Reconstructing the sequence of events leading up to a murder could become easier and more precise thanks to a new system for producing a high-resolution 3D map of a crime scene.
A post-mortem can shed some light on the circumstances surrounding a murder, and vital clues can also be gleaned from an analysis of bloodstains at the scene of the crime. Among other things, this can reveal if the victim was standing, sitting or lying down when they were attacked.
Bloodstain pattern analysis involves studying blood spatters on walls, floors and furniture, using the length and width of elliptical stains to estimate their angle of impact. As seen in TV crime dramas like Dexter, elasticated string is then pinned to each bloodstain, projecting from it at the estimated impact angle. The point in 3D space at which most strings converge is taken to be the position from which the blood was spattered.
The method is both time-consuming and prone to error: a spatter of blood does not travel in a straight line as the strings suggest, but follows a ballistic curve that depends on its velocity. So Ursula Buck and colleagues Silvio Näther and Beat Kneubuehl at the University of Bern, Switzerland, have developed a system that more realistically reconstructs the ballistics of blood droplets at a crime scene.
The first step is to use a laser scanner to make a 3D digital map of every object in the crime scene. This does not include much surface detail, so the team also uses a digital camera to capture the shape of bloodstains. They then use another laser ranging device called a tachymeter to obtain a precise location for each blood spot in the 3D model.
Next, they calculate the mass of each drop from the size of its stain, and use this to calculate its maximum potential velocity - air drag would rip apart a droplet if it travelled faster than this limit. With that information, and an angle of impact estimated from the shape of the stain, their software projects a realistic trajectory backwards in time to locate the origin of the blood spatter (Forensic Science International, DOI: 10.1016/j.forsciint.2010.06.010).
"The 3D results give us good clues about the area of origin, the number of blows, the positioning of the victim and the sequence of events," says Buck. The system has already helped in two murder inquiries, revealing in one that a woman killed by her husband was lying in bed rather than sitting up when attacked.
"This seems a useful way to capture 3D blood pattern information," says Gillian Leak, a blood pattern analyst with the UK Forensic Science Service in Birmingham. But she adds that there is no substitute for human expertise at the scene of a crime: "You still need a scientist to go into the crime scene to interpret which blood stains are the most important."
This article originally appeared in New Scientist.