Bloodstain Evidence is Critical--Part 3

In last month's column we looked at how an accidental smearing of bloodstain spatter patterns could obscure valuable evidence at a crime scene. We also saw how very close analysis of a blood spatter pattern could turn around an investigation. The human body contains between five to six liters (5 to 6 quarts) of blood; adult males having slightly more than adult females. When blood leaves the body whether due to a gunshot wound, knife wound or blunt force trauma, it follows some basic principles of physics. Blood falling from a wound forms a spherical droplet and is not deformed, like happens with many other liquid droplets. The reason for this is the surface tension of the blood droplet, a force which tends to keep the molecules pulled tightly into the blood droplet.

When the blood droplet comes into contact with a surface, it will produce a specific pattern. This pattern is determined by the size of the blood droplet, its angle of impact with the surface, the composition of the surface and most important, its velocity on impact.

Where bloodstains come from

It is this property of blood--its ability to form specific types of stains caused by the physical forces exerted on it--that allows a trained technician to analyze its pattern and come up with answers to what happened at a crime scene. One critical aspect of bloodstain spatter analysis is determining where and how the attack on the victim occurred. When a victim is beaten with a blunt object like a hammer, or stabbed with a kitchen knife, blood is transferred from the wound area onto the object or knife. As the weapon is drawn back usually up above the perpetrator's head, the blood is ejected from the object's surface. The thrown-off blood droplets will impact with surrounding surfaces such as walls, ceilings and furniture. Blood spatter on walls and ceilings provides the most information to the investigator. Bloodstain on furniture like a couch or coffee table is often unreliable, because the furniture may be inadvertently moved by the first responders on the scene such as EMS, if the victim is still alive. Sometimes the first officers on the scene may need to move furniture in order to get to a victim.

Blood droplets released from the weapon and impacted on a wall can determine whether the victim was standing, kneeling or lying on the floor at the time of the attack. A victim may initially be attacked while standing up, and then as the attack continues, be dropped to their knees or the floor. If the victim is attempting to get away from the attacker, then the spatter pattern for each stage of the attack will most likely appear on different part of a wall. This might be across a kitchen and occur on more than one wall of the room or down a hallway as the victim attempted to escape. It is also not uncommon for a victim to crawl along a floor, trying to escape. In this case, there will be wipe bloodstain as the individual crawls through their own blood. The perpetrator may stop for a period during an attack and then drag the victim to a different part of the room or even into a different room, and then continue the beating.

Collecting bloodstain evidence

To determine what happened at each stage of an attack, the CSI investigator photographs all the bloodstain patterns and may make a drawing of specific areas to preserve a record of the actual scene. It is not uncommon for sections of drywall to be cut out of a wall and taken as evidence. Blood droplets that fall passively, as would happen if you cut your finger and let then simply fall directly to the floor, will form a stain pattern that is round and may have some scalloped edges around it. Blood drops that are thrown off from a weapon will be elongated ovals or teardrop shaped, with a red tail at one end. The tail arises because a droplet impacting a surface at an angle of less than 90 degrees will have more blood at the back edge, and gravity will tend to make it run down in a tail-like pattern. In most cases the direction in which the droplet traveled can easily be determined by the location of the tail. The tail will point in the direction from which it traveled, i.e., from the source, the victim.

Bloodstain analysis

Bloodstain analysis requires that a relatively large number of blood droplets be analyzed in order to determine a point of convergence (POC), the location where the blood came from. When these measurements have been made, it is then possible to determine the two-dimensional POC by drawing a line through the long axis of each blood droplet, and extending the line until all the lines converge. The point where all of the lines cross is the POC.

The next part of the puzzle is to determine the angle of impact (AOI) of the droplets with the surface. This requires careful and accurate measurement of many blood spots, determining their length and width using a metric ruler or caliper. This measurement of the stain on the wall can be used to mathematically determine the angle that the blood droplet impacted the wall. These days, there are several computer programs readily available to make these mathematical (trigonometry--sine function) determinations for the technician. Placing the AOI pattern over the POC pattern allows a three-dimensional POC to be determined. This is the position of the victim in relationship to the wall or ceiling when the bloodstain pattern was produced.

Bloodstain analysis equipment

In the past, bloodstain analysis was done using colored string to represent the path of blood droplets as they left the weapon. These would be taped to the wall and floor, creating a three-dimensional pattern representing the blood as released from the attack. Some departments also use colored plastic trajectory rods placed in a similar manner. These methods are time-consuming and labor-intensive. The introduction of laser equipment capable of being placed on a bloodstain and projecting a laser bean in the direction of the source, or placed on a POC and projected to specific bloodstains on a wall, has increased the speed and accuracy of determining bloodstain patterns. The angle of impact for a large number of blood droplets can be determined rapidly.

Bloodstain pattern analysis provides an important piece of the evidence puzzle at a crime scene. The goal of this analysis is to link a specific pattern of bloodstains with a specific source event (attacked victim). In cases where multiple blunt trauma blows are delivered by one or more assailants and the scene is very complex, it is not always possible for this link to be made accurately.