If you're flipping channels one day and come upon a crime scene as depicted on one of the many TV shows that focus on forensic science, such as "CSI" or "Dexter," you might notice something strange. Among the technicians dusting for fingerprints and collecting hair fibers at the murder scene, you spot an array of red strings running from the floor, the wall, the table and the sofa. All of the strings meet at nearly the same point.
Suddenly, an investigator begins recounting aspects of the crime: When it happened, where the assault took place in the room, what kind of weapon the perpetrator used and how close to the victim the assailant stood. How could he have learned all that information from a webwork of strings?
The strings themselves aren't important. They're simply a tool to help investigators and analysts draw conclusions from a substance that's often found at crime scenes: blood. We've become used to hearing how specialists use blood samples to identify suspects through DNA. But the blood itself — where it lands, how it lands, its consistency, and the size and shape of its droplets, or spatter — can uncover a lot of significant aspects of the crime.
Of course, analyzing blood spatter isn't as simple as fictional bloodstain pattern analysts like Dexter Morgan make it appear. Experts in the field often say that it's as much an art as a science. Although important aspects of bloodstain pattern analysis are well supported by research, analysts must be careful not to overreach [source: Iowa State]. If the violent act involves multiple victims and multiple assailants, the challenge becomes even more complex. But a well-trained and seasoned analyst can provide key information that leads to arrest and conviction.
Let's start with the basics of bloodstain pattern analysis — for example, what blood spatters can reveal (and what they can't).
Basics of Blood
When a crime results in bloodshed, the blood left behind functions as evidence for investigators. However, a bloodstain pattern analyst can't simply glance at drips and smears of blood and immediately tell you the who, what and when of a crime scene. Blood spatter analysis takes time and provides only a few pieces of the total crime puzzle. Yet such analysis can corroborate other evidence and lead investigators to seek additional clues. After close analysis, blood spatters can provide important clues to aspects of the violent act such as [sources: Dutelle, James et al., Murray]:
- Type of weapon
- Velocity of blood
- Number of blows
- Position and movements of the victim and assailant during and after the attack
- Which wounds were inflicted first
- Type of injuries
- How long ago the crime took place
- Whether death was immediate or delayed
Blood spatters can guide the recreation of a crime thanks to the same laws of motion and gravity, physics and chemistry that govern all liquids. Blood travels in spherical drops because of surface tension, the tendency of liquids to minimize surface area because their molecules are attracted to one another — it's cohesive [sources: DLS, Rosina et al.]. Also, its drops behave in predictable ways when they strike a surface or when a force acts on them.
Consider what happens when you spill water: The liquid falls to the ground and makes a puddle. The shape and size of the puddle depends on the amount of liquid, the height of the container, and whether you spill on carpet, wood, linoleum or some other surface. In general, more liquid, or a fall from a greater height, will make a larger puddle. Moreover, droplets striking a hard surface will retain a more circular shape than those landing on carpet, which partially absorbs the liquid and causes the edges to spread [sources: Dutelle, Murray, Wonder]. These are but some of the many factors a blood spatter analyst must consider.
Blood behaves not unlike those spilled water droplets, and the speed at which the droplets travel when they strike a surface, known to analysts as a target, affects their shape. This speed, combined with angle and surface characteristics, also determines how far blood droplets skip or bounce after meeting a barrier.
One category of slow-moving blood, called "drips," occurs after, not during, an injury, and has a relatively large footprint of 0.16 inches (4 millimeters) or more. Drips can fall from a bleeding nose or wound, or a motionless, bloodied weapon or object. A moving object produces what's known as a cast-off pattern. Other low-velocity patterns include blood pooling around a victim's body and impressions left by bloody objects. This latter phenomenon, called a transfer, sometimesretains the shape of the object that made it [source: Wonder].
At the other end of the scale are the tiny droplets caused by blood travelling at high speeds. These are usually caused by gunshot wounds, but they can also result from explosions, power tools or high-speed machinery. These fast-moving drops leave stains measuring less than 0.04 inches (1 millimeter) across.
Bullet wounds can produce both back and front spatters. Backspatter, or blowback, refers to blood exiting the entrance wound in the direction opposite the impact [source: Dutelle]. (Actually, thanks to Newton's Third Law of Motion, backspatter can result from other impacts and traumas as well.) Investigators dealing with such small drops must rule out other sources of blood spray, such as respiration or pinhole arterial pressure [source: Wonder]. Forward spatter, in the direction of the impact, occurs only in the case of an exit wound [source: Dutelle].
Between these extremes lies a range of medium-sized droplets. Typically measuring 0.04 to 0.16 inches (1 to 4 millimeters), they can be caused by a blunt object, such as a bat or a fist, or can result from stabbing, cast-offs or even bloody coughs [sources: Dutelle, Wonder]. A number of factors complicate their analysis. For example, during a beating or stabbing, arterial damage can cause the subject to bleed faster or to spurt blood, the latter creating what's known as a projected pattern [source: Dutelle].
In addition to spatters, analysts look for voids, aka blockages. In the case of a high-density spatter, these gaps in the pattern indicate that something in the way, potentially the shooter, caught some of the victim's blowback.
Drop size is only one aspect used in analyzing blood spatters. Next, we'll look at the shapes of spatters and how analysts use strings, trigonometric functions and computer programs to map out a blood-spattered crime scene.
Stringing, Sine and Spatter Shapes
To analyze a bloodstain pattern, an expert relies on three main interrelated elements: the size, shape and distribution of bloodstains. The most visually striking part of the process, and therefore the one most featured in shows like "Dexter," is the technique of stringing. However, this technique can take place only once analysts grasp certain other aspects of the bloodstains. Since we've already discussed stain size, let's dive right into shape.
Blood drops that fall straight down with little but gravity and air resistance affecting them make round stains. Blood moving at an angle and sped along by some force, however, tends to make elongated marks, especially when it strikes a nonporous surface. As a rule, following the long axis of the stain from the blunter end to the sharper, more disturbed edge reveals the direction the blood traveled. If a number of stains radiate outward, analysts can draw lines backward along these axes to an area of convergence. But this gives them an area in only two dimensions; investigators must also determine how far above the floor, or away from a vertical surface, the area of origin lies [source: Dutelle].
The time has almost come to break out the string. First, however, investigators must determine the angles at which the various bloodstains struck their respective surfaces. The lower the angle at which blood strikes a surface, the thinner and more elongated the stain. The converse holds true as well [source: Dutelle].
For example, a 10-degree drop creates a highly elongated stain, whereas a 90-degree (vertical) blood drop leaves a round stain [source: Dutelle]. Thanks to a handy mathematical formula, analysts can use these measurements to calculate the impact angle:
Angle of Impact = ArcSin (Stain Width / Stain Length)
The greater the difference between the width and length, the sharper the angle of impact [source: Dutelle]. For example, imagine a bloodstain 0.08 inches wide by 0.16 inches long (2 by 4 millimeters). The width divided by the length equals 0.5. The ArcSin of 0.5 is 30, meaning the blood hit the surface at a 30-degree angle. In a bloodstain measuring 0.04 by 0.16 inches (1 by 4 millimeters), the impact angle comes out to about 14.5 degrees.
Once analysts know these angles, they can begin to run strings from the rear edges of the bloodstains upward at the appropriate angles to find where they roughly meet — the area of origin. This only provides an approximation, however, and is mainly used to establish whether a victim was seated, standing or lying down when the event occurred. The presence or absence of blood on other surfaces, combined with common sense, also aids in this analysis [source: Dutelle].
Increasingly, analysts are taking advantage of computer programs that allow them to store spatter data, calculate values such as impact angle, and display information in helpful 3-D renderings. Initially, this software required manual data entry, which was tedious but perhaps not as tedious as stringing. Software manufacturers hope to eventually be able to scan crime scenes with lasers or to automate the analysis of digital images, but many hurdles remain. For example, computers will need some way to register all crime scene photos to the same coordinate plane and angle and must acquire an expert's ability to distinguish secondary spatter from primary, among other judgment calls [sources: James et al., Shen et al.].
So far, we've discussed how bloodstain pattern analysis can work when trained law-enforcement officers implement it correctly. Next, we'll look at the history of bloodstain pattern analysis, as well as an infamous case that contains botched bloodstain pattern analysis practices.
History of Blood Spatter Analysis
The first methodical study of blood spatters, titled "Concerning the Origin, Shape, Direction and Distribution of the Bloodstains Following Head Wounds Caused by Blows," was published in 1895 by Dr. Eduard Piotrowski of the University of Krakow in Poland. This early research influenced pioneering investigators in early 20thcentury Germany and France, including Dr. Paul Jeserich and Dr. Victor Balthazard, respectively [sources: Brodbeck, Eckert and James, James].
Although research continued into blood spatter patterns in homicide cases, the watershed moment for using blood spatter evidence in American legal cases would not arrive until 1955, when Dr. Paul Kirk submitted an affidavit of his findings in the highly publicized case of the State of Ohio v. Samuel Sheppard. Kirk showed the position of the assailant and the victim, and his research revealed that the attacker struck the victim with his left hand. Significantly, Sheppard was right-handed [source: Eckert and James].
The field saw vast expansion and modernization through the work of innovative forensic scientist Herbert MacDonell, who published "Flight Characteristics of Human Blood and Stain Patterns" in 1971. MacDonell also trained law-enforcement personnel in blood spatter analysis and developed courses to continue to train analysts. In 1983, he and other attendees of the first Advanced Bloodstain Institute founded the International Association of Bloodstain Pattern Analysts (IABPA). Since then, the field of bloodstain analysis has continued to grow, develop and become standardized [source: Eckert and James].
Prior to the 1970s, blood analysis used a system of categories based on the velocity of blood drops at impact [source: Wonder]:
- Low-velocity impact spatters (LVIS) that resulted from dripping and were assisted by gravity alone
- Medium-velocity impact spatters (MVIS), which were slower than those produced by a gunshot but faster than gravity drips
- High-velocity impact spatters (HVIS), produced by gunshots or fast-moving machinery
After the 1970s, these definitions changed. Instead of "impact" referring to the speed of the droplets, it came to refer to the speed of the weapon or object that sent them flying. These new interpretations assumed too many unknown (and unknowable) factors. Moreover, they tempted investigators to make assumptions based on outside information — for example, to assume that droplets were HVIS because the case involved a suspected shooting. To cope with these problems, analysts today use more specific terms. LVIS, for example, might be called "gravitational drops" or "drips" [source: Wonder].
Bloodstain Pattern Analysis in Action: The Chamberlain Case
One infamous case that comes to the minds of many people when thinking about blood spatter analysis involves a line that has since become a pop-culture catchphrase (thanks to Meryl Streep in "A Cry in the Dark" and Julia Louis-Dreyfus on "Seinfeld"): "The dingo ate my baby."
In August 1980, the Chamberlain family camped near Uluru (formerly known as Ayers Rock) in the Red Centre desert of Australia's Northern Territory. One night, Lindy Chamberlain put two of her children, 4-year-old Reagan and 9-week-old Azaria, to bed in their tent. When she returned, the story goes, she cried, "The dingo's got my baby!" [source: Latson]
According to Lindy, when she got to the tent she saw a dingo with a bundle in its mouth. She wasn't close enough to see what it was, but when she checked on the children she saw that her daughter Azaria was missing [source: Haberman]. As the cry went out, she and her husband, Michael, along with other campers, began searching for the child. A nearby camper, Sally Lowe, went into the tent to check on the still-sleeping Reagan. Seeing a pool of wet blood on the floor of the tent, she thought that Azaria was probably already dead [source: Linder].
When a tourist found the baby's jumpsuit, it was only slightly torn and bloody, but mostly intact. Though an initial investigation backed up Lindy Chamberlain's claim of a wild dog attacking her daughter, it was not long before the parents themselves stood accused [source: Haberman].
The baby had been wearing other clothes that weren't found at the time [source: Latson].
Throughout the case, the local police improperly handled blood spatter and other evidence. Forensics investigators found "blood stains" in the family car and concluded that Lindy had taken Azaria there to cut her throat. Later analysis revealed that the stains came from a spilled drink and a sound-deadening compound that came with the car. One expert identified a "bloody hand print" on Azaria's jumpsuit that later analysis revealed to be red desert sand. However, in 1982, expert testimony — and public opinion — proved enough to convict Lindy Chamberlain of murder and her husband of being an accessory to murder. The baby's knit jacket, found in 1986 near a dingo lair, helped exonerate the Chamberlains after Lindy had served three years of a life sentence, but several years of trials and hearing were yet to come [source: Latson]. In 2012, 32 years after the event, a coroner finally pronounced that a dingo was responsible for the death [sources: Haberman, Latson].
The Chamberlain case shows what can happen when people involved in handling and analyzing blood evidence lack proper training, or when investigators allow public opinion or preconceived notions to influence their analysis.
What's involved in crime scene cleanup? Learn about the various hazards of crime scene clean up in this interactive gallery from HowStuffWorks.
More Great Links
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- Shen, A.R. et al. "Toward Automatic Blood Spatter Analysis in Crime Scenes." Proceedings of the Institution of Engineering and Technology Conference on Crime and Security. Page 378. 2006. http://mi.eng.cam.ac.uk/research/projects/BloodSpatter/BloodSpatter_ShenBrostow.pdf
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