Crime scene reconstruction is a favourite of CSI-type television shows. After all, most people like a good “whodunnit”. And finding out who did it usually starts with finding out what happened. It’s a very broad topic, but here we’ll introduce you to blood spatter analysis, and see how it can help forensic investigators go back in time to unearth a few very important clues.
You might recall a post from a while back titled Forensic Techniques: where’s the blood? detailing how police or forensic investigators determine whether a particular stain is blood, or something else: using luminol or, for a more conclusive verdict, the Kastle-Meyer test, blood can be distinguished from, say, paint or wine.
Since our tests (for the purpose of this post) came back positive for blood, it’s time to see if more clues can be unearthed.
Blood spatter analysis is the examination of the shapes, locations, and distribution patterns of blood stains. When the analysis is complete, forensic investigators should be able to determine the direction a drop of blood was travelling prior to impact with a surface as well as the angle of impact.
The nature of the force involved and what type of weapon used can also be determined along with the relative position of both the victim and the perpetrator. Seasoned forensic investigators may also be able to piece together the events surrounding the incident.
The impact spatter
The impact spatter is a type of blood stain caused when force is applied to a blood source, such as a person. Examining the spatter, it can be determined whether impact occurred at a high velocity, medium velocity, or low velocity.
A low velocity impact spatter is blood that simply drips to the floor. In other words, it’s under the influence of gravity alone. Interestingly, the diameter of the spatter increases as the height increases, which may provide clues as to whether the victim was standing, kneeling, or lying down when the incident occurred.
A medium velocity impact spatter, as pictured, has an elongated shape, indicating that the blood was travelling at some speed before coming to rest on a surface.
A high velocity spatter. This spatter resembles a fine mist due to droplets breaking up under great speeds.
If there is no victim or weapon present at the crime scene, the spatter type may give forensic investigators some idea of what to look for. This may include hammers or knives for low velocity spatters, items such as baseball bats for medium velocity spatters, or a firearm for high velocity spatters.
The Angle of Impact
Take a look at the medium velocity blood spatter to the right. The direction in which the narrowing tail of the blood spatter is pointing, is opposite to the direction of travel. Forensic investigators will calculate the angle of impact (which is the angle between a blood droplet and the surface it strikes) to determine the flight path of these droplets. The more elongated the droplet, the smaller the angle at which it hit the surface.
When a line is drawn through the long axis of a group of blood stains on a surface, a point of convergence is determined where these lines intersect. This is often illustrated in CSI-type television shows with pieces of string attached to the surface. The point of origin is determined where the strings intersect, and indicates the position of the victim when the incident occurred.
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