This type of evidence is often misrepresented on television. How many times have you seen a murder victim lying in a pool of blood on a TV show? These victims are usually in a thick, glossy, bright red liquid pool, right?
Not so much.
Pools of any depth begin to separate. Thin layers of blood tend to dry quickly and become dark brown. These TV victims would realistically be lying in dried, sticky brown stuff or in a sticky mess of separated plasma and browning goo. It may be one of the forensic elements on TV shows that irritates me most, but I’ll get off my soap box now.
Blood is a fascinating substance. It’s three to four times thicker than water. It has a metallic scent and taste. And it’s something we’ve all seen at some point. This “liquid life” also carries with it evidence of poison in many cases, our DNA, information about our health, our blood type, and sometimes our state of mind (adrenaline and cortisol). We’ve used the ABO blood group typing system since about 1930. It was, before DNA, what we used to eliminate possible suspects from crime scenes.
Beyond type and content, how blood is left at the crime scene gives great details about what occurred. For example, blood may be dripped, transferred, or splattered.
Dripped blood is passive, dropped in a round or elliptical pattern depending on whether the bleeding person is standing still, moving slowly, or moving fast. Read by a knowledgeable investigator, direction and rate of movement become clear.
Transfer happens when, say, a hand or sleeve comes in contact with blood and then touches another surface such as a wall or towel. If both parties have been injured (not uncommon in stabbings), their blood may mix before transfer.
Spatter is the sexy part of blood evidence for most people interested in forensics. Spatter comes in three types: low, medium, and high velocity.
Low velocity spatter is often “cast off,” or flung from an object used to strike or stab the victim. Imagine shaking water from your hands to get the idea.
Medium velocity spatter leaves larger patterns, such as arterial spray or blood erupting from the wound when struck.
High velocity spatter is a fine mist and generally caused by bullets.
The pattern of blood spatter, including voids in spatter that may mark the location of the perpetrator, reveals such things as the attacker’s height, left or right handedness, where they were positioned, and possibly how hard the victim was struck. It may also reveal the type of weapon (independent of the wounds), height, strength, and angle of the blows.
Blood is corrosive and really soaks into things. Even with a good scrubbing, traces will remain on most surfaces. It can be painted over, smeared with cleaners, even removed from the naked eye, but it cannot be eradicated completely unless you care to replace every element of the room including those floor joists.
To assist crime scene technicians in uncovering evidence of blood, the traditional method of luminal is used in conjunction with a black light. These days, lasers are used in some cases and a new forensic camera will eventually replace luminal all together.
Beyond walls, carpet, and furniture, blood can end up in the smallest of nooks and crannies: behind unsealed baseboards, the undersides of lamp shades, on the ceiling across the room. In one case blood, absorbed through carpet and subfloor(both of which were replaced), only to be discovered along the floor joist. It’s been found in sink drains and plumbing pipes, in soil and on plants, under furniture, in the tiny screws of eyeglasses, between a cleaned knife’s hilt and blade, and even inside the tiniest cracks of a cell phone.
There’s one more thing blood can accomplish. It can, in court, “prove” murder or death if enough blood is present to show that life could not be sustained.
Blood evidence, to my mind, is fantastic for writers. We can hold the tension of lack of evidence and then reveal a whole crime scene with luminal. We can provide what appear to be inconsistencies for our characters to resolve. If set in history, a brilliant detective might use the budding technique of studying blood evidence to solve a crime that is otherwise unsolvable. And the finding of the tiniest speck of blood in the oddest of places can make a case the reader thought was lost.