Dealing With Ambiguous Bloodstain Patterns
Ross M. Gardner
Classification of bloodstain patterns based on recognition of specific criteria is a critical component of the bloodstain pattern analyst’s efforts, but it is just one part of a process that may allow the analyst to offer a conclusion as to the source of an unknown pattern found at the scene. Proper protocol demands we evaluate the stain and decide what kind of pattern it is, its classification, then we must consider the stain within the unique scene context.
There is no question that the more certain the classification of a scene pattern, the more certain any subsequent conclusion offered by the analyst will be. If the analyst has confidence the pattern was created by a streaming ejection that significantly limits the potential source mechanisms that may be present in the scene. But what if the analyst is not confident of the specific classification? Does that prevent them from offering a specific conclusion?
The answer is no, not always. Which brings us to the ever-present ambiguous pattern! Not all bloodstain patterns found in a given scene will demonstrate sufficient physical characteristics (taxon) that allow the analyst to isolate the pattern to an absolute classification. Substrate issues in particular and other variables often prevent the taxon from appearing. In those instances only a general or ambiguous pattern classification may be possible.
As an example, lets imagine encountering a series of spatter oriented in a line on a vertical surface (a doorway facing) where all of the stains are very elliptical, all are oriented with their long axes parallel to the ground and where there is no volume indicator present (e.g. no flows emanating from individual spatter). There are analysts out there who will and have testified that such a pattern “could be anything”, but that is not technically correct. The pattern is some form of linear spatter.
The mere fact that we have spatter eliminates all of the non-spatter mechanisms in and of itself. The presence of the linear characteristic eliminates any radiating or random form of spatter (e.g., impact, expiration, mist, drip) leaving us with only three potential source events. As the pattern is made up of linear spatter, the source event must be either: a form of drip trail, a form of cast-off, or a spurt. There are no other options.
Based solely on this classification of “linear spatter”, no further conclusion is possible. The stain itself is ambiguous and to force a more refined “classification” based on the limited taxon present would be subjective. But that is not the end of the story for the analyst. Context is a factor in offering any ultimate conclusion. Under what circumstances did we find the pattern and what, if anything, does that tell us?
How about considering a potential claim of drip trail? In our hypothetical, the spatter are oriented in a line, but their respective long axes are parallel to the ground. This demands the stains impacted while moving across the door face. Drips are a function of gravitational force acting on the blood mass, if associated with drips the long axes of the pattern would be oriented up and down in some fashion. The lack of this aspect effectively eliminates drip trail. That knowledge alone isn’t bad, we have now eliminated all potential source mechanisms except two: cast-off and spurt.
Lets further refine our context. Lets imagine the pattern is found on the outside facing of a doorway that opens inward. The door is standing open. The stains start on the door face near the hinges and move across the doorway towards the knob. Based on other stains, it appears the door was open when the bloodletting occurred.
Under this context, lets evaluate cast-off as a potential source event. If the pattern were caused by cast-off the directional aspect and location of the stains tells us the object was swung from outside the room, the stains impacting across the door at knob height. The individual stains in the pattern are all highly elliptical; cast-off rarely begin with highly elliptical stains (the patterns typically go from less elliptical to more elliptical), under this circumstance we can make a relatively valid prediction: If the pattern is cast-off, the there will be additional linear stains on the adjoining wall next to the doorway hinges. Before the spatter could be deposited on the door facing, droplets would have ejected from the object and struck this adjoining wall and that pattern should lead directly to the door pattern.
In our hypothetical scene no such staining is present. Does that absolutely refute cast-off? Not really. Potential explanations for this lack of stains are limited, but one possible explanation would be that something else was present blocking this adjoining wall (e.g., a person) and our predicted additional cast-off were deposited onto this unknown intermediate surface.
How about considering spurt in this context. If a spurt, the stains were directed from a position outside the door and ejected effectively in the same plane as the open door face. Under these circumstances, some valid predictions include: spatter stains may be present on the immediate adjoining wall, with their long axis oriented up and down, stains that were slightly offset and did not reach the door. If present these spatter may well demonstrate volume indicators. Additionally beyond the door, inside the room on the floor we could also expect to see spatter that struck the floor in line with the pattern on the door. Such stains might well have an evident directional component or they could be generally circular. These being spatter that were offset the other way and missed the door face completely and then simply continued on to the terminal aspect of their parabolic flight path, with the stains falling downward at the end of the arc.
In our hypothetical scene stains are found on the adjoining wall, at and below the height of the pattern on the door facing with the long axes oriented up and down. In one of the several stains present, there is an indication of volume. Circular stains are also present on the floor beyond the door. They are oriented in a line that is parallel to the pattern on the door.
Given such a context, the analyst should be quite comfortable in concluding the linear spatter pattern was caused by a spurt source. The lack of the additional indicators of cast-off and the presence of the additional indicators of spurt provide sufficient foundation for this conclusion.
So having begun with what was an ambiguous linear pattern and by considering context the analyst is able to isolate a more effective explanation for the pattern.
The pattern classification itself did not change; the pattern remains ambiguous, it is still just a “linear spatter” pattern. But the conclusion offered as to its source has changed dramatically by considering the context in which the pattern was found.
Unfortunately not every ambiguous pattern can be dealt with as effectively. At times an ambiguous pattern is just that, an ambiguous pattern. Analysts have to accept that and the reality that no effective conclusion may be forth-coming in such instances. The lack of specific taxon in any scene pattern and a subsequent ambiguous classification for that pattern may well prevent the analyst from offering any conclusion as to the source event. But keep in mind that classification is just one part of the analysis of a bloodstain. An ambiguous pattern may well be a hindrance, but it is not always a show-stopper. Once a classification is achieved, the analyst must look objectively at the unique context in which the pattern is found and applying all of their knowledge to determine if that context allows a better understanding of the pattern.