Blood spatter patterns can vary from a few drops on a bed to pools on the floor or sprays across the wall. In his book, Invisible Evidence, Bill O’Brien describes the ways blood spatter analysis can lead investigators to accurate conclusions about the manner of death and facts about the killer.
Confirming the Presence of Human Blood at a Crime Scene
When blood is present at a crime scene, investigators will examine it and ultimately confirm that it is human blood by testing it in the laboratory. A test strip is used at the scene and a positive result will turn it green.
In some cases, the offender will have cleaned up the crime scene and removed all visible traces of blood. This is commonly done by washing the area, using bleach or removing stained furnishings. Crime scene investigators may search the area using a high-powered light that reveals tiny specks of blood or they may use luminol. Luminol is a reagent that reacts with the iron found in hemoglobin to cause luminescence. It is sprayed across the crime scene and will show up the finest traces of blood.
Analyzing Blood Spatter Patterns at Crime Scenes
Bill O’Brien describes the way that blood spatters are analyzed. If a person has been beaten, one of the blows will draw blood and subsequent ones will spatter the blood. Here are some basic facts about blood spatter patterns:
Medium velocity spatter is caused by fist fights, boots and baseball bats Medium velocity droplets are caused by knives High velocity spatter is caused by bullets When a blood spot falls straight down to the floor, it will be circular A blood spot falling at an angle will be more elongated like an exclamation mark The tail of the exclamation mark will always point in the direction of travel Arterial bleeding causes spurts of blood caused by the contractions of the heart. This can cause a curtain-like patterns on walls Cast-off patterns are created when blood is flicked off a moving object such as a baseball bat
What Blood Spatter Patterns can Reveal
A skilled investigator can analyze a crime scene and enter facts and figures into a computer programme to determine things such as the height of the killer and the type of bleeding. Here are some of the facts that can be revealed through careful investigation:
Whether the victim was sitting, lying or standing during the attack The type of weapon used and the type of injury caused Where in the room the victim was attacked and if he was moved afterwards The track of the blood curve can reveal if the assailant was right or left-handed The amount and pattern of blood can reveal if the attack was short and quick or long and frenzied
Blood spatter patterns are extremely important in crime scene investigation and by using a combination of human experience, mathematical knowledge and computer technology, they can reveal much about the homicide or attack. Even if the body is not present or found, blood spatter patterns can present an accurate picture of what happened at the crime scene.
O’Brien, Bill, Invisible Evidence, David Bateman Ltd, 2007