You and your partner are the first ones on the scene. There is blood everywhere. It looks like two victims in the living room and another in the long hallway leading from front to back of the house. Your partner yells, "There's another one in the kitchen. Looks like she was bludgeoned with a bat or something." He indicates that she is still alive.
You make your way down the corridor towards the kitchen. Your partner has called for EMS and you can hear the sirens in the distance. The hallway floor is covered with blood in several locations. There is considerable blood spatter on the walls. You try to navigate the hallway without stepping in any of the blood or areas of blood wipe where the victim has either crawled or was dragged. You reach out to put your hand on the wall to steady yourself but there is blood spatter and a smeared hand print on the wall and you pull your hand back quickly. All of this blood spatter and wipes are critical evidence and clues to determining what happened at this crime scene. It must be preserved until the analysts from the CSI team arrive.
Securing the scene is not always easy
Now you have a new problem. The EMS team has arrived and is entering the front door of the house. Immediately you have to redirect them to the back kitchen door before they walk thru any of the blood evidence in the rest of the house. As they enter the kitchen and attend to the victim there you have to attempt to keep them from compromising any of the blood evidence, realizing of course that their primary mission is to remove the living victim and get them to the hospital.
As the investigation team arrives, you brief them on the scene and then work to secure the scene from neighbors and the press all of whom will want to see what is going on. You have managed to do all this without disturbing any of the vital blood evidence, but what if you hadn't? What vital evidence might have been destroyed?
Bloodstain evidence is critical
Stepping in blood would spread blood to areas not directly involved in the crime, or potentially cover up and obscure the interpretation of other shoe patterns in the blood. Accidentally putting your hand on a bloody wall, even to steady yourself might have disturbed some valuable blood spatter pattern or wiped thru a bloody thumb- or palm print on the wall. For these reasons it is highly important for officers in the field to understand what type of information can be obtained from the application of modern blood spatter analysis.
Advances in blood spatter analysis over the past decade have allowed CSI technicians the ability to reconstruct even very complex crime scenes where multiple victims are involved, as well as identifying situations where more than one perpetrator was involved.
Some old analytical reagents for detecting blood have been packaged in more convenient formats that allows for their easier use in the filed. These reagents--Luminol, Phenolphthalein, O-Toluidine and combinations of these--provide very accurate detection of readily observable blood as well as latent blood and blood patterns where the perpetrator has cleaned the crime area in an attempt to hide the evidence. One product, Hexagon OBTI (Evident Co., Union Hall, Virginia) can provide a determination right in the field that blood is indeed human. The hemoglobin in the blood sample reacts with an antibody to human hemoglobin and produces a blue line on the detector wand. The detection device looks similar to an over-the-counter pregnancy test kit.
Colored string is often used at a crime scene to demonstrate the path of blood droplets cast off from a knife of object used in a beating. These devices can give investigators a preliminary determination of the location of victim and perpetrator at the time of the crime. They become even more important in situations where there is more than one perpetrator or where a victim manages to crawl from one location to another or is dragged. However, the definitive interpretation of blood patterns comes when the information is analyzed back at the laboratory using computer programs like BackTrack or Crime Scene Command.