Applying Bloodstain Pattern Analysis in the Crime Scene
Written by Ross M. Gardner & Donna R. Krouskup   

COMPLETE ANALYSIS OF BLOODSTAIN PATTERNS requires specific training and experience. When presented with situations involving such evidence, specialists will typically be brought in to assist. A specialist, however, is not always available to come to the crime scene while processing is ongoing. As with all evidence the basic responsibility of a crime scene investigator (CSI) is to collect the evidence in a fashion sufficient that subsequent analysis can be accomplished. In terms of BPA, collection effectively entails photographic documentation and sampling. Therefore, at a minimum, the technician must recognize and properly document this evidence. General recognition of the various bloodstains also allows a general understanding of what was happening within the scene, which may aid in on-scene efforts. Though somewhat advanced, these evaluations are common in many violent crime scenes, and it is in the best interest of the crime scene technician to develop skills in BPA.

Bloodstain Pattern Analysis

The consideration of the bloodstains at the crime scene can provide important information to the crime scene technician. The discipline of BPA considers the location, shape, size, distribution, and other physical characteristics of bloodstains in the scene, and from this derives information regarding the nature of the event that created the pattern. Thus, bloodstain patterns tell us “what” happened. This information, when combined with the information derived from any deoxyribonucleic acid (DNA) analysis, may allow an investigator to corroborate or refute specific investigative theories as well as subsequent statements offered by suspects, victims, and witnesses. Although the crime scene technician may not be trained in depth in BPA, it is imperative that she or he be able to recognize critical classifications of stains and know how to properly document bloodstained scenes.

Theory and underlying principles of bloodstain pattern analysis

The theory of BPA is relatively simple. Blood as a fluid (a complex fluid, but a fluid nonetheless) responds to variations of internal and external forces in a predictable fashion. The bloodstain pattern analyst evaluates various mechanisms and collapse mechanics of blood masses with respect to the patterns produced under controlled conditions (known patterns). This knowledge is compared against the stains found in bloody scenes (unknown patterns). The general predictability of how blood behaves under basic known conditions allows the unknown patterns to be compared via class characteristics. This predictability also allows the analyst to recognize other aspects, such as directional and impact angle and the alterations brought about by environmental conditions.

There are three underlying principles that guide the behavior of the bloodstain pattern analyst. They are

• The Pattern Diversity Principle
• The Principle of Stain Shape and Vector Correlation
• The Physically Altered Bloodstain (PAB) Principle

The pattern diversity principle

Pattern diversity is the core concept behind BPA and was recognized more than one-hundred and fifty years ago. One way of articulating this principle is to say:

The variations in combinations of blood volumes and forces acting on those volumes lead to recognizable classes of patterns.

When a blood mass comes under external force, the fluid’s behavior under this force leads to the creation of recurring patterns. Different mechanisms produce different patterns with different class characteristics. These basic mechanisms are not specifically crime-oriented. For example, a beating mechanism does not produce a “beating” specific pattern. The pattern produced by beating is the same as a pattern produced when static blood is impacted by any external force. The basic pattern types are driven by principles based in physics and relate to the way the fluid mass is affected. These event mechanisms can be distilled into six primary categories. These six mechanisms are

Static blood dispersed from a point source. When a volume of blood is present at a point source and the blood mass is exposed to an external impulse or force it produces a radiating pattern of small circular and elliptical shaped stains. The blood is incompressible and under the impulse/force it must displace. The blood shears into small droplets at the source and these small spatters radiate outward from the source. The resulting spatter stains and their radiating nature are the primary characteristics of the pattern. These patterns are referred to as “impact patterns,” but they also include blood dispersed by air such as expectorate.

Blood dispersed in a jet (streaming ejection). When a volume of blood is released in a stream, (referred to as a “jet” in fluid dynamics) it produces patterns consisting of stains, that are often deposited either in linear patterns or in accumulations with associated spines and secondary spatter. It is important to understand that the underlying force behind the ejection may be nothing more than gravity, such as when a large volume of blood is flowing heavily from a source or it may include pressure such as that produced by the circulatory system. The blood volume escapes a point source (e.g., the wound) flowing as a jet, in the short time it is in free flight the mass of the jet often breaks up as a function of air resistance and gravity. The breakup of the jet leads to both stable droplets and large undulating masses of blood, resulting in large spatter stains. If the stains land across a surface, the result is a linear pattern of relatively large volume spatter. When the jet is directed into the same area the result is a large accumulation. The interaction of the blood landing into this accumulation also produce spines and secondary spatter that radiate out from the primary pattern. Spatter stains deposited in linear form are referred to as “spurts,” large accumulations with irregular margins are called “gushes.”

Blood dispersed as a function of accelerated motion. When a volume of blood adheres to an object and the object is put into accelerated motion (e.g., swung), the result is a linear pattern of spatter. The inertial force imparted by movement of the object overcomes the adhesive force holding the blood to the object, this results in the release of either a small jet of blood flowing off of the object or individual droplets that shear directly from the object’s surface. In the case of the small jet, this structure shears into small droplets as well. As the small droplets are released from the object over time and space, they land across surrounding surfaces and are deposited in linear orientations. These patterns are referred to as “cast-offs.”

Blood that accumulates or flows on a surface. When a volume of blood collects and/or flows across a surface it produces patterns consisting of accumulations with generally regular margins. These are the simplest mechanisms to understand, as all humans are aware through common experience of the pooling and flow behavior of fluid. These patterns are known as “flows,” “pools,” and “saturations.”

Blood dispersed as a function of secondary contact with a surface. When blood is deposited or displaced on a surface through some form of contact it produces patterns of blood that demonstrate the interaction of the two surfaces. The blood may adhere to a non-absorbent surface or be wicked into an absorbent surface. As this surface interacts with another surface, the fluid is displaced on the original surface, transferred to a second surface or any combination of the two. These patterns are commonly referred to as “smears,” “wipes,” or “swipes.” An additional pattern produced by this mechanism is the “pattern transfer.”

Blood dispersed as a function of gravity. When blood drips from an object it produces patterns consisting of a relatively consistent spatter. The source may be replenishing (e.g., a bleeding individual) or non-replenishing (e.g., a bloodied object), in either case, gravity draws the fluid downward where it collects on some aspect of the object’s surface. As the volume of the collecting fluid increases, its weight forces it away from that surface resulting in a small column of fluid that connects the forming drop and the surface. When the weight of this mass at the end of the connecting column overcomes the surface tension of the fluid, it shears producing a large primary droplet and satellite droplets. If the dripping source is moving, the result is a series of similar stains deposited on a surface either randomly or in linear patterns. If the dripping source is static, the result is an “accumulation.” These patterns are commonly referred to as “drips,” “drip trails,” and “blood into blood.”

By recognizing these basic pattern types, the bloodstain pattern analyst recognizes the underlying source event of the pattern. This recognition allows the elimination the other events. For example, having found a castoff pattern, all other basic mechanisms (impacts, streaming ejection, drip, contact, and accumulations) are eliminated as having produced the pattern. The analyst then looks for what similar mechanisms were in play in that unique crime scene and through context attempts to isolate a specific mechanism, if possible. Whatever the situation, this analysis extensively limits the possible scene specific source events that must be considered.

As distinct as these pattern types are, not everything encountered in a scene fits neatly into a single type. Complex patterns are often encountered, patterns that exhibit characteristics of more than one of the six mechanisms. Even in the instance of a complex pattern, when the basic pattern types are discernable within the complex pattern, the bloodstain pattern analyst can still eliminate certain actions as a source for the pattern. Once a stain classification is identified the stain is then considered in a scene specific context. These basic patterns are described in detail further in the chapter.

The principle of stain shape and vector correlation

The principle of stain shape and vector correlation is a significant consideration in the examination of a variety of stains found in crime scenes. It involves two primary sub-principles: directionality and impact angle. The general principle can be articulated as:

The shape of certain bloodstains provides indicators as to the direction of deposition as well as to the spatial origin of the blood.

The first sub-principle is Directional Angle. This principle is articulated as:

The collapse of a free flight droplet on a surface produces a stain with a circular or elliptical shape. These stains may have spines, scallops, or satellite stains. These characteristics if present, may define direction of travel for the droplet at the moment of impact.

An examination of the individual spatter stains (stains that are shaped as small ellipses or circles) within any pattern will likely establish the direction the droplets were traveling at the moment they struck a surface. This is referred to as directionality, the directional angle or the gamma angle of the stain. This determination is based on the collapse of the fluid droplet; the resulting long axis of the stain; and the creation of scallops, tails, and satellite stains that appear on or around the stain. In elliptically shaped stains, these scallops and satellites will appear on the side opposite the initial contact point of the stain. In the more spherically shaped stains, the scallops and tails will appear in a heavier concentration on the side opposite the initial contact point. See Figure 1. By considering the directionality of a number of stains in a pattern, the technician can visualize the general area from which the droplets originated. The reverse vectors defined by the individual stains’ directionality may (if the stains are related) converge in the scene. This convergence is a two-dimensional area referred to as the pattern’s convergence point, as seen in Figure 2.

The second sub-principle is Impact Angle. This principle is articulated as:

The collapse of a free flight droplet on a surface produces a stain with a circular or elliptical shape. The ratio between the length of the long and short axes of the resulting stain has an empirical relationship to the angle at which the droplet struck the target.

Figure 1. Directionality of a bloodstain is defined by both the long axis of the stain and the presence of scallops, tails, and satellite spatter. These small characteristics appear in a greater concentration on the side opposite where the droplet first struck the surface. The directionality of the stain in the inset photo is upward and to the left. The origin of the stain then must be somewhere along a line aligned with this directional angle, but extended in an opposite direction.

Figure 2. By visualizing a line aligned with the directional angle and extended back into the scene for a number of related spatter, a point of convergence will form where the lines cross each other on a surface (e.g., a floor). The area of origin for the pattern would be located in three-dimensional space above the convergence point.

When a stable droplet strikes a surface creating a well-formed bloodstain, the resulting stain has both a major and a minor axis. Victor Balthazard demonstrated in 1939 that there was an empirical relationship between the ratio of the length and width of a stain and its impact angle, establishing the basis of the principle. In 1982, MacDonell expanded on the principle offering a mathematical model for determining this relationship.

In practice, given a suitable stain and surface, the crime scene technician can establish the approximate angle at which a droplet struck a surface. A droplet of liquid in flight is spheroid. As a result, any measurement of the diameter of the droplet will be equal. Refer to Figure 3. As the droplet collides with a target, a right triangle can be visualized. This triangle is visualized by considering the diameter of the droplet, the path of the droplet, and the area on the target where the droplet first touches and then terminates movement. See triangle abc in Figure 3. The internal angle of lines acb in this triangle is the angle of impact (the same as angle i in Figure 3). If we transpose the triangle and compare it to the bloodstain, there is an analogy and relationship between the two, as seen in Figure 4. The major axis of the bloodstain is analogous to the hypotenuse of the triangle (line bc), and the minor axis of the bloodstain is analogous to the side opposite (line ab). Using the trigonometric relationship Sin and the two known measurements from the bloodstain, the technician can solve for the unknown angle (angle i), as seen in Figure 5. As confusing as all of this might sound, the act of determining the angle of impact is quite simple in practice. The technician simply measures the long and short axis of the stain. This measurement does not include any portion of the tail, scallops, or satellite spatter. The technician then divides the short axis by the long axis, which will always result in a number of 1 or less. Using a scientific calculator, the technician then determines the inverse sine of this number, which is the angle of impact. See Figure 6. This same relationship is true of defects created by bullets, which was discussed in the Chapter 12. The impact angle for any given stain can also be approximated by simple examination of the shape of the stain. The more circular-shaped stains indicate impact angles between 70° and 90°. Bear claw-shaped stains usually indicate an impact angle between 40° and 60°. Long, elliptically shaped stains indicate acute angle impacts of 30° or less. See Figure 7.

Figure 3. The vector a droplet is following combined with the surface it impacts creates a right triangle (abc). By drawing certain relationships between the stain and this right triangle, the technician can identify the angle of impact (angle i ).

Figure 4. The major axis of the stain is analogous to the hypotenuse (line bc). The minor axis of the stain is analogous to the “side opposite” portion of the triangle (line ab). A measurement of the stain’s width and length provides two known values that allow the technician to solve for the unknown angle.

Figure 5. The formula used to define the impact angle is based on the trigonometric function of SIN. The inverse SIN of the angle at i is equal to the minor axis divided by the major axis.

Figure 6. A verbal explanation of the impact angle formula may seem complicated. In practice, however, it requires a simple division of two numbers and the conversion of that number using a scientific calculator. The width of the stain divided by the length will result in a number equal to 1 or less. The inverse SIN of this number is the impact angle.

When we combine the information offered by the two sub-principles of Directional Angle and Impact Angle, they set the basis of a corollary called Area of Origin. This corollary can be stated as:

Considered together the impact and directional angles for a number of stains associated with an impact event (a point source dispersion of blood) may define the origin of the stains in three dimensions.

Figure 7. There is an empirical relationship between the shape of a stain and the associated impact angle. The more elliptical the shape the more acute this angle is. The more circular the shape, the more likely it is that the droplet struck to a 90° angle.

If the analyst can establish both the directional angle and the impact angle for a well-formed stain and do so for a number of such stains associated to the same impact event, the vectors defined by these angles will usually converge in three-dimensional space. The area where the stain vectors converge provides indications of where the corresponding droplets originated their flight. See Figure 8.

Figure 8. By considering the directionality and impact angles of several related spatter stains, the technician can establish an area of origin for the pattern.

Information derived from directionality, point-of-convergence, and an area-of-origin analysis can assist the crime scene technician in understanding the direction and origin of the event that created the impact pattern and can effectively place individuals in the scene at discrete moments in time during the crime. This process is generally known as “stringing” where strings are oriented in line with the impact and directional angles of the involved stains, isolating this area-of-origin. With the advent of software, stringing may involve physical stringing or virtual stringing within such software.

A final aspect of this principle is that various stains provide indications as to motion of deposition. Swipes, wipes, and flow stains often assist in understanding the motion associated with deposition.

The physically altered bloodstain principle

The final principle used in BPA is the Physically Altered Bloodstain or PAB Principle. This principle can be articulated as:

Once exposed blood will react to environmental conditions (e.g., air flow, temperature, humidity, variations of surface) in a predictable manner.

This principle is a simple recognition that blood dries, wicks, permeates, and coagulates in and on surfaces in a somewhat predictable fashion. That information may be probative to the investigation. For example, it may allow an estimation of the volume of blood in a scene or allow estimation of the minimum time since the blood was first exposed. There is significant research on-going in the area of PABS in an effort to better understand this complex aspect of BPA.

These three principles are the basis of all action and analysis accomplished by the bloodstain pattern analyst. Any conclusion offered with regard to BPA must be consistent and within the limitations imposed by these principles. The analysts practically apply these three principles using a functional methodology.

This article appared in the Winter 2018 issue of Evidence Technology Magazine.
Click here to read the full issue.

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