For fans of the Showtime series Dexter, the concept of blood spatter will be nothing new. Dexter, a forensic scientist and serial killer, is able to dissect crime scenes by analysing blood spatter, and instantly tell his colleagues precisely how someone died. Of course, what his detective friends don’t know is that is preternatural skills in the art of killing make him very experienced at reading a crime scene. They never seem to catch on to this.
However, determining how “spatter” (bloodstains) on various surfaces have formed (and what these mean) is not quite as simple as shows like Dexter and CSI would have you believe: we’ve put together a reliable primer on how this is achieved.
Although it certainly is as much an art as it is a science, spatter speaks volumes: it can distinguish the type and velocity of weapon; the number of blows; the handedness of your suspect; the movements of suspects and victims during and after the attack; and may even approximate when the crime took place. All this information can be immensely useful in constructing a suspect profile, which is collated using three key factors (discussed in detail below).
1. Flying through the air: viscosity and velocity.
Fresh blood behaves much like another liquid you’re familiar with: water. That is, until it clots. Low velocity spatter is usually the result of dripping blood. Think about the pattern droplets of water would make from a faucet in a dry sink. This might occur where your victim is wounded, and is stumbling around with blood seeping from sustained injuries. These are known as passive spatters.
A medium velocity spatter might be caused by an injury with a blunt instrument, such as a repeated beating with fist or a blade. If the wound has pierced a major artery, a medium velocity spatter may also have a distinctive spray pattern (imagine, if you will, a super soaker type spray).
A high velocity spatter is typically caused by a gunshot wound or an extreme amount of force. These travel a long distance and look like a fine spray of small droplets; this type of spatter can travel over a long distance. Skilled spatter analysts look to voids in the high velocity spatter, where empty places indicate that someone or something has caught some of the blood and has since been moved.
2. That’s no ordinary waterfall: how blood behaves.
If we take a droplet of water from a cup on our finger and drip it onto a surface from, say three feet, we will notice that this action creates a small droplet. If we repeat this process, from approximately six feet, its radius will significantly increase due to its final velocity when it hits the floor.
Fortunately, a breadth of research already exists on how blood behaves when it is dropped from various heights and it’s quite easy to compare the diameter of the dropped blood to a known baseline and to determine roughly from what height it fell. Put simply, this means that the larger the blood droplet, the higher it has fallen. We might expect to see blood droplets at a crime scene gradually becoming smaller as the victim dropped closer to the floor. Observing and charting this phenomenon may supply us with a good idea of where a wound was inflicted.
3. Trigonometry, my dear Watson.
You may have seen TV shows recreating crime scenes filled with strings, and this is the time to deploy them. “Stringing” or attaching strings to spatter to determine the source of convergence, gives analysts an indication of where the victim was when the blood was drawn. By measuring the length and width of the spatter, analysts can use trigonometry to “draw” a straight line through the crime scene using string to determine the position of the victim, depending on the angle the blood fell at.
For a down-and-dirty analysis, droplets that fall perfectly vertically will appear largely round. All others will have a slight or long tail, which will indicate which direction they came from. Generally speaking, the longer the tail, the more shallow the angle of trajectory.
There will be some relief for the mathematically-challenged, however, in that computers are there to assist. By making accurate measurements of the crime scene, it can be modelled effectively in forensic reconstruction programs, which create three dimensional models and show the area of convergence.
Determining the point of convergence is a critical step, as it gives you a ballpark estimate of not only where the blow was inflicted, but of what the actions of the suspect and victim were prior to the blood being drawn.
Spatter analysts have to undergo unique training and undertake accredited qualifications in blood analysis, which is complex and challenging to be sure but ultimately very rewarding. The recreation of a crime scene, particularly where a suspect is unknown, can be pivotal in making or breaking a case, and spatter analysis is a method that has effectively held the public’s fascination.
Your Turn: Any would-be Dexter Morgans (or perhaps any practically-minded aspiring forensic scientists) can leave us their views on additional techniques that are utilised to determine what spatter patterns actually mean. We’d love to hear from you.