Saturday, September 19, 2009

TIm's question

In a typical forensic sweep of a crime scene are there certain things that are sampled as standard procedure and others which might be overlooked unless you were specifically searching for them, or are samples of carpets, dust, etc just collected across the board and then analyzed later?

So if there was a violent struggle and a body at the scene, would the focus solely be on blood samples, hair and tissue, and so on, or would random dust or surrounding matter be scooped up. I'm wondering in the context of poisons being used, something that might be left behind as a trace powder, but not something you'd look for if there seemed to be a more obvious, physical (as opposed to chemical) cause of death.

How much trace evidence, like hairs, fibers, dust and powders are collected at a scene will depend upon the circumstances of the death, the appearance or lack of likely suspects, and the habits and training of the investigating agency.

If there’s a visible streak of powder on the carpeting near the body, with no apparent explanation, it would probably be collected. If it’s next to a spilled jar of scouring powder or something that it is likely to have come from, it would probably be ignored. If it’s on the other side of a generally filthy room, it would probably be ignored. Once collected, it would be tested as a potential drug like cocaine, and then stored. It would probably not be tested unless the investigators were really grasping at straws, because it’s a long story to test a substance when you have no idea what it is. You have to test things for things. Narcotic? Adhesive? Accelerant? Blood? If it’s curare or Cover Girl Shine Free translucent eye shadow, we probably don’t have the supplies or procedure to identify it. Never mind that they do it in CSI every week. In real life, we don’t have a Bat Computer.

When I worked at the coroner’s office, we would tape the clothing and the scene for hairs and fibers, but only if the person died of strangling or bludgeoning. We didn’t tape gunshot victims when there was no reason to believe there had been any physical contact with the suspect. (Incidently, taping is generally better than vacuuming because adhesive tape picks up only the most recently deposited debris. A vacuum might suck up stuff that’s been there for years.) However, now that hair and fiber analysis is such a dying art, agencies may no longer do even that. Ditto for collecting samples of carpeting or upholstery at the scene to be compared to anything found on the suspect. At the moment my boss here in Florida is looking into getting me a comparison microscope so I can start doing hair/fiber analysis again, as even the state labs discontinued the practice. With this in mind, it’s likely that hairs and fibers would not even be collected at the scene, certainly not dirt or dust.

So an obvious powder might be collected, but only as a possible illegal drug. With signs of a violent struggle present it’s unlikely that the idea of poison would enter anyone’s mind.

But if your character needs a hair and fiber expert, well, let me give you my number….

Lisa Black spent the happiest five years of her life in a morgue. Strange, perhaps, but true. In her job as a forensic scientist she analyzed gunshot residue on hands and clothing, hairs, fibers, paint, glass, DNA, blood and many other forms of trace evidence, as well as crime scenes. Now she’s as a latent print examiner and CSI working with fingerprints and crime scenes. She has been published in Germany, the Netherlands, France, the United Kingdom, Spain and Japan.


Shane Gericke said...

Why is everyone abandoning hair and fiber analysis, Lisa? Is there new science that works better? If it still works, why wouldn't they do the analyses?

Lisa Black said...

Because all you can say with fibers and (microscopic analysis of) hairs is that "all characteristics are consistent and could have had a common origin." We can't say the hair 'matches' this person or the fiber 'matches' their clothing. We can't even lay odds on what are the chances this fiber came from their shirt, because even if we knew how many shirts of that type were manufactured, even if we knew how many were sold in the area, and even if they were very recently sold so probably still in use as clothing and not car wash rags, we STILL couldn't say 'there's a 1 in 2 billion chance that this fiber could have come from someone else's shirt', the way they can with DNA results. We can't give the jury a handy yes or no with such evidence, so they're getting information without any clear idea of what to do with it. So most attorneys (and cops) feel it's better to leave it out.

Rebecca Cantrell said...

Fascinating discussion! I understand why it might be useful at trial because it can't be statistically quantified, but it seems like it might still help the investigation, leading to different suspects, etc. But maybe folks are just shedding so much DNA that it's not necessary? (seems wrong, somehow)

Lisa Black said...

It seems wrong to me, too, to discard information just because it isn't convenient.