How Bloodstain Pattern Analysis Works

By: Shanna Freeman & Melanie Radzicki McManus  | 
What can drops of blood tell investigators about a crime? A lot more than you might think. Chris Dale/Getty Images
What can drops of blood tell investigators about a crime? A lot more than you might think. Chris Dale/Getty Images

If you're flipping TV channels one day and come upon a show depicting a crime scene (think "CSI" or "Dexter"), you might notice something strange. Among the technicians dusting for fingerprints and collecting hair fibers, there's an array of red strings running from the floor, the wall, the table and the sofa. All 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 they have learned all that information from a webwork of strings?

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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, especially since the evidence is typically interpreted strictly by humans, which can result in error. And if a violent act involves multiple victims and multiple assailants, the challenge becomes even more complex. So while important aspects of bloodstain pattern analysis are well supported by research, analysts must be very careful not to overreach [source: Iowa State].

That being said, a well-trained and seasoned analyst can provide key information that leads to arrest and conviction. And with new, computerized methods of analysis coming online, there may be more consistency and reliability in the future.

Let's start our investigation into this topic with the basics of bloodstain pattern analysis. For example, what blood spatters can — and can't — reveal.

Basics of Blood

blood spatter
The diameter of a blood drop will increase as the height from which it falls increases. HowStuffWorks

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:

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  • type of weapon
  • velocity of blood
  • number of blows
  • position and movements of victim and assailant during and after the attack
  • which wounds were inflicted first
  • type of injuries
  • when 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, 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. In other words, it's cohesive [source: 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 from which it fell, and whether it landed on carpet, wood 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 a softer surface like carpet, which can partially absorb the liquid and cause the edges to spread [sources: Dutelle, Murray, Wonder]. These are just some of the many factors a blood spatter analyst must consider.

Something else they must consider is blood dries over time. How quickly this happens depends on the surface on which the blood landes, how much blood the spatter contains, and the heat and humidity at the crime scene. But as a rule, the outer edges of the stain dry first. Consequently, after the interior portion flakes off or is smeared by an object, a dry blood spatter can skeletonize, leaving behind a ring similar in appearance (if not color) to a water ring on a coffee table [source: James, et al].

Patterns of drying help analysts determine how long an assault went on, detect whether it took place all at once or in stages, and nail down possible crime scene contamination [source: Wonder]. Clotting patterns in blood provide similar information and can help nail down the time factor if analysts arrive at the scene before blood can dry. Clotting generally begins within three to 15 minutes, although actual times vary by the amount of blood, surface type and environment. Mixed levels of clotting can indicate that multiple blows or gunshots occurred over time [sources: Dutelle, Wonder].

Blood Spatter

blood spatter
Blood spatter from a high-velocity incident like a suicide bombing will create a cast-off pattern much different from a low-velocity pattern like a gunshot or stabbing. David Silverman/Getty Images

Blood behaves not unlike 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 pattern of slow-moving blood, called "drips," occurs after an injury, and has a relatively large footprint of 0.16 inches (4 millimeters) or more. Drips, which result from blood dripping onto blood, 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, sometimes retains the shape of the object that made it [source: Wonder].

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At the other end of the scale are the tiny droplets caused by blood traveling 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. Back spatter, 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, back spatter 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].

Several 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 assailant, 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

blood spatter
Many things will determine the shape of a blood droplet, including gravity, force and the surface it lands on. HowStuffWorks

To analyze a bloodstain pattern, an expert relies on three main interrelated elements: the size, shape and distribution of 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 the blood's impact angle [source: Dutelle].

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The lower the angle at which blood strikes a surface, the thinner and more elongated the stain. The converse holds true as well. For example, if a blood droplet has a 10-degree drop, it creates a highly elongated stain, whereas a droplet with a 90-degree (vertical) drop leaves a round stain [source: Dutelle]. Measuring a stain's width and length, analysts use the following mathematical formula to calculate the impact angle:

angle of impact = arcsin (stain width/stain length)
blood spatter
Blood droplets elongate the higher the angle from which they strike the surface.
HowStuffWorks

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.

If the bloodstain has a tail, as can occur in droplets striking a surface at certain angles and speeds, it should be left out of this calculation [sources: Dutelle, Eckert and James].

Once analysts know all three angles, they can move on to the most visually striking part of the process, and therefore the one most featured in shows like "Dexter" — the technique of stringing. Stringing involves running strings from the rear edges of the bloodstains upward at the appropriate angles to find where they roughly meet — the area of origin. This technique provides only 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 3D renderings. Initially, this software required manual data entry, which was tedious but perhaps not as tedious as stringing.

blood spatter
Stringing involves running strings from the rear edges of the bloodstains upward at the appropriate angles to find where they roughly meet, also known as the area of convergence.
HowStuffWorks

Developers are working on software that can automatically reconstruct a single coordinate frame from several images, limiting user input. Others are working on using laser scanning and even machine learning to analyze the blood spatter.

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

blood spatter
The late criminologist Dr. Paul L. Kirk is seen here examining the bloodstained pillow of Marilyn Sheppard. The pillow was part of the prosecution's exhibits in the murder trial of her husband, Dr. Samuel Sheppard, who was eventually convicted of second degree murder in her death. Bettmann/Bettmann Archive

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 20th-century Germany and France, including Dr. Paul Jeserich and Dr. Victor Balthazard, respectively.

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.

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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 [sources: Eckert and James].

Before 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, the case must involve 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

blood spatter
Poorly analyzed bloodstains on 9-week-old Azaria Chamberlain's knit jacket helped wrongly convict her mother Lindy of the baby's murder. Lindy was later exonerated. Fairfax Media Archives/Fairfax Media via Getty Images

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].

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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'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.

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 handprint" 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 four years later 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 hearings were yet to come [source: Latson]. In 2012, 32 years after the event, a coroner finally pronounced that a dingo was responsible for Azaria's 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.

Originally Published: Apr 24, 2008

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