Police procedures at death scenes-6: blood spatter

From Scott:

I was trying to decide what aspect of fingerprints I should cover as the next part of my continuing series of crime scene blogs when Rick reminded me that I had promised some information about the study of blood spatter patterns. So without further ado, let’s dive right in.

In many violent crimes where blood spatter patterns are present, there aren’t any living witnesses to the crime who can tell us about the sequence of events. It falls to the investigator to study the signs and determine exactly what happened. Much like Aragorn in Tolkien’s Lord Of The Rings, a trained investigator can take a series of signs and use them to reconstruct the crime.

Any time someone bleeds at a crime scene, certain patterns of bloodstains will be left behind. Often, the patterns will tell us the type of injury sustained, the severity of the injury, how many times a victim was struck, and even the point of origin of the blood spatter, in three dimensions (how high off the floor and how far from the wall). The simplest pattern to discern is arterial bleeding. When an artery is sliced open, the force of the blood’s pressure against the walls of the artery will force blood out in a spray, timed to the beat of the victim’s heart. Each time the heart beats, blood is ejected through the wound, but each successive beat will have less force than the one before. Blood is being depleted at a high rate, so the victim’s blood pressure is dropping. Less pressure, less blood shooting out. On a wall, the pattern is typically a horizontal line with drips descending from it. Picture taking a paintbrush soaked in paint and flinging it along a wall without making contact. The pattern of the paint would be just like arterial spray. Arterial wounds can quickly turn fatal due to a loss of blood.

Drop patterns are common when an injured person is moving around. If, for instance, the victim has a deep laceration to the arm, the blood will flow down the arm to drip from the fingers. If the person is stationary, the drop patterns will be almost perfectly round, with jagged edges where small amounts of blood splashed out of the drop on impact (called satellite spatter). If the person is moving, the drops will have some forward momentum, and therefore the drops will elongate in the direction of travel. The size of the drops will be determined by the severity of the bleeding.

A subset of the drop patterns is the castoff pattern. Let’s say the killer is striking the victim in the head, repeatedly, with a baseball bat. On the first impact, blood will begin to pour out of the wound, with some transferring to the bat. When the killer whips that bat back to strike another time, some of that blood will be cast off, usually landing on a wall or ceiling. By counting the number of castoff patterns, we can determine the likely number of impacts. If there are seven castoff patterns, it would indicate eight strikes with the weapon (since the first strike had no blood on the weapon to cast off). In the case of stabbing weapons, the blade will definitely be bloody, so again there should be castoff.

There is also a likelihood of spatter coming from the victim with each successive strike. In the case of blunt instruments, the second and successive strikes will likely splatter blood away from the point of impact. We can study these patterns to determine where the victim was when he was struck. We photograph the field of patterns, then photograph it again with paper rulers taped in place. By measuring the width and length of the individual drops, along with determining points of convergence, we’re only a bit of trigonometry away from figuring out where the victim was. Why is this important? We can often use it to impeach the killer’s statement. If he claims he hit the victim in self-defense, but we determine the victim was on his knees at the time, we just destroyed the killer’s alibi.

Another pattern is expirated blood. This is indicative of internal injuries, resulting in blood in the victim’s lungs (another fairly fatal injury). Every time the victim exhales, blood comes out in tiny droplets. Picture yourself with a mouthful of red Kool-Aid, standing in front of a white wall. Blow out through your mouth, as in the cliché “spit-take.” This pattern can appear at natural death scenes, as well. I’ve seen a number of cases where a person dies of cirrhosis of the liver. The house looks like a murder scene, with blood all over the walls from every exhaled breath in those final minutes.

Two patterns that are often confused with each other are swipes and wipes. It doesn’t help that they were given such similar names, for similar patterns. Both involve the smearing of wet blood. The difference: when the surface (called the “substrate,” if you want to sound fancy) has wet blood, and an object wipes through it, smearing the blood, you have a “wipe.” Example: the killer’s sleeve brushes against a bloody wall, smearing the blood (and incidentally transferring some blood to his shirt). The “swipe” is the opposite situation. The killer has the victim’s blood on his sleeve, and swipes up against a clean wall, smearing the blood onto the wall. Again, this can impeach an alibi. If the killer has the victim’s blood on his sleeve and claims he got it by rubbing against a wall, but we show that the blood is a “swipe,” we have a broken alibi. To tell the difference between the two, let’s return to the paint analogy. Paint a single vertical stripe on a wall, then drag your hand through it, smearing it across the wall. This “wipe” pattern only disturbed a portion of the whole pattern. For a swipe, dip your hand in the paint and smear it across the wall. The smear is the pattern.

Sometimes, we will encounter oddly shaped patterns, or ghost images of a recognizable shape such as a shoeprint. These patterns are called transfers. These can be from bloody clothing pressed against a wall, a bloody knife pressed against another surface, bloody fingerprints, or shoeprints after the killer stepped in blood. Bloody fingerprint and shoeprint evidence is very damning. Obviously, the person who left those transfer patterns was present, not only after the victim was killed, but while the blood was still wet. Bloody fingerprints can be difficult to identify, because they are so wet that the blood tends to smear the ridgelines together and blur the pattern.

In the case of gunshot wounds, if the bullet exits the body, you have a blood pattern of miniscule droplets due to the high speed of the impact. The drops are nearly vaporized, reduced to such a fine size that they are almost a mist.

One final note is a pattern called “shadowing.” If we have an area of blood pattern on a wall, but there is a void in the pattern (a clean spot where there is no blood), then we know that something was removed after the crime. Whatever it was, the blood landed on that object instead of the wall, leaving a clean space behind it. This could prove to be useful information later, especially if an object that fits the size of the void should be found in the killer’s possession.

Sometimes, the killer will clean up a crime scene, scrubbing the floors and walls to remove the blood evidence. There are chemicals that can still detect the blood, even after all visible traces have been removed. The most commonly used chemical is Luminol. This is what you see television detectives using, usually with tremendous success. The truth is much more disappointing. Luminol glows in the presence of blood, but also has a number of false positive results. If you spray it on metal, you can count on it glowing. It also has a very dim glow, so unlike television and movies, the room must be almost completely dark in order to see it. This makes photography extremely difficult. Luminol will also destroy the DNA in the blood, which is a major issue. My department uses a chemical called Bluestar. The glow is brighter, there are fewer false positives, and DNA is preserved.

I’ll finish off with a glossary of general terms associated with blood spatter patterns. Next time, I’ll return to the exciting world of fingerprints.

Glossary of Key Bloodstain Pattern Analysis Terms
Angle of Impact: the angle at which a blood droplet strikes a surface

Arterial Gushing: the large pattern of blood that is created when blood escapes an artery under pressure; the increase and decrease in blood pressure is apparent

Arterial Spurts: large patterns created under pressure, but with less volume and usually more distinctive evidence of blood pressure rising and falling

Clot: a mass of blood and other contaminants caused through clotting mechanisms

Cast-Off Stains: blood that has been thrown from a secondary object (weapon or hand) onto a target other than the impact site

Drop Patterns: characteristic patterns present when blood drips into standing, wet blood

Expiratory Blood: blood which is spattered onto a target, as a result of breathing; typically, this occurs when an injury is sustained to the throat, mouth, or airway

Impact Site: usually the point on the body that received the blow or applied force, from which the blood was shed

Origin: the point in space where the blood spatter came from

Parent Drop: the droplet from which satellite spatter originated

Projected Blood: blood under pressure that strikes a target

Satellite Spatters: small drops of blood that break off from the parent spatter when the parent droplet strikes a target surface

Shadowing/Ghosting/Void: a pattern that helps to place an object or body in the scene; normally, the area in question lacks blood even though areas surrounding it show blood

Skeletonized Stain: the pattern left when an object moves through a partially dried stain, removing part of the blood, but leaving the outline of the stain intact

Spatter: bloodstains created from the application of force or energy to the area where the blood is

Spines: the pointed edges of a stain that radiate out to form the spatter

Splash: pattern created when a volume of blood in excess of 1 mL strikes a surface at a low to medium velocity

Swipe: the transfer of blood onto a target surface by a bloody object that is usually moving laterally

Transfer Pattern: the pattern created when a wet, bloody object comes in contact with a target surface, leaving a pattern that has the features of the object making it useful for identifying the object

Target: the surface where the blood ends up

Wipe: pattern created when a secondary target moves through an existing wet blood stain on some other object

–Scott