Documentation in Forensic Cases
SFU Museum of Archaeology and Ethnology
Once you have mastered the process of the scientific method you become very aware of the need for data or as it is called in the forensic world "evidence". There are two general types of evidence used in both science and law - Physical and Testimonial evidence. Physical evidence is comprised of those forms of data that can be measured or quantified. Examples include fingerprints, accelerants, hair or fibers, etc. These types of evidence can be measured, weighed, and defined by a number of other physical methods. Testimonial evidence is comprised of observations either by the scientist or witnesses. Physical evidence or data is often called "real" evidence or "hard" data. The reason for this distinction is that physical evidence can be measured numerically while testimonial evidence is more subjective in nature. Each individual remembers events in a slightly different way and even observations of things like color depend on the individual's visual ability to discern different hues. Another way to look at the two types of evidence or data is that physical evidence is objective and testimonial evidence is subjective.
Crime Scene Evidence
Eric Liou
Different Types of Evidence Require Different Collection Techniques
As you saw on the previous slide there are many different types of evidence and as a result there are many ways in which to collect and preserve these types of evidence. The stress on the word preserve is deliberate. Just collecting evidence does not mean that it will remain intact and unchanged between the time it is collected and analyzed. Steps must be taken to be sure the evidence does not degrade. The collection and proper preservation of evidence will now be discussed. Pay close attention since in just a few weeks you will need to put these techniques into practice.
Blood
Blood Stain
Blood and crime scenes seem almost synonymous, you can't have one without the other. This is not completely true but Blood is one type of evidence that can provide large amounts of data for an investigation (blood type, DNA, etc.) and it is therefore imperative that the blood that is collected be kept pure and undecayed.
Blood that is in a liquid form should be picked up on a gauze pad or other clean sterile cotton cloth and allowed to air dry thoroughly, at room temperature. Once dry, It should be refrigerated or frozen as soon as possible and brought to the Laboratory as quickly as possible. Because DNA and other components in the blood decay rapidly, a delay of more than 48 hours may make the samples useless.
You obviously cannot dry a blood stained sheet or shirt at the crime scene itself so you must prepare these samples for transport. At the crime scene, you should label and roll the evidence in paper or place it in a brown paper bag or box and seal and label container. Never place more than one piece of evidence in a container. Any never use plastic containers. The lack of ventilation in plastic promotes decay and will destroy the blood evidence before you can get it to the lab.
You might think that drying the stain faster would be a good way to preserve the stain but you cannot heat the stained material or place it in bright sunlight to dry as this will degrade the biological value of the stain. Once the evidence is in the lab, you should hang clothing and similar articles in a room or cabinet where there is adequate ventilation but low light is the best environment for drying with minimal degradation.
Collecting Samples
What do you do if the blood stain is already dried in place? Well, if the object that is stained is small enough to package and move, the entire object should be taken to the lab. If the object cannot be moved, like a tree or floor or carpet then there are several good ways to collect the evidence. For a solid object that cannot be moved or cut, the stain should be scraped off over a paper envelop, sealed and labeled. For a surface like a concrete floor where scraping will not work, a damp swabbing is the best collection method. Then you follow the same protocols from there on that you would use for any other wet blood sample. For blood stains on carpet or cloth furniture or even wall paper, the area containing the stain should be cut out and transported whole to the laboratory. Each piece recovered should be labeled and packed separately as always.
Semen, Spit, Urine and Other Fluids
Semen Under UV Light
Other biological fluids have some of the same issues in collection as blood. Containment in plastic will cause degradation, so again air drying the object is the best method of preservation. Once dry the object can be refrigerated or frozen to further preserve the evidence. Freezing should only be used if you are sure the object containing the stain will not "sweat" as it is thawing. If the object will produce condensation (like a metal or plastic object) while it thaws do not use this preservation technique.
Hair, Fibers and Threads
Forensic Evidence: Hair & Fiber
Gerald Baldwin
Hair and fibershair are another valuable type of evidence. Hair can often tell investigators the race of an assailant or the part of the body from where the hair was lost. If the root is intact, hair can also provide DNA evidence. Fiber evidence is used to connect suspects to crime scenes. This type of evidence is often found torn off in windows or on jagged surfaces or fences. Matching fibers from crime scenes to suspect clothing, while not conclusive evidence of the participation in a crime, can often add to an evidence pile to place the suspect at a location or at least prove the possibility they were there.
Collection of these tiny pieces of evidence generally pertains to not losing them. Unlike blood evidence the hair or fiber evidence should not be added directly to a mailing envelope as it can often fall through the seams in these types of envelopes. Hair and fiber evidence should be picked up with gloved fingers or tongs and then wrapped in folded paper and then placed into a coin envelope. It should be labeled as usual. Also as usual, samples from different sites, even if they look the same, should be collected and stored separately.
Hairs and fibers that are combined with blood or other fluids should be collected and preserved intact whenever possible. Use the previously discussed protocols for fluids as the correct procedure to follow as hair and fibers do not suffer from decay unless allowed to mildew or mold from exposure to wet.
Glass, Rocks, Guns, Bullets, Shoes, Paper and All Other Types of "Hard" Evidence
Gun as Evidence
Object evidence like bullets, shoes, guns, knives etc. are solid objects that simply need to be collected, wrapped in paper, labeled and stored in paper bags or boxes to preserve their condition so that it remains unchanged from the crime scene to the lab. Again, if biological evidence is on the object then those protocols take precedence but a bullet or spent cartridge at a crime scene can simply be collected. Each piece of evidence requires its own container and label.
Foot Prints, Tool Mark and Tire Tracks: Impression Evidence
Leave No Trace - Footprint Evidence
BBCWorldwide (YouTube)
Tool Marks, Footprints and Tire Tracks fall under the category of Impression evidence; that is evidence left behind as an impression in a softer material. Collection and preservation of these types of evidence requires special techniques not used for other types of evidence. Because the impressions in soft materials like dirt or sand can easily be disturbed and thereby lost, the upmost care must be taken not to walk on or around the area of the impression without extreme care. Preservation of tire and footprints can be made by casting the impression using specially formulated gels or plasters. Tool marks can also be cast but this requires a high level of expertise since these marks are often on vertical surfaces and can be damaged or lost in the process if not done properly.
Whenever possible tool marks should be collected by cutting them out of the surface in which they are located or by simply collecting the object they are found on. Again we use the process of wrapping the evidence in paper and bagging or boxing it so that it cannot be lost or damaged in transport. The location of a tire or footprint should be thoroughly photographed before attempting to cast it. This establishes both the location of the print and also provides a backup of what the impression looked like if the cast is not successful. Once dry, the cast should be wrapped in protective material such as cotton or bubble-wrap and then bagged or boxed to keep it safe and whole.
A Crime Scene: Too Little Evidence or Too Much?
Crime Scene under Investigation
As you saw on the original listing there are many other types of evidence and we may revisit the process for collecting and preserving them as the need arises but for now, I think that you are starting to see the similarities in the processes used regardless of the type of evidence being collected.
Common sense in collection of evidence should always be used. Wet materials should be allowed to dry in air-able containers to prevent rot, objects should be isolated to prevent transfer contamination, labels should be provided for each piece of evidence and each piece should be sealed in a container that prevents loss and then delivered to the lab as soon as possible.
Each and every piece of evidence at a crime scene must be collected, labeled and preserved. In most cases, the scenes provide plenty of objects to collect, most of which will provide little to no evidentiary value but a crime scene technician will collect many such objects as the old adage that it is better to be safe than sorry really does apply at a crime scene and until more is known about the crime it is often hard to determine what has value and what does not.
In each scene you should keep in mind that every piece of evidence is chemical in nature and therefore can be analyzed by chemical processes. We will explore many of these methods in the lectures to come.