Blood pattern analysis – principles

The main points relating to blood pattern analysis are the following. Drops falling vertically onto a horizontal surface will leave a round stain. Drops striking a surface at an angle will create a stain that is elliptical with a tail. The blood and tail shape will show the direction in which the blood was travelling. The ellipse will narrow as the angle of impact becomes more acute. The angle can be calculated by dividing the width of stain by its length excluding the tail, which gives the sign of the angle of impact. This information can be used to estimate the position of the point of impact.

If the stains impact on two or more surfaces that are in different planes to each other, the point of impact can be estimated from the point of convergence of strings in three dimensions. We saw the results of this technique in the case earlier in the case narrative in week three. Note that the car is the one used in the case study. And the blood stains are not from a real incident. However, the stringing shown mirrors what was obtained in the reconstruction of the real incident in which our case study is based.

Blood spatters resulting from a forceful impact such as being hit by a club or a bullet wound would be smaller than freely dropping blood. The more force, the smaller the spatter. We have seen how this was used in the previous case we discussed– the body in the woods case, which we’ll come back to later– to lead to the conclusion of suicide and not murder. The physical nature of the recipient surface will affect the shape of the stain. For example, a drop of blood falling onto a rough surface will show more marked spines and satellite formation than one falling onto a smooth surface. Biologists or crime scene examiners who specialise in BPA recognise several types of patterns.

But we will only discuss three of them which are bloody footwear marks and how they can be enhanced, castoff marks, and void marks. Bloody footwear marks can be classed as positive or negative images. If someone steps into shed blood that is still wet while wearing footwear with a patterned sole, they will leave a negative impression in the blood. If they now walk away, they will leave positive prints of the sole pattern on the flooring. As the blood picked up by stepping in the pool is now transferred to the surface being walked on. Interpretation of such marks is generally straightforward. They can be used to associate footwear with the incident scene. Whose footwear and where did the marks lead us.

And perhaps also, some indication of when, since the shed blood must have been liquid at the time. These are essentially two dimensional marks and are best recorded by photography. Associating the marks with possible source footwear is not always easy. The original mark in the shed blood will be poorly defined if the blood was liquid. The positive marks left as the person walks away from the blood may also be indistinct, depending on the amount of transferred blood and the nature of the surface being walked on. Faint marks can be enhanced by a variety of techniques based on the presumptive tests for blood we have already discussed. Luminol and leuco crystal violet are among the more commonly used reagents.

Cast-offs refers to the patterns created by blood droplets thrown off of a weapon as it’s forcibly swung to and fro in an attack. In principle, these stains will create a more or less linear pattern of smallish stains showing directionality. The patterns can be used to estimate the number of blows, the movement of the arm, vertical or lateral blows, and where at the scene of the incident the blows were struck. In practice, it’s not that simple. Try doing it with water and see what you think. There is a link in the resources that you can look at for some material about castoff blood stains.

The divergence of opinion on the interpretation of blood stain patterns is nothing new but is particularly the case when discussing voids. The theory is simple. If someone is the victim of a forceful attack such as repeated blows from a club or from a close range gunshot, then some of the blood spatters will hit the perpetrator. And if he is between the victim and a wall, there will be an area on the wall that has no blood stains because it was shielded by the attacker. Some crime scenes do indeed contain patterns that fit this concept. Unfortunately, there is no law of physics that says there will be a uniform density of blood spatters emanating in all directions from the victim.

There is also an epistemological issue regarding drawing conclusions from the absence of something. We know that a stain free area could be a void and may believe that to be so during an investigation. But there is no way that we can know this as a certainty. Let’s look at an example. These photographs are from the body in the woods case that we spoke about earlier. And show an example where voids, or their absence, produced valuable evidence to challenge a hypothesis.

This case was investigated as a murder. But when the crime scene examiners began their work at the scene of the shooting, they soon found something did not fit. Blood on the floor by the couch shows where the deceased must have been sitting when shot. The first photograph shows one of the crime scene examiners in that position. On the right side as we look at the photograph, there is an exit hole through the curtain. The black line and arrow show the trajectory of the fatal shot. The second photograph shows the scene examiner standing where the gunman must have been to fire the shot on that line. But the third photograph shows the wall behind the scene examiner.

The stuck on labels show blood spatters. And if events had been as described, there should have been a void caused by the body of the gunman shielding the wall from the spatters. The only way to explain the trajectory and spatter patterns is a self-inflicted injury– suicide, not murder.

We shall close this discussion of BPA with a reminder of the Sherlock Holmes quote. “It is a capital mistake to theorise before one has data. Insensibly, one begins to twist facts to suit theories instead of theories to suit facts.” Almost everything that we have considered in regard to the creation and interpretation of blood stain patterns is logical and reasonable and can be replicated in a test environment. However, it is not so simple in practice. Some of the influences are the movement of the people involved– the obvious example being the victim trying to escape.

The nature of the surface onto which the blood droplets are deposited– any rough surfaces will deform the stains as they strike it, and smooth ones will result in deformation of droplets hitting them by allowing the wet blood to run, with the exception of blood falling vertically onto a smooth surfaced floor– weapons involved, and the actions of the assailant.