CHAPTER 11 BLOOD SPATTER ANALYSIS
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Chapter 11 Blood Spatter Analysis
Blood spatter analysis is based on mathematics and physics, which can be used to determine the velocity and angle with which blood has hit a surface, as well as the direction from which it came. Experts can also determine what type of impact caused the blood, based on the size of the blood drops.
Once these basic facts are known, however, the job of the analyst becomes more complex. There may be blood drops from more than one source that are intermingled. As the analyst attempts to reconstruct the likely position of people and objects, the science of blood spatter analysis moves into a more speculative realm.
In 2009, the National Research Council identified bloodstain pattern analysis as an analysis that requires multiple types of scientific and experience qualifications:
• an appropriate scientific education;
• knowledge of the terminology employed (e.g., angle of impact, arterial spurting, back spatter, cast-off pattern);
• an understanding of the limitations of the measurement tools used to make bloodstain pattern measurements (e.g., calculators, software, lasers, protractors);
• an understanding of applied mathematics and the use of significant figures;
• an understanding of the physics of fluid transfer;
• an understanding of pathology of wounds; and
• an understanding of the general patterns blood makes after leaving the human body.1
Needless to say, most blood spatter "experts," particularly those who have gained experience as police officers, lack qualifications in all of these areas. However, today, most courts accept the reliability of blood spatter analysis and allow questions of qualifications to be handled by cross-examination. However, some courts have refused to let experts testify about blood spatter, when the conclusion of the expert seems speculative. Some experts are permitted to testify as to a hypothetical set of facts, but are not permitted to conclusively relate the reconstruction to the defendant.
Blood spatter experts are frequently used when a defendant offers a version of facts, typically one in which the victim was the aggressor, which the blood spatter shows is inconsistent with the facts.
Experts are generally not permitted to testify about experiments they conduct to recreate the crime scene unless the court is satisfied that the conditions are exactly the same. Some police investigators have not been permitted to testify about crime scene blood on the theory that they lack training and experience in blood spatter.
Based on the chapter, students will be able to:
1. Understand and explain how the measurement of blood spatter can identify angle and speed.
2. Explain the difference between low, medium, and high velocity blood spatter.
3. Define cast-off, arterial spurt, and transfer pattern.
4. Explain methods blood spatter experts use to reconstruct events at a crime scene.
5. Explain legal issues with admissibility of blood spatter testimony at trial.
6. Identify the reasoning behind cases that hold blood spatter testimony is reliable and those that hold it is not.
7. Find ways in which blood spatter testimony can refute a defense of self-defense.
8. Explain why the court in Crawford v. County of Dauphin admitted some blood spatter testimony and refused to admit other testimony.
9. Discuss the blood spatter issues in the case of Sam Sheppard.
We tend to think that blood spatter (that's spatter, not splatter) evidence is basically a matter of physics. The scientist measures the blood spots, determines the speed by which they were deposited and from what direction, and then can reconstruct the crime.
Spattered blood is defined as a random distribution of bloodstains that vary in size and that may be produced by a variety of mechanisms. The quantity and size of spatters produced by a single mechanism can vary significantly, depending on the quantity of available blood.... Spatter is created when sufficient force is available to overcome the surface tension of the blood.2
Figure 11.1 Low velocity blood spatter, courtesy Connecticut State Police, Department of Records.
In practice, the process is not so simple or certain. Here is what Louis L. Akin, a licensed professional investigator with 23 years' experience in crime scene reconstruction, says:
[Blood spatter analysis is the term] most commonly used to describe the process of examining bloodstains at crime scenes for the purpose of determining what happened to who by whom. ... However, the procedure is far more akin to a tracker reading a trail sign than a hematologist working in a lab. The analyst interprets the evidence at the scene just as if it were tracks in the same.
In fact, the analyst uses every item of evidence at the scene, as well as the autopsy reports, the police reports, witness statements, and knowledge that he brings to the scene himself such as knowledge about the dynamics of the behavior of blood, knowledge of guns and ballistics, and knowledge of wounds to the human body. The analyst looks at the evidence, and based on what he sees in the blood spatter patterns and other evidence, makes a pronouncement about what he, or she, believes happened. Seen in this light, blood stain analysis is more of an art than a science and is always open to interpretation.3
This does not mean that the laws of physics are not important in the reconstruction, but it does mean that if the scientist is influenced by facts beyond the pattern of blood spatter itself, the results are going to be influenced by those facts.
Blood Spatter Analysis — What Is the Scientific Hypothesis?
All blood pattern analysts agree on certain fundamentals based on science and mathematics.
A free-falling drop forms a sphere or ball. The sphere breaks up when it strikes another object or when acted upon by some other force. The angle of impact can be determined by looking at the shape of the drop. A drop of blood striking a surface at right angles will produce a nearly circular stain. As the angle decreases, the stain appears more elongated. Therefore, the shape of blood drops can indicate the angle of impact. The tail of blood spatter drops will point in the direction of travel.
Therefore, just looking at the drops can tell the angle of impact and the direction from which the blood was moving.
Another part of the analysis of blood spatter involves the fact that blood travels through the air at different speeds, depending upon the force with which the blood is expelled from the body. The resulting size of the droplets will determine the speed, and therefore can be linked to the kind of weapon or object that produced the blood flow.
Blood spots are typically grouped into three types, depending on the different velocities at which the blood travels:
• Low velocity stains are produced by a force less than 5 feet per second (fps), which is the force of gravity. The resulting blood drops are 4 mm or larger and circular.
• Medium velocity stains result from a force between 5 fps and 25 fps. They are 1 to 3 mm in size. This kind of stain is commonly believed to result from blunt force trauma such as that caused by a fist or baseball bat.
• High velocity spatter is produced by a force of greater than 100 fps. Those stains are generally less than 1 mm. This pattern is usually the result of a gunshot or explosive. As the drops are small, they do not travel far because of the resistance caused by the surrounding air.4
The size of the blood drops coupled with the impact pattern they create can help the investigator determine the object that caused the bleeding. In addition to the size of the drops, the analysts must examine the pattern of the drops:
• The first pattern is called "arterial spurt." Arterial spurts are squirted arcs of blood caused by blood being pumped from the left ventricle of the heart. It starts with low pressure, increasing and decreasing with the rhythm of the heart muscle, and generally is concentrated near the victim, who typically dies quickly from blood loss.
• Cast-off patterns typically result from blood being thrown off a weapon. After the first penetration, the assailant may yank the bloody knife blade out of the victim. As he does so, the victim's blood is cast off the blade, forming a pattern on items in the surrounding environment: perhaps a ceiling. Successive stab wounds will add to the number of separate spatter patterns. Analysis of these patterns can help determine how many blows were struck. The total will be one more than the spatter patterns, as the first blow will not create cast-off. The pattern tends to be oval or elliptical as the weapon is swung in an arc. One of the most useful conclusions from cast-off analysis is based on finding a pattern in a place that is inconsistent with the defendant's version of what happened. For example, if the suspect says he was defending himself from attack, but has blood cast-off on the bottom of his shoes, the suspect was probably on his knees bending over a prone victim when the bleeding occurred.
• A transfer pattern occurs when a wet, bloody surface contacts another surface. The resulting pattern may indicate the source of the bloody surface. This is frequently the case with bloody fingerprints or handprints.
Investigators can reconstruct the crime by placing people and things at the scene based on the spatter patterns:
There is a mathematical correlation between the length and the width of these blood spatters that can be measured. We can then determine what angle they came in at and by using a set of strings and thumbtacks and large protractor we are able to reconstruct the scenes of crimes many times and actually place people where they were at the time they were injured ... or shot.5
Of course, most blood scenes are not neat, and a variety of patterns can occur together, combined with smears or swipes as the victim or suspect touches blood, struggles, or wipes blood off on clothes or objects...
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