10.2: Bloodstain patterns

Some important bloodstain patterns to document are impact spatter, cast-off and cessation cast-off, transfer and transfer impressions, voids, drip-trails, flow, pooled, and projected blood. Of course, these are only a few of the bloodstain patterns the technician will need to understand.

Impact spatter

When force is applied to a blood-letting source, blood will take flight primarily out and away from the source causing impact spatter. The faster (higher) the force (energy) applied, the smaller the blood droplets. When a high-velocity force such as a gunshot is applied, the resulting impact spatter will be mist-like, whereas a blunt or sharp force blow may produce impact spatter that is much larger. Remember, in order for a mist-like pattern to be associated with impact spatter resulting from a gunshot wound, there must be an exit wound and a corresponding bullet defect on the target. Direction of the force can be determined by the angle the blood impacts the surface. Forward spatter is blood that takes flight in the direction of the force of energy. Back spatter is blood that takes flight back towards the force of energy. Round stains indicate the blood impacted the surface at or near 90-degrees. Elliptical stains indicate an impact of less than 90-degrees. The more elliptical the stain, the lower the angle of impact. The direction of the stain can be determined by noting the “tail” of the stain known as the wave cast-off. The impact angle is determined using the same mathematical principles used to measure bullet defects. It is essential the bloodstains must have a scale, preferably in metric, to facilitate the calculations.

Figure \(\PageIndex{1}\): The above stain impacted the surface at 40 degrees. The direction of the energy was from right to left. – Author’s collection

Figure \(\PageIndex{2}\): The above stain impacted the surface at 60 degrees. Author’s collection

Figure \(\PageIndex{3}\): Impact spatter caused by a bat striking a blood source. Note the force of energy was from left to right. - Author’s collection

Figure \(\PageIndex{4}\): Forward mist-like impact spatter created by the perforation of a high-velocity bullet. Note the bullet defect in the center of the pattern. – Author’s collection

Another misting pattern, called an expectoration pattern, is the by-product of blood being expelled from the nose, mouth, or a wound and caused by the blood being mixed with the flow of air. If you look carefully, you will see air bubbles, called bubble rings, in the resulting stain.

Figure \(\PageIndex{5}\): Expectoration pattern. - Author's collection

Determining Area of Convergence and Point of Origin

By analyzing the directionality of stains associated with impact spatter, it is possible to determine the two-dimensional source of the blood-letting event by calculating the area of convergence of the pattern. The area of convergence is the area containing the intersections generated by lines drawn through the long axis of individual stains.

Figure \(\PageIndex{6}\): Determining the Area of Convergence. - Author's collection

Once we have determined the common convergence point several spatters may have made, we can then determine the point of origin of the stain by measuring the furthest stain in the common stains within each line back to the area of convergence and applying the average of the mathematical equation Tan1 of the width/length x the distance from the stain to the area of convergence. In the above example, the point of origin of the blood-letting source is approximately three inches.

Void patterns

A void pattern will result when an object intervenes with the distribution of blood. The resulting pattern is the absence of blood from an otherwise continuous stain. These voids can provide valuable information to the crime scene analyst and criminal investigator regarding weapons that might have been removed from the scene or the positions of victims, witnesses, or suspects.

Figure \(\PageIndex{7}\):Void area with corresponding finger impressions and transfers on the lower bunk and wall suggest the victim was kneeling in this position during the assault. – Author’s collection

Figure \(\PageIndex{8}\): The void pattern suggests the cardboard was at one time pressed against the wall during the blood-letting event. – Author’s collection

Cast-off and Cessation Cast-off

Spatter resulting from blood being flung or “cast-off” from an object can also providevaluable information to the investigation. Case in point, the sweatpants of a suspect accused of a homicide involving the use of a club are collected and analyzed by a forensic technician trained in bloodstain pattern analysis. The inspection of the clothing in the laboratory revealed small spatters on the front and rear left leg. White reinforcement tabs were placed over the spatter to improve visualization. Spatter was noted on the front and back left legs indicating the suspect used his right hand to deliver the fatal blows. In the downward swing, blood cast-off of the club landing on the front left leg. As the club was brought back for another strike, the cessation of inertia caused the blood to cast-off onto the back of the left leg. This pattern is called cessation cast-off. Although the actual details of this case have been modified for this chapter, the gist of the investigation remains the same.

Figure \(\PageIndex{9a/b}\):The suspect’s gray sweatpants with cast-off patterns on the front and cessation cast-off on the rear left leg. – Author’s collection

As mentioned before, when looking for cast-off spatter, the investigator should inspect the ceiling and walls of the crime scene. Cessation cast-off is likely to be found low on walls and on the floor.

Figure \(\PageIndex{10}\): Cast-off spatter being flung from a broken mop handle during an assault. – Author’s collection

Transfer and Transfer Impressions

A transfer occurs when a bloodstained object presses against another surface thus transferring the blood from one surface to another. A bloodstain transfer impression occurs when a bloodstained object presses against another surface, leaving behind an image of the bloodstained object.

Figure \(\PageIndex{11}\): There is no doubt what created this transfer impression. – Author’s collection

Figure \(\PageIndex{12}\): Blood drops transferred to the bottom of a shoe heel.- Author's collection

Drip Trail

Drip trails occur as the result of the movement of a blood source between two points. This can be passive blood-letting falling from a victim who is ambulatory, being carried or removed from the scene on a gurney, or blood falling from an object like a knife or blood-soaked garment. This is often the first clue to determining direction of travel. Dripped bloodstains that are close together can represent a blood source that is slowly moving through the crime scene. Dripped bloodstains that are further apart may indicate the blood source was moving quickly through the scene.

Figure \(\PageIndex{13}\): The blood drip-trail of a stabbing victim. – Author’s collection

Flow

Blood flow occurs as the result of the movement of a volume of blood on a surface due to the pull of gravity, or the movement of surface upon which it was deposited, and before the blood had dried. Flow patterns can be an indicator that a victim has been moved after death.

Figure \(\PageIndex{14}\): The inmate in this photograph was discovered in the supine position, but the flow patterns on the left side of his head indicated he was in the prone position during the blood-letting event.- Author’s collection

Pooled Blood

Pooled blood occurs when a blood-letting source remains stationary in one area for a period time sufficient to result in the passive accumulation of blood. Pooled blood is often found in the final resting area of a victim of a blood-letting event.

Figure \(\PageIndex{15}\): Pooled blood from the final position of a gunshot victim. Note the emergency medical services left something behind. – Author’s collection

Projected Bloodstains

Projected blood stains occur when blood is forced out of an opening under pressure by the breech of an artery or large vein. Because this pattern is produced by the pumping action of the heart, the technician can deduce the victim was alive at the time of the blood-letting event.

Figure \(\PageIndex{16}\): Blood projected through an open wound by a breech in a tributary vein of the jugular.

Figure \(\PageIndex{17}\): Blood projected from the nose following a bullet wound to the head.

Wipes and Swipes

Two passive bloodstain patterns often found in crime scenes are wipes and swipes. Wipes occur when an object moves through a preexisting wet blood source, such as the arm of the victim moving through pooled or dropped blood.

Figure \(\PageIndex{18}\): A wiped pattern caused by an object moving through dropped bloodstains. - Author's collection

Swipes occur when a bloodstained object moves across a clean surface thus transferring the blood onto the clean surface with the characteristics of relative motion between the two surfaces.

Figure \(\PageIndex{19}\): A swipe pattern created by blood-soaked hair being dragged across a clean surface. - Author's collection