Cast-off trails are the bloodstain patterns that are created by cast-off blood . When a blood source moves, drops slide towards its end and may be flung off if the object is moving fast enough. This is called cast-off blood and it moves away from its source as a spray of droplets. When these land on a nearby surface, a cast-off trail is formed. Analysis of the trails may tell the forensic investigator some key facts about the nature of the attack.
When droplets of cast-off blood land on a surface, they do so in a linear trail described as in-line staining. The actual size of the spherical splashes depends on many factors such as the size and shape of the blood source, which is often the weapon, as well as its velocity and the amount of blood it produces.
Cast-off trails occurring some way from the presumed attack, such as on a ceiling, may be indicative of the use of a long, relatively light weapon. Droplets cast-off from a forward swing of a weapon tend to be smaller than those from a backward swing. Pronounced cast-off, consisting of large drops at some distance from the attack, often indicates the end of a backswing from a weapon. This is a point at which particularly large forces are applied to any cast-off blood.
The physics of the trajectory of cast-off blood is quite complex. Generally, the droplets are flung at any angle from radial to tangential to the arc along which the source is moving. This means that the cast-off trail is usually found outside the arc of swing and the attacker is protected from it. One exception can occur, however, when the attacker is kneeling, and cast-off trails might be found on the lower leg. Sometimes cast-off trails are also found on the attacker's back.
There are so many factors affecting the nature of a cast-off trail that the investigator has to take great care in its interpretation. Nevertheless, the patterns can often provide some useful information. For instance, the minimum number of blows in an attack might be estimated from looking at the number of trails of in-line staining on a surface. Geometric analysis of the trails might also help assess the location of the attack.
see also Blood; Blood spatter; Bloodstain evidence.