Note: Please refer to gallery when text below refers to a photo.
Dispatch instructs you to respond to a call in your community regarding a violent home invasion. Upon arrival, you are met by a gentleman who claims he found his wife inside their residence badly injured. The woman is being transported by EMS and appears to be critically wounded with injuries to her head and face, with a large amount of blood on her face, head and hair. A blood-stained piece of metal pipe has been found near where the victim was located. The husband states that upon returning home from running some errands and going to the grocery store, he discovered his wife injured and bleeding in the living room. The husband tells you there have been several burglaries in the area, and he fears his wife may have been attacked by an intruder.
While speaking to the husband, you observe several bloodstains on his pants and shirt. The husband claims he got the blood on his clothing when he went to his wife and tried to comfort her. He says he held his wife in his arms until he heard the ambulance arrive. Is this man a distraught husband and victim, or is he your prime suspect? A quick and accurate assessment of his possible involvement may prove much easier than you realize.
Blood subjected to force or trauma will behave in a predictable and consistent manner. In addition, blood and blood-stained objects that come into contact with other surfaces will leave identifiable stains and patterns.
The principles associated with bloodstain recognition and analysis are simple enough that all officers and detectives can quickly grasp them. Although this article can never replace advanced bloodstain instruction, officers and detectives can still incorporate this information into their respective patrol and investigative job functions.
A sufficient quantity of bloodstain evidence associated within a crime scene or traumatic event location can allow you to determine the following:
Understanding the principles of fluids in motion and other phenomenon associated with bloodstain evidence will enhance the abilities of first-responding officers and detectives twofold. First, you will be able to quickly recognize bloodstain evidence within a crime scene that can help you assess what may have taken place and confirm or refute the validity of statements. Second, you will be able to recognize bloodstain evidence that requires interpretation or documentation on the part of an expert within this field, thereby putting this evidence to greater use.
Principles of Fluids in Motion
Because of cohesive forces, blood, like all liquids, is drawn unto itself and forms a “skin” as the result of surface tension. This skin works to reduce the surface area and to resist penetration and separation. When a source of blood is acted upon by a force stronger than the surface tension, drops of blood will either drip from the source or break away and become airborne. The amount of force exerted onto the blood source will dictate the size of these blood droplets. The greater the amount of force applied to the source, the smaller the drops will be. In other words, as the force increases, the drop size or diameter decreases.
These airborne droplets will then obtain a spherical shape as they travel, a direct result of the cohesive forces and surface tension mentioned above. These drops will retain this spherical shape until another force acts on them or they strike an element of the environment, thereby creating bloodstains.
Understanding this concept will help you determine what level of force was involved during an incident.
Levels of Force
The levels of force associated with bloodstain analysis are broken down into three different categories. The first level: low velocity, often referred to as 90-degree spatter. At this level, drops of blood fall onto an object at a 90-degree angle, meaning gravity (the force) is stronger than the surface tension created by the blood’s cohesive forces, thereby causing drops to fall from the blood source. These stains are also sometimes referred to as dropped bloodstains. See photo a.
The second level: medium velocity. Medium-velocity spatter produces a pattern of bloodstains that measure 4mm–6mm in diameter or larger. A force of 5–25 feet per second (fps) impacting onto a blood source—e.g., punching, hitting, kicking, stabbing, stomping, etc.—produces medium-velocity spatter. See photo b.
The third level: high velocity. High-velocity spatter produces a mist or spray-type pattern consisting of bloodstain diameters of less than 1mm. A mist pattern of blood requires an impact force of at least 125 fps, such as gunshots, power tools and explosions. See photo c.
Important: When analyzing bloodstains and using these definitions, remember to focus on the majority of the pattern stains or spatter size within a bloodstain. In other words, you will rarely see singular-sized stains within a pattern, and the predominate stain-size defines the pattern.
Direction of Travel
As opposed to the circular stains found with 90-degree spatter, blood droplets that strike an object at an angle other than 90 degrees create an elongated stain with a tail. This tail will enable you to determine the blood drops’ direction of travel.
By understanding the blood drops’ direction of travel and flight path, you can then recreate the location of the original blood source at the time of impact, also known as the area of origin. Once you know the area of origin, you can assess the victim’s positioning at the time of the assault.
Example: Say you locate a series of elongated stains on the wall approximately 20"–28" from the floor at the scene of an assault. The tails on these stains indicate the blood source was below the pattern, and that the blood was traveling upward. The suspect admits to striking the victim, but claims the victim was charging toward him with a baseball bat, and that he acted in self defense. If you can determine the directionality of the bloodstains, you could show that the victim was below the stains on the wall, and most likely actually down on the floor itself, seriously bringing into question the validity of a self-defense claim.
An individual with advanced training in this field could recreate a three-dimensional area of origin, allowing the officer to show exactly where the victim was when being struck. A process referred to as the String Technique and specific computer programs can replicate the actual flight paths of bloodstains and identify an intersection where the paths converge. This area of convergence represents the area of origin of the impacted blood source.
Stain & Pattern Recognition
A number of other bloodstains and patterns found within crime scenes can also provide critical information. Note: I will only briefly describe some of these patterns and stains, so you should consult with crime-scene officers or bloodstain examiners to view actual crime-scene photographs of these and other stains and patterns to gain a greater understanding of their appearance and significance.
Swipe or Wipe
The stain created when an object with blood on it moves across a surface, or when an item moves through an existing bloodstain or pool of blood. The swipe stain might be found when a body is moved or removed by dragging. The wipe stain might represent an attempt to clean the crime scene. See photo d.
The stain or pattern created when a bloodstained item comes into contact with a surface. Use the pattern of this stain to identify the object by studying the stain’s shape, size, pattern, design, etc. See photo e.
An area within a generally continuous bloodstain pattern that lacks stains. This interruption is caused by an intermediate target that disrupts the stain or blood pattern.
Consider two important questions when you observe a void or transfer stain: 1) What object created the pattern? and 2) if the object has been moved or removed from the crime scene, why? See photo f.
The stain pattern resulting from blood droplets that fly off of an object being swung or moved. These stains are often located on the ceiling within a scene where an assault took place. (For instance, blood drops strike the ceiling as the suspect raises the bloodstained weapon during repeated blows.) The location of these stains can often help you recreate the suspect’s position during the assault. See photo g.
Blood which impacts a surface under pressure. This stain most often represents arterial spurting, created by the breaching of an artery, a serious and life-threatening injury. See photo h.
Sequence of Events
When two or more stains or patterns intermix with each other, assess each stain’s affect on the other. Example: 90-degree stains on a wipe pattern.
90-Degree Stains on a Wipe Pattern
These bloodstains can allow you to determine a chronological order of events, which you can then use to attempt recreate or reconstruct the crime scene. Look at the 90-degree drops located within the wipe pattern in photo i. The fact that the drops appear to be undisturbed indicates the wipe pattern was present before the drops of blood struck the surface. A smearing of these same drops by the action that created the wipe pattern would indicate the opposite.
Analyzing & Applying
Now let’s get back to the crime scene sketched at the beginning of this article. Look at the stains present on the husband’s pants and shirt and try to identify them (see photos j, k, l and m). Then determine whether the stains present on the husband’s clothing are consistent with his account, and whether stains you expected to find based on his account are absent.
Could the transfer stains on the front of the husband’s shirt and pants have come from holding his wife? Or, do those stains appear to have more likely been created by the husband rubbing his bloody hands on his own clothes? Did you notice the impact spatter located on both the husband’s lower pants leg and shirt tail? Would you expect the husband to have impact spatter on his clothing if he came home and found his wife after the event?
Caught Red Handed
The blood evidence on the husband’s clothing tells a very chilling story, does it not? If this were your case, you should now have an interview strategy in mind you may not have had before you analyzed the bloodstains.
Remember, you must legally collect, photograph, and properly preserve any bloodstained clothing or other items of evidence either worn by or associated with the suspect in such cases.
Understanding and using bloodstain evidence will without question make you a better detective, a better interviewer and a better police officer.