by Sue Carney
In this week’s #cclivechat we focused on blood pattern analysis. The previous week we’d touched on the issues of whether it might be possible to age blood stains (see here and here for some recent projects) and this led to questions about blood patterns and my eventual volunteering to chat about BPA this week.
BPA is a fascinating but incredibly detailed area of forensic science and I was excited but apprehensive as the chat began. Trying to keep up with the stream of questions and to formulate answers in less than 140 characters quickly enough was, as I’d anticipated, a challenge to focus the mind! I like a challenge. But, as @PennStateCSI pointed out (see the comments after Vidster’s recap) some of my answers lacked sufficient detail, resulting in ambiguity & inaccuracy, and that’s not acceptable in forensic science, even in a live chat on Twitter. Those who work with me, especially those whose work I’ve peer reviewed, will gladly tell you that I’m a pedant. I make no apology for that, and they’ll tell you that too!
In order to provide some additional clarity to my brief answers, I will share here some details of the BPA I learnt at the Forensic Science Service. This post is not an exhaustive review of BPA by any means. People have written vast tomes on the subject. Instead, think of it as an overview especially for DCC, that attempts to penetrate the surface of a vast and complex subject. I will gladly answer any additional questions and maybe there’ll be a part two. Let’s deal with part one first.
Training in BPA is not just theoretical. It’s physical! The FSS implemented a thorough training programme in which its fresh faced trainee reporting officers were packed off to Sheffield, a city in South Yorkshire, UK, to spend a few exhausting days simulating blood patterns. You need a place that can be easily cleaned if you’re going to chuck vast quantities of blood at the wall, so where better than a pristine white-tiled city morgue?
You can’t simulate blood patterns without blood. It simply doesn’t work with any other liquid in quite the same way. Blood has a unique composition comprising cells, mostly red cells containing haemoglobin, and a lesser proportion of white cells containing DNA (there’s no DNA in the red cells), smaller structures called platelets that are involved in clotting, and numerous complex proteins with a variety of functions. All are suspended in a liquid, plasma. The result: blood reacts to physical forces in a slightly different way to other liquids. For this reason, we use real blood in this training. For those worried about health and safety, it’s routine to use treated, pathogen free horse blood that can be legitimately purchased from scientific reagent suppliers.
In order to investigate how blood is transferred from a wounded individual to an assailant during an assault, we instigated various assault situations: punching, kicking, striking with a variety of weapons. This was where we encountered the first limitation of the exercise. You can’t beat up a real person to investigate how their blood might transfer. Well, more accurately, one would not be justified in doing so! Instead, we used the closest and most readily available approximation to human tissue: a variety of joints of pork. We ‘wounded’ them, filled up the wounds with blood and proceeded to assault them in a controlled manner for scientific purposes. (Whilst educational, this might also be considered fun. I’ll tell you some time about the other experiments we forensic types get up to such as simulating damage to clothing whilst it’s being worn!)
One thing I learnt immediately was that it’s actually quite difficult to punch something with any degree of force. It must hurt to punch a real person. The only drawback to using pork is that it’s no longer alive, hence no longer has blood circulating. This means that eventually the blood needs to be replenished because it doesn’t continue to flow like it would from a live person. However, the results of our various assaults gave a good indication of how and where blood might transfer to an assailant when they kick or punch a wounded person. We’ll get to those blood patterns presently, but we’ll begin with some basics.
A 1939 publication by Balthazard, in which he describes the characteristics of a single blood drop, is considered a significant work on BPA. The characteristics of the single blood drop is BPA 101. There’s enough information on just this to fill the rest of this post. However, I’ll provide the highlights. In studying the variations in the patterns caused by single drops, we’ll first consider those falling vertically, due to the action of gravity alone. These might be from a blood-stained weapon, a wound dripping blood, or any surface on which there is sufficient volume of blood for a drop to form (i.e. if the force of gravity acting on a large enough volume of blood exceeds the surface tension holding the blood to the surface.) The size of the drop varies according to the site of the drip, with smaller drops forming from a smaller or more confined site such as a knife blade tip.
The shape of the stain when a drop falls onto a flat (perpendicular) surface varies according to the volume of the blood drop, the distance fallen and the nature of the surface. Absorbent surfaces result in smaller stains, whilst non-absorbent surfaces cause the blood to spread to form a larger stain. Many of these stains are approximately circular but rougher surfaces cause spines to radiate out from the central stain, or produce secondary stains known as satellites, around the edge. An example of this kind of stain, blood dropped onto sandpaper, can be seen here. Very generally speaking then, a circular stain is a good indication that the blood dropped from directly above, but the size of the stain gives no real indication of distance. These type of stains might be expected if a person is bleeding and stands still dripping blood onto the floor.
I remember discussing and viewing the scene photos from a particularly brutal murder case in which a whole family had been bludgeoned to death with a cricket bat. The beatings were so severe that that none of the victims were recognisable. The scene was completely covered in blood: The bodies lay in pools of blood, spatter covered most of the walls and cast-off staining (we’ll come to that) was all over the ceiling. Those images will stay with me, and I only saw the pictures, I wasn’t there at the scene, it was my colleague’s case. As we viewed the photos, my colleague asked what was out of place in one particular photo. On the clothing of that victim we saw a single circular spot of blood. It wasn’t part of any other blood pattern on her clothing and DNA testing showed that it wasn’t from any of the victims so it hadn’t dripped from a weapon. It had been dropped from directly above as the killer stood over this victim and bled, presumably from a wound he had sustained during the attacks.
In the ‘person standing still dripping blood onto the floor’ scenario, we must also consider that if there’s sufficient bleeding, blood will start to land on blood, and the pattern of spots will be obscured. Blood drops landing in wet blood will cause splashing, producing secondary spatter.
If the surface onto which a blood drop falls is angled, then the stain becomes a tapered ellipse with the tapered end pointing in the direction of travel. The steeper the angle, the more stretched the ellipse. Eventually the stain resembles a tadpole until the angle is so steep that, as the drop lands, a small part of the drop keeps moving and tumbles over the leading edge of the stain to form a secondary stain. This is called wave cast off, and the stain resembles an exclamation mark (or exclamation point, I think, for US readers.)
This ability to determine direction is useful in interpreting blood patterns. Let’s say our bleeding man doesn’t stand still but walks or runs away from the crime scene. He’s cut his finger, and as he swings his arms blood is dripping off. It will land on the floor at an angle and we’ll see roughly elliptical shaped stains (depending on the surface, some might be more irregular) rather than circular spots. There will likely be disruption at the leading edge of each stain in a blood trail which might help to tell us which direction he ran.
When an object impacts into a surface wet with blood, then blood becomes airborne and flies off. This blood lands on a surface in an impact spatter pattern. Although these blood spots are traveling in arc-like trajectories rather than a straight line downwards due to gravity, the shape of the resultant spots can still indicate the direction the blood was traveling. There are many types of impact spatter pattern, dependent on the force of action that caused them, the shape & size of the object causing the impact and the amount of blood at the site of impact. Some general characteristics are: the number and distribution of blood spots is random; the spots may appear to show direction to a common area of convergence; the greater the force, the smaller the droplets; and generally speaking, smaller blood drops travel shorter distances than larger ones because of air resistance.
In an assault scenario, an attacker may not always be heavily blood-stained, even if there is a large amount of blood at the crime scene. Some spatter might be so small that it is not readily visible on the attacker’s clothing. It’s also possible for spatter to occur in predominantly one direction, possibly away from the attacker, depending on how it was caused.
In the forensic examination of clothing, we may not always observe what we might describe as classic distributions for particular actions, because there are so many variables in a real assault scenario. Back in the morgue, we created some of the classic impact patterns under controlled conditions. Here’s what we found:
In a punching, spatter might be seen around the assailant’s cuff. Blood spots found on the inner surface of a cuff, are particularly significant. Spatter might also be found on the upper sleeve and across the chest. If there’s only one punch in an assault, and the victim isn’t bleeding yet, then no spatter will be generated, even if he starts to bleed as a result of that punch.
In a kicking, blood is forced into the crevices on the front and upper of the shoe due to the force of the kick. There will generally be a contact-type stain on the front of the shoe and spatter on the shoe and lower trouser leg. If there’s blood on the floor, then secondary spatter can form on the shoes of anyone walking through it. The direction of stains may help to distinguish this from impact spatter, but it may not always be clear. Again, the amount of blood on the victim at the time of kicking will affect the pattern on the assailant’s shoe.
There are other types of impact spatter that I’ll try to cover in an additional post.
If a weapon is used in an assault with sufficient force then the spatter produced can be very small and may not be visible on the assailant’s clothing without a microscope. In addition, blood on a weapon may be flung off by the action of centrifugal force when the weapon is swung, forming cast-off staining, or when a weapon comes to a sudden stop as it strikes a surface, forming cessation cast-off. Cessation cast-off can look very similar to impact spatter and the many variables affecting cast-off staining, such as the type of weapon, the amount of blood on the surface and the velocity of swing, will mean that a variety of patterns can be produced. Indeed much cast-off staining at crime scenes may never be identified as such. However, if the rate of swing is constant and sufficient blood is present on the surface of a weapon, then a particular type of cast-off staining can be produced that we refer to as in-line staining. In-line staining appears as lines of blood spots of a similar size and direction. We may find this on the ceiling if a long weapon such as a baseball bat has been used and it may indicate the number of swings of the weapon. However, caution is advised in such interpretations, since real life assault scenarios are inevitably less straightforward, with the weapon describing a more complex path as it is swung. The way the weapon is held and even the anatomy of the arm may play a role here too. Furthermore, there will probably be a difference in force between the forward and backward swings, and there may be more blood available to be cast off on the backward swing, as blood has been replenished.
A distinctive pattern can be formed on the weapon itself at the time of cessation cast-off. I shared a picture of this during #cclivechat. This is percussive staining: wet blood on the surface of a weapon forms into runs during the swing, as centrifugal force acts on it, but as the weapon rapidly halts when it strikes a surface, this blood keeps moving due to its inertia, producing ‘feathering’ in the blood pattern.
We haven’t yet covered arterial stains, contact stains, some of the other types of impact spatter, such as those caused by gunshots, expirated blood, physiologically altered blood stains, stabbings, or any of the testing methods, so it seems a return trip to the morgue for part two of this post is definitely in order!
In the meantime, there are plenty of great books out there with much more detailed information. For those with a professional interest, Blood Dynamics by Anita Wonder is my recommendation.