Bloodstain Pattern Analysis at a Crime Scene
Blood is the fluid connective tissue of our body. It is the only bodily fluid that helps in the overall growth of living beings and supports the life system. It is a highly complex mixture consisting of cells, proteins, and enzymes. Given that blood plays an important role in a living body, from a scene of a crime that perhaps could have involved physical struggles, the most expected evidence is blood.
Among other body fluids which contribute to the category of biological evidence from the crime scene, blood is considered to be of extreme importance, mainly due to its abundance in the crime scene as well as in the human body. It is easy to find from the victim’s body and could provide ample information about the crime.
Blood evidence can be used to determine the identity of a suspect or a victim via DNA tests. But other than DNA and other racial identification, blood spatter analysis helps a lot in crime scene reconstruction.
The interpretation of bloodstains at a crime scene to recreate the actions that caused the bloodshed is referred to as bloodstain pattern analysis(BPA). Analysis of the shape, size and distribution of the bloodstains could produce valid opinions about what did or did not happen at the scene of crime.
Bloodstain Pattern Analysis uses principles of biology(behavior of blood), physics(cohesion, capillary action, and velocity), and mathematics(geometry, distance, and angle) to assist investigators in answering questions such as:
- The direction from which the blood originated
- The angle at which a blood droplet struck a surface
- The location or position of a victim at the time when bloody wound was inflicted.
- The movement of bleeding individual at the crime scene
- The minimum number of blows struck to a bleeding victim
- The approximate location of an individual delivering blows that produced bloodstain patterns.
Bloodstains range in both amount and types of bloodstain patterns. The shape of the bloodstain pattern will depend greatly on the force used to propel the blood as well as the surface it lands on.
Collection of Bloodstain Evidence
They can be collected for pattern analysis by cutting the stained surface or materials without distorting the pattern, photographing the stains, or drying and packaging the whole material consisting of the spatter pattern.
The tools for collecting bloodstain evidence usually include high-quality cameras (still and video), sketching materials, cutting instruments, and evidence packaging tools.
Impact Spatter Bloodstain Pattern & Factors Affecting It
Impact spatter bloodstain pattern is the most common type found at crime scenes. It occurs when an object impacts the source of the blood. Impact spatters that project the bloodstains outward and away from the source are referred to as forward spatter and those that are projected backward from a source are referred to as backward spatter.
Based on the velocity of the spatter, impact spatters can be of three types:
- Low Velocity Spatter: It consists of large, separate, and compounded drops with diameters of 4 millimeters or more. This kind of spatter is usually produced by gravity alone, a minimal force, or by an object dropping into and splashing blood from a blood pool. They can also be a result of an applied force moving at up to 5 feet per second.
- Medium Velocity Spatter: The blood drops are relatively small with 1 to 4 millimeters of diameter and are normally associated with blunt-force trauma. It can also be caused when an applied force is moving at between 5 to 25 feet per second.
- High Velocity Spatter: They appear as very fine drops with a diameter of less than 1 millimeter. It can be a result of an applied force of 100 feet per second or faster. Gunshot exit wounds or explosions commonly produce these kinds of spatters.
Two major factors that affect the origin of impact patterns are:
- Area of convergence: It is a point on the two-dimensional plane where lines traced through the long axis of several individual bloodstains meet.
- Area of origin: It is the location in three-dimensional space from which blood that produced a bloodstain is originated.
Shooting may leave a distinct gunshot to spatter pattern. This can be characterized by both a forward spatter from an exit wound and a back spatter from an entrance wound. Forward spatter generally leaves a pattern of very fine drops characteristic of high velocity spatter.
The location of the injury, the size of the wound created, and the distance between the victim and the muzzle of the weapon all affect the amount of back spatter that occurs. Depending upon the distance from the victim at which the gun was discharged, the back spatter sometimes can strike the gunman and enter the gun muzzle, this is called a drawback effect.
It is created when a blood-covered object flings the blood in an arc onto a nearby surface, like that of a weapon which was pulled out from a stabbed wound and is again pushed in to stab, in between the blood drops from the weapon would fling across the nearby surface. It can also be caused when two people hit each other with bloodstained wrists.
Arterial Spray Spatter
It is created by a victim who suffers an injury to the main artery of the heart. The pressure created inside the arteries due to the pumping of blood by the heart, causes the blood to spurt out if an injury occurs. The pattern shows large spurted stains for each time the heart pumps.
Patterns created by the blood that is expelled from the mouth or nose is expirated pattern and they are usually of very fine velocity. There can be a presence of oxygen bubbles in the drying drops.
They are created when an object blocks the deposition of blood spatters onto a surface or object. The spatter is deposited to the object itself or the person instead.
Contact or transfer patterns are formed when the blood is rubbed off or passed in between people due to their contact with each other. Flows, on the other hand, are formed when a huge internal impact has been caused to the victim and the blood flows due to the action of gravity. The flow direction can help in determining the direction in which the body was pulled post the attack.
A pool of blood can occur when blood gets collected after an assault at a slope less level and in an undisturbed place. Blood that pools on an absorbent surface can get absorbed throughout the surface and diffuse creating a pattern larger than the original pool of blood.
Patterns like drip trail occur when there is a series of blood drops or blood tails that is separate from other patterns and is formed when blood drips from an object or an injury. It can help hugely in the determination of personnel movement.