[ About the author: Emily Nelson earned an M.S. in Electrical Engineering from the Massachusetts Institute of Technology before beginning her career as a science writer ]
When you see images of crime scenes on shows like 48 Hours Mystery, you often see blood. Sometimes, lots of it. Blood is an important form of evidence that can answer many questions about a crime. The pattern of the blood that is spattered around the crime scene, called the bloodstain pattern, contains valuable information about the sequence of events that occurred to cause the victim’s injuries. Unlocking this information requires the expertise of a forensic scientist who specializes in bloodstain pattern analysis.
How exactly does bloodstain pattern analysis work? That is, how do drops of blood tell the story of the crime? It turns out that the location where the blood lands, and the shape of the blood on the landing surface, reveal both the direction in which the blood was moving and how much force was used to wound the victim. Bloodstain pattern analysts use principles from biology, chemistry, physics and math to figure out the location of the victim when the blood was shed and even the type of weapon or impact that caused the victim’s injury. Sometimes, blood from a weapon can even reveal a criminal’s mentality. For example, powerfully projected sprays of blood would suggest that the attack was fierce and was meant to kill the victim.
In the context of crime scene investigation, bloodstain pattern analysis is the process of analyzing the location and shape of all the blood at a crime scene to determine the sequence of events that caused the bloodstains. This involves answering as many of the following questions as possible about the crime:
- What weapon(s) did the perpetrator use?
- How many times was the victim hit, shot, stabbed, etc?
- Where was the victim when each event occurred?
- Where was the perpetrator during and after each event?
In order to use bloodstains at the scene of a crime to reconstruct an attack, investigators first have to find all of the stains. Investigators commonly use lights specially designed to show bloodstains. The evidence is then photographed and carefully recorded. Forensic scientists who analyze bloodstain evidence are typically called bloodstain pattern analysts. It is best if the bloodstain pattern analyst is present at the crime scene to begin the investigation, but it is possible to use photographs, video, and reports from a crime scene investigator for the analysis.
When the bloodstain evidence has been recovered from the crime scene, work begins in the lab to reconstruct the events that produced the bleeding. Bloodstains often appear to radiate from a single point. As it turns out, that point is where the injury occurred. Although bloodstain pattern analysts can roughly see the pattern without doing any calculations, they use computer programs to figure out the exact path that each blood droplet traveled. Even with the power of modern computers, it can be difficult to determine the exact sequence of events at a crime scene, though, particularly when a crime involves a lengthy struggle that smears some patterns and deposits new blood over others. Bloodstain pattern analysts often perform carefully planned control experiments to try to recreate the bloodstains, especially when they need to test different hypotheses about the events.
The bloodstain pattern analyst must take detailed notes during every step of the process. These notes are used to write a full report about the analysis and its conclusions. If the bloodstain evidence is used in a case that goes to trial, the bloodstain pattern analyst may be required to testify in court about the work.
Bloodstain pattern analysis is a challenging and fascinating field of forensic science that will continue to provide exciting career opportunities well into the future.