About six quarts of blood is pumped throughout the entire vasculature in the body of an adult. When that circulation is interrupted by trauma-from bullets, a blunt object, or a knife-some of that blood is spilled. In the event of a violent confrontation or a shooting, that blood is usually spurted forth in scattered drops, or spattered.
The suggestion of a blood-spattered, blood-stained crime scene is horrible. The positions of the blood stains or the patterns of the blood spatter tell a meaningful story to a crime scene investigator. Such evidence can be damning for the defendant in court.
From the number and the location of the stains, it is possible to ascertain the movements of the victim with respect to his attacker. From blood spatter analysis, it is also possible to determine the direction from which the attack came. The proximity of the bloodstain to the victim can indicate the kind of impact the victim received, the kind of object that created his wounds, the number of strokes that were inflicted, and the location of the victim and the offender during and after the crime was perpetrated.
Such evidence can be corroborated by statements taken from suspects or witnesses. There have been many occasions in which a perpetrator asserts that the victim’s death was accidental, but blood spatter analysis contradicts his claim. Whenever blood drips, it produces stains that are circular in shape. However, when blood is thrown forcefully, as from a blow to the head, it creates stains that are elongated, if not ovoid, in shape. The longer the shape of the blood stain, the harder the blow the victim received.
Crime scene investigators take pictures of blood stain and blood spatter evidence and carefully analyze the patterns. In the past, forensic scientists have tried to study blood spatter evidence by what is known as directional analysis. Directional analysis involves using strings attached to certain points in the spatter pattern and tracking these back to a more likely source of the blood. Nowadays, a computer software program called BackTrack enables investigators to analyze spatter patterns digitally and graphically by using computer generated strings. The software user enters into the computer data that includes the width, length, direction, and location of each blood drop. Then the computer calculates an angle of attack for each drop and comes up with a flight trajectory backward from each. Where more than one of the trajectories meet, the criminal investigator can determine the position of the original injury.
Given enough time, an offender may try to remove blood stains and spatter by washing towels, blankets, and clothing, or by wiping down furniture, floors, and other surfaces. Luminol, a chemical reagent that reacts with the protein hemoglobin found in blood, can reveal even the minutest traces of blood. This chemical causes blood to fluoresce, or glow, with a bright green color.
