Forensics

Spatter from blood-soaked sponges could aid gun forensics

Whodunnit? Forensic investigators could soon be able to deduce in much greater detail how specific blood spatter stains were caused at a crime scene.

Detectives can back-calculate some of the details of a shooting by measuring the stains made by splattered droplets of blood. The size of the droplets helps them figure out the speed of the bullet, and their shape can help deduce where in space the victim was hit, whether by a bullet or a fist.

But the current methods for analysing blood spatter patterns are incomplete, says Alexander Yarin at the University of Illinois in Chicago. They only allow investigators to determine general categories of velocity, like low, medium or high impact. The calculations also assume that the drops travel along a straight path.

In reality, gravity and air resistance drag and slow the droplets, causing them to follow curved trajectories. While some newer models incorporate those forces, they still miss another crucial factor, Yarin says – the conditions that affect the creation of the drops themselves. These initial conditions set the scene that the detectives will later investigate, so they are crucial to understand.