The students discovered first-hand over the past couple of weeks that firearm and toolmark identification is much more difficult in real life than it is on TV. The forensic specialty of firearm identification focuses on trying to determine the specific firearm from which specific ammunition was discharged. Since many crimes involve one or more guns as a weapon of choice, being able to identify the actual weapon used is of vital importance in linking a suspect with the crime scene.
When examining firearms and their ammunition, investigators first look at class characteristics to narrow down their search. A fairly casual examination by an experienced investigator can provide many helpful clues: the caliber of the weapon, the manufacturer of the ammunition, and the type of weapon used, such as a handgun vs. a rifle. This is only the beginning, though! For example, hundreds of thousands of Glock 9mm semi-automatic pistols have been produced and sold in the U.S. alone, so how can you determine if it’s YOUR SUSPECT’s Glock 9mm that actually did the deed? This is where identifying individual characteristics comes in.
Even two guns that are of the same make and model can have slight differences in their overall structure that make them unique. For example, the barrel of most guns (except shotguns) is actual rifled; it is not smooth. When the bullet travels down the barrel, certain impressions (known as striations) of the barrel’s rifling pattern are left on the surface of the bullet. These striations can be compared to those found on a test, or reference, bullet that is fired from the suspect’s gun. No two guns have the same exact rifling pattern. Keep in mind that at this point, the investigator has a pretty good idea of what type of weapon was used; now he or she is trying to determine the specific weapon used.
Other important individual characteristics include firing pin, extractor, and ejector impressions left on the bullet casing (or jacket) by the weapon’s breechface. Again, even guns that are of the same make and model may have slight imperfections in their breechfaces, leaving a unique impression on every cartridge that is fired from that gun. This individual characteristic is helpful when only the casing of a bullet is found and not the bullet itself. Since the casing itself doesn’t travel down the barrel, it does not acquire any striations on its surface the way the bullet does.
Since a firearm was involved in our crime on campus, the students went about processing the casings and bullet fragments that were collected from the crime scene. Based on a class characteristic analysis, they have determined that a .22 handgun was used to fire the ammunition. They compared the crime scene ammunition to reference ammunition obtained from the guns belonging to some of our suspects. This past week, they had to submit their preliminary report of evidence collected so far, and it will be interesting to read their conclusions concerning which gun (if any of these tested) may have been used…
Dr. Shelly supervises while Kyle and Grace try to reconstruct the path of the bullets that struck the evidence locker. Apparently, the perpetrator tried to shoot the box open, and bullets ricocheted off of the lock and went through parts of the box. Technically, this is “ballistics analysis,” which is different from firearm identification. The field of ballistics attempts to predict or reconstruct the trajectory of bullets fired in order to determine the location of the shooter, victim, and other relevant objects at the crime scene.
Before working with the “real” evidence, the students practiced identifying class characteristics of several reference cartridges, including those fired from handguns, shotguns, and rifles. Here, Cheryl, Seth, and Brandon are using a stereoscopic microscope to examine the firing pin impression left on this handgun ammunition.
After examining the firearm evidence, the students practiced obtaining toolmark impressions from several objects. A toolmark impression is any cut, gouge, dent, or other alteration that is left on one object by another. The term “tool” is a general term for any object that causes this type of impression, although the most common “tools” are pry bars, bolt cutters, screwdrivers, and other objects typically involved in breaking into something. Since it also appears that our perpetrator tried to break into the evidence locker using a hammer and a wrench to bust the lock, determining exactly which tool he or she used is an important aspect of reconstructing the crime scene. A permanent record of the toolmark impression is obtained by using a putty-like substance, such as Mikrosil, to make a malleable cast of the impression. Just like with firearm identification, if a “tool of interest” has been collected, investigators make reference toolmarks with the tool and compare these toolmarks to those left at the crime scene. This way, the tool itself and the object with the toolmarks are not altered or damaged in any way. The important thing to remember is that any trace evidence (hair, fingerprints, paint chips) that may be present should be collected BEFORE Mikrosil is applied; otherwise, you risk losing this precious evidence when you lift the putty from the tool or object. In this picture, Callie and Jessica are comparing the toolmarks left by several Phillips flathead screwdrivers (all made by Craftsmen) and now realize how just knowing the make and model of a tool is, like firearms, not enough to determine what specific tool made the impression.
Stay tuned for the next lab episode, which will likely be very “hairy”…