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Posts tagged ‘evidence’

Forensics for Writers: Toolmarks and Firearms

A great deal of the forensic scientist’s time is spent peering into a microscope. Our fictional heroes aren’t interested in that tedium, and neither are the readers of their adventures. But understanding what the results of that microscopic inspection reveals, and what it means to an investigation, that’s important. And as with any other aspect of this, a hero or villain getting it wrong through incompetence or malice adds to the conflict in a book, even if we don’t want to see it in real life.

comparison microscope

A comparison microscope allows observation of two objects without bringing them into contact with one another.

I think all of us can grasp the concept of a tool leaving a mark when it is used to, say, pry open a window or a door. But logically, this isn’t going to be terribly helpful in identifying the specific tool that was used – after all, hundreds if not thousands of crowbars have been made in various configuration. However, through the use of a comparison microscope that allows the side-by-side comparison of the marks and the tool, individualization can be made in some – not all – cases.

I’m not sure I covered the two classes of evidence before, so this is a good place to touch on it. Classification is the sorting of physical evidence that will match a class of something. Cotton fibers, that mark was made by a crowbar, those footwear impressions were made by Doc Marten boots. Classification is useful in narrowing the field down, but what every investigator wants and hopes for is individualization. This cotton fiber with cochineal dye belongs to that shirt. This crowbar with the nick in the pry surface made that mark. These boots with the cut on the sole made that impression. Such evidence can tie a suspect or his tools to a scene.

Tools can acquire unique characteristics through wear and tear. I’ve chipped and dented many by dropping them, personally. Tools that have come in contact with paint can carry away traces of that paint, which can be classified or individualized. There is, and this is pretty cool albeit very nerdy of me to call it that, a paint database. The RC Mounted Police maintain PDQ, the pain data query, which is used worldwide to determine where that paint came from, what it was used on, and so forth. Even if you can’t individualize something, enough classifications can resolve into the pointing finger of probability.

Side note for our heroic investigator (or the curling-mustachioed Snidely (sorry, channeling Dudley here)) is that you would never, I don’t care what you’ve seen on TV, attempt to match the tool with the mark on scene. You can alter and destroy evidence that way. The correct procedure is to make a cast of the toolmark with a special kind of silicone called Mikrosil. Later, at the lab, a stereomicroscope will be used to compare the mark with the tool.

Firearm evidence is a huge field, but I’ll cover the basics here, of what marks a gun leaves on bullets and casings. When a bullet is fired, and forced through a rifled barrel, minute striations are left on the soft lead of the bullet by the rifling marks in the barrel, called lands and grooves. In theory, a second test-firing through that same barrel can produce a bullet which will have matching striations, and they can be matched under a comparison microscope. Those of you who are familiar with firearms will immediately see the problems inherent with this. Not all firearms will have rifling. Shotguns, for instance, cannot be matched in this way. Alteration of, or removal of, a barrel can also alter the test firing results. My instructor a while back, an actively serving detective sergeant, would get entertainingly frothy in class about the moron on TV who routinely picked up suspect weapons by inserting a pen into the barrel. Granted, this will not alter the inner map of the barrel much, but it would not take much for the defense to introduce doubts in the jury’s mind.

Sometimes the other end of the action, the casing, can be more helpful than the projectile. While a revolver leaves no casings behind for our intrepid investigator, a semi-automatic ejects them automatically (heh). A savvy criminal might police his brass, if he has time. But the marks left on casings by the ejector, the firing pin, and even fingerprints from loading, can be possible individualized evidence in building a case. It helps if the investigator knows what he is looking for. A while back in a non-investigative CJS class, I had to read and write an analysis of a case. In it, the evidence was listed, and one thing they found in and around the house of the shooter in question were numerous (something like 40 without looking at the case file) unfired casings. That, to me, was hinky. I would have been very unhappy with taking that evidence anywhere listed as it was (the professor of that class got very impatient with my hang-up on the evidence, which I found funny). Do you know why it struck me wrong?

What you could see with the comparison ‘scope allows a very close match of the test fire and the casing collected at the scene.

I will finish up the bulk of the series next week with toxicology (and a foray into serology). You can find the first part here, the Crime Scene here, Evidence and Investigation here, and Blood Spatter and Ballistics here. I’ll wind up the series in two weeks with a linkful post, and a summary with Q&A session in the comments.

Forensics for Writers: Blood Spatter and Ballistics

First thing, before the gory details: it’s blood spatter, no L. Not splatter, even if that is descriptive of how it got there, and there, and ew… there. The technical term is spatter, not splatter. Now that I have assuaged my inner editor…

As a writer the mystery of a crime scene can be worth revisiting, just as an investigator must do to try and recreate what happened there, from the evidence left behind. In the case of a violent crime, that evidence can speak volumes from the blood. The human body only contains 5-6 liters of blood (roughly) but it is under pressure as the heart beats. To an investigator, blood reveals much. It can reveal whether the bleeding person stood still – a passive droplet is completely round, but there are differences in the shape dependent on the surface it fell on, which can lead to ‘satellite’ splashes as the blood strikes a hard surface. The blood will show the direction a person was moving – you look at the ‘tail’ of a drop, and it is pointing in the same direction the drop was moving when it touched down, like a tiny accusatory arrow.

Blood that is coughed up, or expirated, will have tiny bubbles in it, a distinctive indicator of internal trauma that can remain long after the injured person moved on. From a deep wound, arterial spray shows gouts of blood, and if it was directed against a vertical surface (as I have seen a photo of someone trying to make it down a stairwell) you will see peaks in the wave-like pattern, from the beat of the heart, but the peaks get lower, and finally there is the last big smear as the injured person fell against the wall and too the floor…. You can prove death without a body in the case of spilling so much blood the body could not have sustained a heartbeat and would surely have been dead.

When an object strikes the body with sufficient force, blood leaves the body. The level of force, and roughly the size of the object, can be determined from this. A gunshot wound will show signs of high-velocity blood spatter, which is defined as less than 1 mm in diameter. This never travels very far, so you are most likely to find it on the victim, the suspect if they were within a half meter of the wound when it was made, or objects that were close. As the bullet strikes the body, the blood is projected outward in the opposite direction, but this isn’t blowback – that is the powder residue of that gun which will land on the shooter’s hand and clothing.

The size of an object makes a difference in the size of the blood spatter. An object with a smaller diameter, say, a length of rebar, will cause smaller droplets than a baseball bat. In class we speculated about what kind of spatter a cast-iron skillet would cause – I’m going to guess fairly large, low-velocity spatter, more than 4 mm in diameter. Messy… Medium-velocity spatter falls in between, from 2-4 mm in diameter. All those are rough numbers, but they allow the investigator to see patterns, like castoff from an attack where the victim was struck more than once, and the bloody implement slung droplets off as it was brought down again. Long, thin trails of castoff with small medium-velocity droplets indicates a sharp blade like a knife or a sword. These cast-off marks can show what hand the suspect preferred, useful if you need to know you are looking for a sinister rather than a dextral.

Currently, most crime scene analysts (ignoring the TV shows, please) carefully measure and plot the directionality of the droplets, to get an idea of where the victim was, where the assailant was, any movement during or after the attack – this can be told by altered marks, which can show smearing and retain prints of friction ridges, palm prints being as useful as finger prints. Movement after a certain time leads to what are called skeletonized droplets, which dry from the outside in. When touched, the still liquid interior smears, but the dry outline, or skeleton, remains. This drying begins at roughly (depending on atmospheric conditions) fifty seconds. This allows a timeline of the crime to be developed. There is a formula for determining the angle at which a droplet hit the surface, Sin (impact)=width/length.  And if something or someone was at the scene, and then removed, that absence causes missing blood, which is called a void.

Related to the determination of the angle at which blood struck a surface is the determination of what angle a bullet was traveling when it struck a person. Ballistics is only this – it is not marks on the bullet itself or the firearm, those are properly referred to as toolmarks. “Ballistics (from Greek βάλλειν ballein, “to throw”) is the science of mechanics that deals with the launching, flight, behavior, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance.” You can find a cool calculator here.

Using ballistics, your investigator can discover where the projectile came from, how far it was likely to have traveled, based on what sort of bullet it was… pistols and shotguns don’t have a terribly long range, but that rifle bullet could have traveled hundreds of meters, and a sniper round even further. Determining the sort of bullet falls under next week’s post on toolmarks, fingerprints, and other ways criminals leave their signatures at a scene.

You can find week one here, the Introduction, then Crime Scene, and Evidence and Investigation. I am planning two more installments, one on marks and another on toxicology, and perhaps one last post wrapping it all up with a list of the links and resources for people. As always, ask questions in the comments and I’ll answer there, or in the posts where they fit. Hopefully this is being helpful to some!