The success or failure of any criminal investigation often depends on the recognition of physical evidence left behind at a crime scene and the proper analysis of that evidence. Crime scenes that involve bloodshed often contain a wealth of information in the form of bloodstains.
The examination of the shapes, locations and distribution patterns of bloodstains provides an interpretation of the physical events by which they were created. This is based on the premise that all bloodstains and bloodstain patterns are characteristic of the forces that have created them.
Blood spatter analysis takes time and is just one piece of the puzzle when investigators are assembling together the elements of a crime. However, bloodstain pattern analysis can corroborate other evidence and lead investigators to seek additional clues.
Imagine what happens when you spill water droplets on the floor. The water falls slowly to the ground, making a circular puddle. The shape and size of the puddle depends on how much water you pour, how high you hold the water glass, and whether you are spilling it on carpet, wood or some other surface. For example, a lot of water makes a larger puddle, the puddle will be smaller in diameter if the water falls from a distance; a hard surface will retain more of a circular shape, while carpet absorbs some of the water and makes the edges spread.
Although initially it behaves as a liquid, blood eventually begins to clot after it leaves the body within 15 minutes. If some blood spatters are more clotted than others, it can indicate that multiple blows or gunshots occurred over a period of time.
If a gunshot occurred at close range, the victim might have some burns on the skin from gunpowder. Shots fired at very close range can also cause internal muzzle staining. When this happens, the victim's blood is sucked back into the gun's muzzle by the cooling of the explosive gases that are released when a shot is fired. Hence, testing the gun's muzzle for blood can provide an additional clue to solve a case.
A drop of blood that fell perfectly vertically at a 90-degree angle will be round. As the angle of impact increases, the drop of blood gets longer and develops a tail.
The greater the difference between the width and length, the sharper the angle of impact. Another method involves taking the length and width of each blood spatter, as well as other measurements of the area and putting them into a computer program. These programs create three-dimensional models and animations of the crime scene, and show the area of convergence.
References
bloodspatter.com
crime-scene-investigator.net
science.howstuffworks.com