Bloodstain Pattern Analysis
Shedding of blood is the dramatic accompaniment to murder committed by violent means. Blood accounts for about 9 per cent of a healthy person’s body-weight and as many murderers have discovered to their cost, when it is spilled a little goes a long way. Once blood is shed in any quantity, and especially when it starts to clot, it becomes very difficult to deal with. Murderers’ attempts to clean up after their violent handiwork often fail because of blood-traces which adhere tenaciously to their clothing or to the murder weapon. Blood found at the scene of the crime has trapped many killers who thought they removed all incriminating traces. A sensational demonstration of this was provided by the French detective Gustave Mace in 1869, when he was interrogating a murder suspect in the room which he believed had been the scene of a ghastly crime involving the dismemberment of the victim. Convinced that a great deal of blood must have been shed, Mace looked about the room but could see no obvious traces. Then he noticed a marked hollow in the tiled floor. With the suspect looking on in astonishment, the detective took a jug of water and tipped the contents on the floor – the water collected in the hollow area, and when the tiles were lifted their under-surfaces were found to be caked with dried blood. This discovery led to a murder confession by Pierre Voirbo and to a triumph of detection for Mace.
Blood is important forensically, and can yield a great deal of information to the investigator. The first task in examining suspicious stains is to determine whether they are blood, and if so, are they human? Once this is established stains are examined for age, sex and blood group. The shape and pattern of liquid blood-splashes can help in reconstructing the murder; bloody fingerprints and palm-prints tell their own story; dried blood on a suspect’s clothing can be related to the victim, the crime scene and the murder weapon; blood and tissue forced under the fingernails of the victim during a violent struggle can be linked to the assailant.
Thus a single blood-trace can provide a wealth of information, and analytical techniques are improving all the time. Blood dynamics is important not only for narrowing suspicion on the guilty but also in showing a suspect’s innocence. As in many other aspects of forensic investigation, bloodstains are taken into account with a variety of other evidence to build up a pattern of crime.
The investigation of blood at a crime scene can be broadly divided into a biological approach (serology) and a physics approach (blood splatter or bloodstain pattern analysis). This fact file will concentrate on the dynamics of blood evidence.
Examination and interpretation of bloodstains on and around the body, and of blood-spots, splashes and smears at the scene of the crime, are an essential part of a murder investigation. The position and appearance of blood marks on the body and its immediate surroundings will help the investigator to reconstruct the crime.
The theory behind bloodstain pattern analysis is simple: blood is a fluid and will respond accordingly to the laws of physics. Though rarely the dominant piece of evidence in an investigation, bloodstain pattern analysis can be important in the difficult process of reconstructing a violent crime.
Experts begin by taking note of a few key variables:
- spot size
- angle of impact
- target surface
A great deal can be gleaned from the shape of blood spots and splashes found on surfaces such as floor, walls, ceiling, woodwork and furniture. The French criminologist Alexandre Lacassagne noted the correlation between the shape of blood sports and the position of the victim. Blood dropping vertically on to a flat surface form a circular mark with crenated edges, and denotes that the source was stationary at the time. Drops of blood falling from a moving object hit a flat surface obliquely and leave a spot shaped like an exclamation mark. An examination of the shape of obliquely falling blood splashes yields information about the direction and speed of impact. Such evidence helps determine the positions of victim and murderer at the time of an assault, and may also indicate the manner of violence and type of weapon used.
A line of blood spots on the ceiling of a room in which violent murder has been committed is likely to have been made by the killer wielding an axe or bludgeon in an area over his head. Smears and trails on the floor may be produced by a wounded person crawling about or by an assailant dragging the body of his injured victim. Smudges and smears on furniture and doorsteps leaving bloody fingerprints or palm prints may result from similar activities. Blood smears tend to start as drops which become ragged at one edge, indicating the direction of travel.
Even when the blood stain is not evident it may still leave a tell tail fingerprint. To detect invisible blood stains, the luminol test is used, which is a chemical sprayed on carpets and furniture which reveals a slight phosphorescent light in the dark where bloodstains (and certain other stains) are present.
What is the luminol test?
The specialist will try to determine what the position and shape of bloodstains at the crime scene indicate. He/she take measurements to determine the trajectory as well as execute carefully controlled experiments. These experiments will use surface materials like those found at the scene to try to reproduce what has happened.
A leading authority on blood stain interpretation gives the following tips to investigators:
- It is possible to determine the impact angle of blood on a flat surface by measuring the degree of circular distortion of the stain. In other words, the shape of the stain tends to change depending upon the angle of impact which caused the stain. For example, the more the angle decreases, the more the stain is less circular and more long.
- Surface texture is one of the key components in determining spatter type. The harder the surface is, the less spatter will result. It is therefore extremely important to duplicate the surface in a controlled test.
- When a droplet of blood hits a surface which is hard as well as smooth, the blood usually breaks apart upon impact. This in turn causes smaller droplets. The smaller droplets will continue to move in the same direction as the original droplet.
One of the classic murder cases in which blood evidence played an important, if controversial part was the trial of Dr Sam Sheppard. The doctor’s wife was found dead in their Cleveland Ohio, home in July 1954. Her body, with the head brutally battered by over thirty blows from a heavy object was found in the master bedroom. The room, which had been ransacked, was heavily spattered with blood, and a trail of stains led down the stairs and out on to a terrace.
Dr Sheppard, who had been awakened from sleep on the living room sofa by his wife’s screams, claimed to have been knocked unconscious by an intruder as he rushed upstairs. His behaviour was judged to have been suspicious, and there was considerable prejudice against him, not least on account of his alleged infidelity. He was sent for trial and found guilty of second degree murder, for which he was sentenced to life imprisonment. The coroner had made much of bloodstains on the pillow in the murder room, and a bloody imprint which he suggested had been caused by a surgical instrument which had served as the murder weapon. This instrument was never specified, but the imputation was plain that Sheppard, himself a doctor, had used it to murder his wife. The murder room abounded in blood evidence which if properly examined would have led to other conclusions. It was left to Dr Paul Leland Kirk, Professor of Criminalistics at Berkeley, to make a thorough assessment of this evidence several months later in order to reconstruct the murder. As the bedroom ceiling showed no traces of blood, Kirk reasoned that the murder weapon had been wielded in a more or less horizontal fashion. This was borne out by the state of blood splashes on the walls, some of which had been flung from the murder weapon as it was swung backward and forward to make contact with the victim’s head. Other blood spatters had come directly from the battered head. The Professor carried out experiments which suggested the most likely weapon to have caused the pattern of blood splashes was a heavy flashlight. He also judged that the murderer stood between the twin beds, having noted blood drops which had been smeared into streaks on the right side of the victim’s bed. This interpretation was supported by blood free areas on two of the walls behind the murderer which had been protected from flying blood spatters by his body. A killer standing in that position must have swung the murder weapon with his left hand – Dr Sheppard was neither left-handed nor ambidextrous.
By implication, the murderer must have been thoroughly spattered with blood. Yet apart from a bloodstain on the knee of Sheppard’s trousers, which got there when he stood close to the bed to take his wife’s pulse, there was no evidence of other blood staining on his clothes A number of factors similarly pointed away from Dr Sheppard as the murderer – it was certainly the case that the examination of the blood evidence had been bungled in the first instance. There was no better illustration of this than the admission during a second trial that the trail of stains leading from the bedroom through the living room and out on the terrace had not even been properly tested for human origin, nor was blood groupings attempted. Professor Kirk’s interpretation of the blood evidence went a long way towards securing Dr Sheppard’s eventual freedom.
Film and television aficionados may recognise the recognise in this case the opening premise of The Fugitive. Obviously the series would have been a lot shorter with the help of a good bloodstain expert.
Some common terms used in bloodstain pattern interpretation.
- Angle of Impact — The acute angle formed between the direction of a blood drop and the plane of the surface it strikes.
- Cast-Off Pattern — A bloodstain pattern created when blood is released or thrown from a blood-bearing object in motion.
- Drip Pattern — A bloodstain pattern which results from blood dripping into blood.
- Flight Path — The path of the blood drop, as it moves through space, from the impact site to the target.
- Flow Pattern — A change in the shape and direction of a bloodstain due to the influence of gravity or movement of the object.
- Impact Pattern — Bloodstain pattern created when blood receives a blow or force resulting in the random dispersion of smaller drips of blood.
- Misting — Blood which has been reduced to a fine spray, as a result of the energy or force applied to it.
- Projected Blood Pattern — A bloodstain pattern that is produced by blood released under pressure as opposed to an impact, such as arterial spurting.
- Spatter — That blood which has been dispersed as a result of force applied to a source of blood. Patterns produced are often characteristic of the nature of the forces which created them.
- Target — A surface upon which blood has been deposited.
- Transfer/Contact Pattern — A bloodstain pattern created when a wet, bloody surface comes in contact with a second surface. A recognizable image of all or portion of the original surface may be observed in the pattern.
- Wipe Pattern — A bloodstain pattern created when an object moves through an existing stain, removing and/or altering its appearance.