Wednesday, May 9, 2012

CSI: Collecting Evidence at a Homicide

Even those naive to the inner workings of a crime scene investigation know that the collection, preservation, and analysis of evidence can all be the difference between a conviction and an acquittal. Tantamount to this is the fact that all of these evidentiary factors can also mean the difference between convicting a guilty person and convicting an innocent person. Crime scene investigating is an amalgamation of science, logic, and law. Although physical evidence is only part of the equation—with the ultimate goal being to convict the guilty party(s) of the crime—it does deserve special attention and training in and of itself in order to ensure the evidence is effectively used to help solve the crime.

At this particular crime scene (hypothetical crime scene), the investigation involves four key pieces of physical evidence that must be accounted for: dried blood, a handgun, shell casings, and hairs. When collecting these pieces of evidence, the investigator must follow strict protocol to ensure the preservation of such evidence.

Dried Blood

Dried blood will show up at just about every crime scene involving murder. There are several protocols used by investigators to ensure that this evidence gets to the lab with as little harm done to it as possible. If the dried blood is on clothing, for instance, it should be wrapped in clean paper, placed in a brown paper bag or box, sealed and labeled; the investigator should not attempt to remove the stain(s) from the cloth. If, by chance, the dried blood is on a small solid objects; the investigator should send the whole stained object to the Laboratory, after labeling and packaging it.

If the blood happens to be on large solid objects; the investigator should cover the stained area with clean paper and seal the edges down with tape to prevent loss or contamination. If it is not practical to transport the entire object to the lab, the blood stain should be scraped onto a clean piece of paper that can be folded and placed in an envelope. It should not be scraped directly into evidence envelope; rather, it should be scraped from objects using a freshly washed and dried knife or similar tool. The investigator should take precaution by washing and drying the tool before each stain is scraped off, before sealing and marking the envelope. Dried blood stains should not be mixed, but rather they should each be placed in a separate envelope, and they should never be wiped from an object using a moist cloth or paper.

Using various testing methods at the lab, such as the restriction fragment length polymorphism (RFLP) process or the polymerase chain reaction (PCR) process, blood can be examined and amplified in order to determine the genetic profile of the individual who left it behind.


Ballistic evidence can lend a lot to an investigation, answering questions such as what type of gun was used, how many shots were fired, and even where the shooter was positioned when firing in relation to the victim. The most obvious rule of collecting a gun from a crime scene is to never submit a loaded gun to the lab unless it is delivered in person. Unfired cartridges should be left in the magazine of a weapon, provided the magazine is removed from the gun—a firearm with the cartridge in the chamber should never be shipped by any method, even if the weapon is not cocked or on safety.

The serial number should be recorded as well as should be the make, model, and caliber of the weapon, and it should be marked in some inconspicuous manner so it does not detract from its value before sending it to the Laboratory. Weapons should be placed in strong cardboard or wooden boxes and be well packed to prevent shifting of guns in transit. Also, rifles or shotguns should not be taken apart on the scene; this should be left to ballistics experts. If blood or any other material, which may pertain to an investigation, is present on the gun, the gun should be wrapped in clean paper and sealed with tape to prevent movement of the gun and loss of the sample during shipment.

Investigators should never clean the bore, chamber, or cylinder before submitting a firearm, and never attempt to fire the gun before it is examined in the lab. A handgun should be picked up with gloved hands (with index finger and thumb) on an area of the weapon that is unlikely to produce useful fingerprints, such as by the curvature around the trigger. A submerged weapon should be sealed in a plastic container while still under water.

Shell Casings

Shell casings can tell investigators several things about how the crime took place. Shell casings can also tell investigators what caliber, brand, and model of the gun used, as well as if the marks on the casings may show that the gun is involved in other investigations. Casings should be wrapped and sealed in separate labeled pill boxes or envelopes.

Striation marks (or microscopic scratches) inside of the barrel of the gun transfer onto the bullets when they are fired. These marks are unique to each barrel, which allows forensic investigators to identify a weapon from the bullets fired from it. A lack of shell casings at a scene could mean that the shooting took place elsewhere, the weapon didn’t eject the spent shells, or the shooter took the time to clean them before leaving.

The gun can also be fired back at the lab by ballistic experts in order to determine if the bullets found at the scene came from the gun in question and how close the gun was fired from. Firearm and tool mark examinations (called “ballistic fingerprinting”) is also done by ballistic experts.


Hair left at the crime scene can also be a valuable piece of evidence in an investigation. Along from the many things that it can reveal, it can sometimes reveal the possible race of the individual from whom it came and the part of the body from which it originated. All hair should be recovered from a scene. Hair should be collected using gloved fingers or tweezers, placed in paper bindles or coin envelopes, folded and sealed in larger envelopes, and then labeled on the outer sealed envelope.

If hair is attached, such as in dry blood, or caught in metal or a crack of glass, the investigator should not attempt to remove it, but should rather leave the hair together with the object. If the object is small, it should be marked, wrapped, and sealed into an envelope. If the object is large, the area containing the hair should be wrapped in paper to prevent loss of hairs during shipment. In the lab, human hair can be compared to determine whether or not two samples could have had a common origin.

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