop of blood hitting a surface perpendicular (90 degrees) will result in a circular stain (the length and width of the stain are equal) - Blood that strikes a surface at an angle less that 90 degrees will be elongated or have a teardrop shape - Directionality is usually obvious as the pointed end of the stain (tail) will always point in the direction of travel
Impact Angle Determination - Angle of Impact is the acute angle formed between the direction of the blood drop and the plane of the surface it strikes - A drop of blood at a 90° angle gives a nearly circular stain. As the angle decreases, the stain elongates - By accurately measuring the length and width of a bloodstain, the impact angle can be calculated using the SIN formula below
Blood Spatter Forensic Files: Garden of Evil As you watch, focus on the role blood spatter played in solving this case!
Blood Spatter Forensic Files: Where the Blood Drops As you watch, focus on the role blood spatter played in solving this case!