BLOOD SPATTER EVIDENCE
AND RECONSTRUCTION OF EVENTS
Forensic Consultant Services
P. O. Box 11668
Fort Worth, TX 76110
With increasing frequency the legal practitioner is turning to expert testimony to establish what might have happened in a given situation. Engineers and police officers have for many years offered testimony about automobile accidents based upon crush factors, road conditions, skid marks, and other bits of evidence. More recently other areas of expertise are being explored in an attempt to reconstruct a sequence of events in a given questioned fact situation. One area of increased activity among experts is the use of blood spatters and other blood patterns to provide information about possible scenarios in crime scene investigation and other situations, as well.
In the overall picture, bloodstain interpretations are but one means by which investigators attempt to understand the events that transpired in a given setting. The study might be considered part of a field of what is perhaps erroneously called "crime scene reconstruction." It is, of course, impossible to "reconstruct" or "recreate" a crime or other event with unfailing accuracy. However, careful examinations of bloodstains, as well as other types of physical evidence, might well allow a qualified examiner to determine the possibilities of what might have happened, as well as to determine the impossibilities of other explanations.
Bloodstain pattern interpretation has been a much-neglected area of crime scene investigation until recent years. While numerous documented uses were made of bloodstain patterns even in the nineteenth century, relatively little had been done until Paul Kirk presented his findings in an affidavit concerning the bloodstain evidence in the highly publicized murder case against Dr. Samuel Sheppard in 1955. A wealth of empirical data were published in 1971 by Herbert MacDonell under the title "Flight Characteristics and Stain Patterns of Human Blood." Further information was presented in a later book. Others have subsequently done considerable work in the field. In recent years bloodstain recognition and preservation have been stressed by many instructors in the training of police officers and crime scene technicians.
THEORY OF BLOODSTAIN EVIDENCE
As much as human blood varies from one individual to another, as shown by serological differentiation and more particularly by DNA analysis, the physical properties are, nonetheless, quite uniform. The underlying basis of the theory is that the uniformity of blood produces uniform behavior when bloodstains are formed. This uniformity lends itself to a reproducibility that undergirds the "reconstruction" of events by bloodstain interpretation. This basis is grounded in the laws of physics.
The physical properties of blood that produce this uniformity give rise to phenomena including the following.
THE BLOOD DROP IN FLIGHT AND ON IMPACT
The drop is formed and accelerated by the action of some force or forces that pull it from the body or the mass of blood. Gravity begins accelerating the drop downward, and the trajectory will take the drop to the floor or ground or whatever other object intervenes in its flight. While in flight, the drop is held together by the surface tension of the blood. This surface tension, caused by intermolecular attractive forces, serves as a "skin" to hold the drop intact. A useful analogy is a water balloon. The surface tension holds the drop intact, much as the rubber will hold the water inside the water balloon. Upon impact, the drop will go through considerable deformation and contortion, but it will be pulled back together by the surface tension, unless a sufficient force "breaks the skin." The rougher the surface texture on the impact surface, the greater the chance of the "skin" being broken, allowing the drop to break up into smaller drops. In general, a rougher surface will cause a larger stain and will be more likely to cause the drop to break up. Figure 1 shows the influence of different surface textures on how the final stain will appear.
DETERMINING THE DIRECTION OF BLOOD DROPS
When drops hit a surface, the resulting stains will have varying shapes, depending upon the angle at which they impact the surface. In a perpendicular collision, a round stain is observed. As the impact angle decreases, the bloodstain that results is more elongated. The elongation, if pronounced, gives a teardrop shape to the stain. When this teardrop pattern is observed, the point of the teardrop points in the direction of travel of the drop at the time of impact (See also, however, secondary cast-off stains, below). See Figure 2.
VELOCITY OF BLOOD DROPS
The velocity of a blood drop will be a factor in the size of the resulting bloodstain. All other things being equal, higher velocity will result in a larger stain, on a given surface. However, it must be noted that the velocity is generally much less important in determining the size and shape of the stain than is the surface texture. A comparison of blood dropped from different heights onto a given surface is shown in Figure 3.
TYPES OF BLOODSTAINS
Bloodstains may first be classified as being from blood in flight or from direct transfer from one surface to another. Blood in flight gives rise to the following types of bloodstain patterns:
Bloodstains formed by direct contact include:
Blood in flight: low-velocity blood
This type of stain results from blood drops formed only by the force of gravity. It is accelerated downward at a rate of 32 feet/second/second until it acquires its terminal velocity. The drop will always have a volume of 0.05 mL. On a perfectly smooth surface the drop upon perpendicular impact will form a circular stain that will spread in proportion to the speed of that impact; higher velocity drops will cause bigger stains. The size of the drop, then, indicates the velocity, which in turn will indicate a height from which the drop fell, since gravity is the only acceleration factor. Note, however, that the drop, whose terminal velocity is 25 feet/second, will attain this velocity after having fallen for less than one second.
Consider also that the impact with a non-horizontal surface will result in an appropriately elongated stain. In this case the point of the resulting teardrop will indicate the downward side of the surface. Further, if the drop falls from a moving person, it will have some horizontal velocity as well as the vertical acceleration of gravity. This, too, can cause some elongation of the stain.
Blood in flight: medium-velocity blood
Stains in this category are formed by forces greater than gravity, but less than the forces that form high-velocity patterns, described below. Stains most commonly classified in this group result from the forceful impact of one object with another, with at least one being bloody. This type of stain is often seen in the crime scene setting involving bludgeoning or stabbing. Since the force required to form the stain is greater than the force of gravity, smaller drops will result. It naturally follows, then, that smaller drops observed at the scene indicate a greater force, indeed. See Figure 4.
The different types of actions that can lead to medium-velocity stain patterns are nearly limitless. Basically any type of forceful contact that will exceed the force of gravity and yet be less than the impact of a fired bullet will cause the formation of a medium-velocity stain.
Blood in flight: high-velocity blood
This type of bloodstain is strictly defined by the size of the resulting drops; the majority of drops in a high-velocity or atomized stain will have a diameter of less than 1 mm. A simple, cursory glance at such a stain might reveal many drops of greater diameter, and there is a tendency to give greater weight to those larger drops that tend to dominate the pattern visually. However, a detailed examination of the stain will reveal that most (>50%) are 1 mm or smaller. Such a stain requires a great force to break up the blood to this degree. In a typical crime scene setting, the only force encountered sufficient to atomize blood is that which results from a fired bullet. As the bullet strikes the source of the blood (typically a body), it atomizes the blood into a fine spray. These small droplets have small mass and thus low momentum; they generally will not travel downrange laterally farther than two feet. Back spatter of atomized blood may also be observed, which will carry the droplets uprange in the direction of the shooter. See Figure 5.
Blood in flight: cast-off blood
When a bloody object (for example, a bloody nightstick) is swung, centrifugal force tends to fling the blood from the object. This cast-off blood will tend to form a track of blood drops. A study of the overall pattern of the blood track will tend to define the arc of the swing. A closer inspection of the individual drops will indicate the direction of the swing. It should be noted that, typically, when a person is struck with a blunt object, the first blow will not transfer blood to the object, due to its recoil. Subsequent blows struck in the same area after the skin is broken will transfer blood to the object, which can be flung off on the backswing. Each separate swing of the instrument will produce its own individual track of blood. If one counts the number of separate tracks of blood, and adds one to account for the first blow, the minimum number of blows delivered will be determined. Also, a detailed study of the blood drops along the track will indicate which ones hit at 90 degrees, which will tend to locate the position of the person who swung the instrument. See Figure 6.
Blood in flight: secondary cast-off blood
When cast-off blood hits a wall at a low angle, secondary droplets, or satellites, may form. Unlike the primary drops, the teardrop of these secondary drops will not point in their direction of travel; rather, they will point backward to the primary drop from which they originated. A detailed study of the cast-off pattern will allow the investigator to determine which drops are primary drops and which are secondary. A secondary drop in a cast-off pattern is shown in Figure 7.
Blood in flight: ricochet blood
When a blood drop hits an edge or corner at a low angle, the drop often will break up, with a smaller amount continuing onward. In such a case, the blood that ricochets from the surface will form its own stain in another area downrange. When this occurs on a wall near a corner, the association of the ricochet with the original stain usually doesnt present a problem. If the ricochet hits some distance away, of course, the interpretation is more complicated. See Figure 8.
Blood in flight: projected blood
This type of stain results when a relatively large mass of blood is flung, or projected, against a surface. Such a stain can result from blood being thrown from a container, from being forced from a hypodermic syringe, or, more commonly in a crime scene setting, from arterial bleeding. When arterial bleeding is seen, it generally forms a continuing track of "pulsing" stains to indicate the path of the bleeder. Such bleeding would, of course, cease with the death of the bleeder. See Figure 9.
Blood in flight: blood on blood
These stains result from dropped blood continuing to drop into the same area so that the drops hit in pools of blood. When a drop of blood hits a standing pool, a substantial amount of breakup occurs, so that the resulting stain will typically show a number of very small drops, each tracing back to the pool from which it came. Such a stain could result from a person standing in one location while blood drops from an open wound. See Figure 10.
Contact stains: transferred blood
When a bloody object is touched against a surface, blood will be transferred to that surface. Often, inspection of the transfer pattern will be helpful in determining what type of object touched the surface. For example, a bloody handprint is often easily interpretable. Often, fabric impressions or footwear imprints will be transferred. Study of these transfer patterns in the context of the crime scene may be useful in determining the locations and movements of persons at the scene. See Figure 11.
Contact stains: smeared blood
Reason dictates that a transferred stain will be associated commonly with a smeared stain on the bloody surface which gives rise to the transfer. This, then, is the smearing of blood on an already-bloody surface by touching against another surface. Like the corresponding transfer pattern, these stains may be useful in determining locations and movements of persons or objects. See Figure 12.
Contact stains: absorbed blood
When blood is deposited onto an absorbent surface, such as fabric, the blood is absorbed into the cloth, and the stain spreads by the process of capillary action. The resulting stains will have been affected, then, by this absorption of blood. Interpretation of the resulting stain should account for any absorption that would change the nature of the stain.
Contact stains: running blood
When blood is deposited on a non-horizontal, nonabsorbent surface, there is a tendency for the blood to run downhill. This is, of course, dependent upon the size of the pool on the surface, the surface texture, and the angle of inclination form the horizontal. This running changes the bloodstains and must be considered in the interpretation. It is sometimes useful to note that blood appears in a temporal crime scene setting to have run uphill. Since this is impossible, it points to the rearrangement of the scene or object after the blood was deposited, but while the blood was still wet. See Figure 13.
Contact stains: pooled blood
By continued dropping or flowing of blood, substantial pools sometimes collect. The pooled blood will spread by flowing into the lower areas. It is useful in some cases to consider the volume of blood in the stain, since large pools will indicate substantial bleeding at a given location. This again can explain the locations and movements of persons or objects.
CLOTTING OF BLOOD
Within a few minutes after it leaves the body, blood can begin the clotting mechanism. As the clots form the blood begins to retract from the edges of the pool, leaving the yellowish serum at the periphery. In some cases the time of pooling can be estimated due to the degree of serum separation.
POINTS OF ORIGIN AND CONVERGENCE
The use of simple trigonometry allows the examiner in some cases to determine the point of origin of multiple drops. Study of the elongation of a well-defined stain allows one to determine the impact angle. Similar examinations of other drops that are demonstrably associated with the same event lead to a point of convergence. This common point can be established as the point of origin for several drops.
CHEMICAL ENHANCEMENT OF BLOODSTAINS
There are many chemical reagents that will react with some of the components found in blood to form colored complexes or to luminescent species. Many of these tests are extremely sensitive, so that even stains that are invisible before treatment are readily visible after the enhancement. Some of the more commonly employed chemical reagents are luminol, leucomalachite green, ortho-tolidine, tetramethylbenzidine, and phenolphthalein. Each of these reagents may be used as presumptive tests for blood; that is, a positive response upon treatment with the reagent allows one to presume that the stain is blood. Each, however, should be treated as only a presumptive test, and not as an absolute identifier of a stain as blood.
While in the interpretation of these tests results must be used with care in the identification of blood, they are, nevertheless, very useful in the documentation of hidden bloodstains at the crime scene. In many violent crime settings, the identification of blood at the scene is of relative unimportance; there may be large and very obvious bloodstains. A bloody handprint is still a handprint. If it is used to identify the suspect or to define the movement of a participant, that may be much more important than deciding if it is blood, given that the scene is covered with blood.
INTERPRETATION OF BLOODSTAINS AT CRIME SCENES
All bloodstains found at the crime scene are there because of the specific events that occurred. The examiner must study all of the stains found, both individually and in their contextual relationship to the other stains. As always, the overall "big picture" can be understood only after careful, painstaking analysis of the tiniest parts.
EXPERT TESTIMONY FOR BLOOD SPATTER EVIDENCE
Texas trial and appellate courts have allowed expert testimony regarding blood spatter interpretation. The field of blood spatter examination is ill-defined, as compared to some other disciplines. There seems to be no recognized minimum amount of education and experience that would qualify one to testify as an expert. There are crime scene technicians and investigators who lack any formal scientific education that have attended courses and conducted research into blood spatter evidence. Their testimony has been admitted in many instances, the trial courts having deemed their credentials to be adequate to qualify as experts.
As in other areas, two experts may arrive at different conclusions after study of the same bloodstain patterns. In some cases there might be more than one hypothesis that could account for the deposition of the observed stains. If both experts are properly qualified before the court, it becomes a matter for the fact trier to decide issues of credibility and inferences that may be drawn from the testimony.
COLLECTING AND DOCUMENTING BLOODSTAIN EVIDENCE
Assuming that the crime scene will be altered considerably as a function of time, an accurate interpretation of bloodstain evidence requires that all relevant stains be located and exactly oriented. This can be done by a combination of (1) general and close-up photography, (2) video recording, and (3) measurements and diagrams. Photographs of bloodstains that are properly scaled with the inclusion of a suitable reference (generally a ruler) and that show no distortion are valuable tools of interpretation. All scale photographs should be taken with the film plane parallel to the target surface. Photographs with no scaling or with spatial distortion may be of limited value.
Particular problems commonly observed in crime scene photographs include too few photographs, no scale photographs, distortions, and lack of photographs of important areas. Crime scene technicians frequently fail to photograph ceilings, which often have valuable blood-spatter clues, especially including cast-off patterns.
Eckert, W. G., and James, S. H., "Interpretation of Bloodstain Evidence at Crime Scenes," New York: Elsevier, 1989.
Kirk, P. L., "Criminal Investigation," 2nd Ed., J. I. Thornton, ed., New York: John Wiley & Sons, 1974.
MacDonell, H. L., "Flight Characteristics and Stain Patterns of Human Blood," Washington: U. S. Government Printing Office, 1971.
MacDonell, H. L., "Bloodstain Pattern Interpretation," Corning, NY: Laboratory of Forensic Science, 1982.