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Bloodstain Pattern Analysis

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Bloodstain Pattern Analysis
Bloodstain Pattern Analysis (BPA) is the examination of the shapes, locations, and distribution of patterns of bloodstains, in order to provide an interpretation of the physical events that gave rise to their origin. When BPA is done properly, many things can be discerned including distance from the blood source to the target, direction of travel and impact angles, nature of the force used to cause the bloodshed, sequencing of multiple bloodshed events, and interpretation of contact or transfer patterns. When it has been properly documented, BPA can also be used to confirm of refute the position of a victim, witness, suspect, or defendant, determine if there is evidence of a struggle, or if the assault was “one sided”, and confirm or refute statements made by the principles in a case, as in telling if the patterns match accounts given by victims, witnesses or defendants.
Blood has specific characteristics that are exhibited in the way it behaves when separated from the blood source. Blood is always spherical in shape until it impacts a particular surface. This is due to the fact that blood naturally pulls in on itself, both horizontally and vertically, due to its specific surface tension.
Spatters of blood occur when blood is acted upon by force and travels through the air before landing on a target surface. Transfers occur when a blood source comes in direct contact with the target surface area. Depending on the surface texture of the area, bloodstains will show a certain type of spatter pattern. If blood lands on a smooth surface, it will make a more uniform or regular circular shape and if blood lands on a rough surface, it will make an irregular shaped stain with rough or jagged edges.
There are three different types of spatter pattern types. They are low, medium, and high force impact. Low Force Impact Spatters (LFIS) is also known as passive drops. Characteristics of low force are as follows: * Blood that falls at the speed or force of normal gravity * These spatters usually fall from an open wound or from a surface that is saturated with blood * The majority of LFIS are large, circular spatters with diameters of 4mm or more * LFIS will increase in size as the distance fallen increases, however the size of the spatters will remain a constant after approximately 4 feet
Characteristics of Medium Force Impact Spatters (MFIS) are as follows: * Produced with more energy or force than gravity * The force of the impact causes the blood to break into smaller sized spatters relative to the amount of force applied * This type of spatter is usually seen in blunt force, stabbings, and secondary spatters * Produced when the majority of larger drops of blood are broken into smaller spatters with diameters of 2-4mm * The force associated with this type of spatter is greater than 25 fps (feet per second)
Characteristics of High Force Impact Spatters (HFIS) are as follows: * Impact spatter that measures less than 2mm in diameter * The force necessary to produce this size spatter is greater than 100 fps * This type of spatter is usually associated with gunshots, explosions, and high speed collisions * HFIS takes on a “mist like” appearance
The angle of impact is another useful way to help determine the events that lead to the spatter. The steeper the impact, the more elliptical or elongated the blood drop is. Angle of impact is told by measuring the width and length of the stain or spatter and an example is shown below: * Divide the smaller number by the larger, i.e: * Sine = width = 9mm; length = 18mm * 9 divided by 18 = 0.5000 * Arcsine 0.500 = 30 degree angle of impact * Do not measure the “tail” as it is a secondary force or event
Back spatter, otherwise known as “blowback” is when a bullet strikes a target and some of the HFIS may be directed back toward the gun that fired the shot. If the bullet exits its target, a larger amount of HFIS may be directed in the same direction as the bullet. This is known as “forward spatter”. The amount of HFIS created will depend on the size or caliber of the bullet fired, as well as the distance from the gun to the target. The larger the bullet or projectile, the greater the spatter will be. The closer to the target that the gun is when fired, the greater the spatter.
There is an amazing variety of other terms and associations for BPA that could be discussed, but as for the purpose of this paper, this should be a sufficient window into BPA.

References
Crime Scene Forensics, LLC. (2012). Crimescene-Forensics.com. Interpreting Bloodstain Patterns. Retrieved from http://www.crimescene-forensics.com/Crime_Scene_Forensics/Bloodstains.html

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