Bloodstain pattern analysis

Bloodstain pattern analysis has been used informally for centuries, but the first modern study of blood stains was in 1895. Dr. Eduard Piotrowski of the University of Kraków published a paper titled "On the formation, form, direction, and spreading of blood stains after blunt trauma to the head."[8][9] He conducted his first experiments on rabbits.[10] A number of publications describing various aspects of blood stains were published, but his publication did not lead to a systematic analysis. LeMoyne Snyder's widely used book Homicide Investigation (first published in 1941 and updated occasionally through at least the 1970s) also briefly mentioned details that later bloodstain experts would expand upon (e.g., that blood dries at a relatively predictable rate; that arterial blood is a brighter red color than other blood; that bloodstains tend to fall in certain patterns based on the motion of an attacker and victim).[11] A 1952 episode of the police procedural radio series Dragnet made reference to bloodstain pattern analysis to reconstruct a shooting incident.[12]

Between 1880 and 1957, courts in Michigan, Mississippi, Ohio, and California rejected expert testimony for bloodspatter analysis, generally holding that it added nothing to the jurors' own evaluations of bloodstains submitted as evidence.[13] In 1957, the California Supreme Court became the first American court to accept expert testimony examining bloodstains, accepting as evidence the testimony of Paul L. Kirk, a professor of biochemistry and criminalistics.[13] He would also testify in the Sam Sheppard case in 1966, when the wife of an osteopathic physician was beaten to death in her home, interpreting bloodspatter evidence as proof that the murderer was left-handed (Sheppard was right-handed).[13] However, bloodstain pattern analysis would not begin to enter wide use until it was promoted by Herbert Leon MacDonell. MacDonell researched bloodstains with a grant from the United States Department of Justice, and which also published his research in the book "Flight Characteristics and Stain Patterns of Human Blood" (1971).[13] MacDonell testified in court on multiple occasions as an expert of bloodstain analysis, and the legal precedent set by these cases led to its widespread use in American courts, although as early as 1980 some judges expressed strong doubts about its reliability, and it was not always accepted as evidence, especially in states with no prior rulings that relied on such evidence.[13]

The first formal bloodstain training course was given by MacDonell in 1973 in Jackson, Mississippi. MacDonell taught workshops on how to conduct bloodstain analysis, and the newly trained bloodstain analysts, who often had received as little as 40 hours of instruction, in turn would give expert testimony in court cases.[13] In 1983, the International Association of Bloodstain Pattern Analysts was founded by a group of blood stain analysts to help develop the emerging field of bloodstain pattern analysis.[14]

Starting in 1995, court cases where bloodstain analysts disagreed with each other raised concerns of the discipline's prediction's admissibility as evidence in court.[15][16][17] In 2009, the National Academy of Sciences published an examination of forensic methods used in United States courts which harshly criticized both bloodstain pattern analysis and the credentials of the majority of the analysts and experts in the field.[13][16] Judges have largely ignored the report's findings and continue to accept bloodstain pattern analysis as expert evidence.[13]

In 2013 Daniel Attinger, a fluid dynamics researcher at Columbia University, published a paper on bloodstain pattern analysis in Forensic Science International, finding that many of the central hypotheses of bloodstain analysis remain untested, and that existing analysts often made incorrect assumptions or other errors in their analyses. The paper also proposed fluid dynamics as a theoretical framework for solving these problems, and Attinger has continued to publish several papers exploring these concepts (as have other scientists as well). However, these papers are largely theoretical, and have had little impact on the use of bloodstain analysis in courts.[13]

Blood is composed of three components suspended in plasma: erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).[18]

When a blood vessel is damaged, the blood starts moving outside the circulatory system, following the laws of physics. If this leaking blood ever reaches the surface of the skin, it will be externalized and will likely come into contact with the person's surroundings. Some of the most common interactions are:[19]

• Gravity
• An internal source applying pressure on the blood (expiration, blood pressure, etc.)
• External force applied at the source of the bleeding (e.g., being hit with a weapon)
• Transfer or alteration resulting from two surfaces coming into contact (swipe, wipe, etc.)

Since blood is 55% plasma (and 45% solid components suspended in plasma), it is an aqueous liquid and behaves as such. More specifically, it is a shear-thinning, non-Newtonian fluid. Here are its three main physical properties:[19]

• Viscosity (denoted by μ) describes the relation between force applied on the fluid and the rate at which the fluid particles can be separated from each other, or deformed.[19] If a fluid has a high viscosity, it will not flow as easily as a fluid with a lower viscosity. Because this relates directly to the strength of the attraction between the fluid molecules, viscosity of blood decreases with increasing temperature and decreasing hematocrit. It also lowers with a higher shear, which is why it is classified as a shear-thinning, non-Newtonian fluid. This is why blood appears more watery when it hits a surface than right after it leaves the body.[19] The closer the shear rate is to zero, the more viscous the blood is.[20] Some other factors can also influence blood viscosity.[19]
• Surface tension (denoted by σ ) describes the energy required to change the shape of a fluid when it comes into contact with another fluid in which it is not miscible (e.g., blood and air). This plays a big role in the formation of spherical blood droplets as they fall, as well as the way they react when they hit a surface.[19] Surface tension can also be affected by outside factors such as temperature and the presence of chemicals in the blood.[21]
• Density (denoted by d) represents a fluid's mass per unit volume, which allows for comparison between substances. The reference for comparison is usually water since it has a density of 1 g/cm³.[22]

Cast-off blood stain pattern

Impact spatter is the most common bloodstain pattern type in a crime scene. It occurs when an object hits a source of blood.[29] In impact blood spatter patterns, blood is often circular and not elongated.[30] There are two types of impact spatter, back spatter and forward spatter.[29] Back spatter occurs when blood is projected back at an attacker, while forward spatter is blood that exits directly from the victim's wound and projects onto nearby surfaces.[29][31] The speed of the weapon used in the attack can cause changes in the size of blood spatter. The speed of the attack is classified into high, medium and low velocity attacks.[29] High-velocity spatter (e.g., gunshot wounds) create small-sized droplets. High-velocity spatter usually travels 100 feet per second and creates blood droplets sized 1 millimeter or less. Medium-velocity spatter (e.g., blunt force trauma) is often made with a weapon and can create cast-off patterns. They are often made at between 5 and 25 feet/second the blood droplets ranging from 1 to 4 millimeters in length.[32] Low-velocity spatters are usually created just as a result of blood dripping from the individual (i.e., gravity).[25] They can also be referred to as passive or gravity bloodstains, and are separated further into four categories: transfer/contact stains, flow artefacts, drop stains, and pooling.[33] Transfer stains occur when two surfaces come into contact and at least one has blood on it, and it includes swipe and wipe patterns, which can give information regarding sequence of movement in some cases.[33] Pooling occurs when the source of the bleeding remains static for a certain period of time, the blood continuously dripping in the same location and resulting in an important accumulation. If the individual who is actively bleeding moves while blood is dripping, the resulting pattern will allow for determination of direction and relative speed of movement at that time. Cast-off patterns are associated with impact spatter, these patterns are created when the object that is used in the attack is swung and the blood on the object is swung onto a nearby surface.[34] These patterns may be used to guess the direction of a weapon swing. In these cases, the length and the shape of the bloodstain patterns can help determine the speed of the swing.[35] These patterns create elongated or elliptical shapes in blood when it hits the surface of an object.[29] In 1895, Dr. Eduard Piotrowski's experiment showed that these patterns are often created after the second hit using the weapon.[10] In some cases, void or shadow patterns can be obsrved. It is the result of a person or an object shielding an area from the blood coming toward it, and it is characterized by a clean area where bloodstains are expected.[36] It can help determine if whatever intercepted the blood has been moved since the incident occurred.

A 1995 murder case against Warren Horinek was largely decided based on bloodstain evidence that has been hotly disputed after the fact.[15] The police and the district attorney's office believed in Horinek's innocence. The appointed attorneys for the prosecution found a bloodstain pattern analyst who testified that, rather than being a suicide as believed for a number of reasons by police, it was a murder due to the pattern of small blood flecks found on the accused, which according to the analyst had to have come from "high velocity" blood from a gunshot, rather than blood that simply got on him through his attempts to provide medical aid to the victim. Other bloodstain pattern analysts have since disputed this claim and said that the bloodstains were consistent with medical aid. The original analyst has conceded that his claim is not as strong as he originally presented it as being, although he still believes in Horinek's guilt.[15] As of 2017, Horinek remains in prison.[38]

In the criminal case against David Camm, who was tried three times for the murder of his family largely on the basis of blood spatter evidence, both prosecution and the defense used expert bloodstain pattern analysts to interpret the source of the approximately 8 drops of blood on his shirt. The prosecution's experts included Tom Bevel and Rod Englert, who testified that the stains were high-velocity impact spatter. Paul Kish, Barton Epstein, Paulette Sutton, Barrie Goetz, and Stuart H. James testified for the defense that the stains were transferred from his shirt brushing against his daughter's hair.[16] Dr. Robert Shaler, Founding Director of the Penn State Forensic Science Program, decried blood spatter analysis as unreliable in the Camm case. "The problem, in this case, is the number of stains [ ⁠is ⁠] minimal," he said. "I think you're really on the edge of reliability." All of the blood spatter analysts involved in the case are "experts" in the traditional sense. Quoting Shaler, "We have two opinions in this case. That, in essence, is a 50 percent error rate. " This would be ⁠an unacceptable level of reliability in a court case given that the perception of guilt beyond a reasonable doubt is what is required.[39]

Further complicating matters was the testimony of Rob Stites, who testified for the prosecution as an expert blood spatter analyst. It was later uncovered that he had no training and his credentials were fabrications by the prosecutor. His testimony that the blood on Camm's shirt was high-velocity impact spatter aided in the conviction of David Camm. Dr. Shaler pointed out that one limitation of blood spatter analysis testimony is that "you do not have the supporting underlying science" to back up your conclusions. When Stites testified, the jury had no way of knowing that he was not the expert that he purported to be. Even among the expert witnesses, it is unknown which set of experts interpreted the stains accurately as there is no objective way of determining which bloodstain pattern analyst has applied the science correctly.[40]

Other times, bloodstain patterns from different causes can mimic each other. In the 2008 trial of Travis Stay for the murder of Joel Lovelien, prosecution witness Terry Laber testified that the blood spatter on Stay's clothing came from blows to Lovelien during a fist fight. After a review of the evidence by Paul Kish, another bloodstain pattern analyst, Laber reviewed the report submitted by Kish and revised his findings to include the possibility that the blood came from expiration by Lovelien.[17]