Crime Scene Investigator Training

Many violent crime scenes involve trauma due to explosive force. A firearm or a bomb detonation are types of explosive force. The tremendous force impacting liquid blood disperses the blood into tiny droplets. The old school of thought was that anything with a force of 100’ per second or greater created high-velocity impact spatter (HVIS). Stain sizes created from HVIS were generally 1 mm or less, with many of the stains approximately .1 mm, creating an atomized or misting effect on nearby surfaces.

One problem with the HVIS methodology was that a force of less than 100’ per second could produce less than 1 mm stains. Another issue was that bloodstains of varying size ranges were found within patterns created by explosive force. The analyst must know the weapon used and the injuries of the involved subjects before stating, with any degree of scientific certainty, that the bloodstain pattern was created from explosive force.

A bullet that perforates a liquid blood source may create forward spatter and backspatter patterns. A forward spatter pattern results from an external force striking liquid blood, causing the breakup into individual droplets that travel in the same direction as the applied force. The droplets strike a surface, creating stains that define the pattern. Forward spatter patterns are consistent with a projectile creating an exit wound. A backspatter pattern results from blood droplets that travel opposite the direction of the applied external force, as in the case of an entrance wound created by a projectile.

An impact pattern’s stain sizes are consistent with the amount of force impacting a liquid blood source. Stains created by blunt force are generally larger than those created by gunshot or explosive force but smaller than those created by gravity; however, blunt force can cause stains that resemble those caused by explosive force or gravity. When analyzing the cause of impact patterns, one should know the weapon(s) involved and the injuries the parties sustained. Just because a stain pattern has spatter stains of 1-4 mm does not necessarily mean the pattern was created by blunt force. These types of patterns are created when a blunt object impacts liquid blood with a certain amount of force. They can often be analyzed for an area of origin.

The shape of individual bloodstains found at crime scenes can range from entirely circular to very elliptical. The different shapes correspond to the angle of impact with a target surface. Being able to interpret a stain’s impact angle allows the analyst to correlate multiple stains in an associated impact pattern and determine their area of origin.

An analyst at a crime scene may notice linear or semi-linear trails of blood with a slight curvature on the walls, ceilings, or other surfaces. Such “cast-off” patterns are typically created when blood is released from an object in motion, such as the swinging of a hammer, pipe, or hand.