Recall the importance of maintaining a chain of custody. It is a critically important legal requirement for the introduction of physical evidence in court that the evidence item is identified as the same item collected at the scene and that the item has not been altered in any significant way. Thus, any person in physical control of the evidence, no matter how briefly, must be recorded so that they can testify in court that the evidence was not altered. Any break in the chain of custody will likely result in the exclusion of the evidence in court.
All personnel responsible for the chain of custody of evidence items should maintain written records documenting the sample number, a description of the evidence, the date and location where the evidence was found, the name of the person collecting the evidence, and any other information required by policy. All transfers of custody should be documented, including the name of the recipient as well as the date and manner the evidence was transferred. The final disposition of the evidence should be documented as well.
Another critically important element in the scientific validity of the analysis of physical evidence is preventing contamination of the evidence. Contaminated evidence is useless in court. This is why securing the crime scene is a critical responsibility of the first responder. In addition, it is critically important that evidence is protected from environmental factors that may cause damage to it, and the evidence must be packaged properly. Proper packaging and storage ensure that the evidence remains in the state that is was found at the crime scene.
It is also important to remember to collect comparison standards where appropriate. The analysis of many types of evidence involves comparing something found at the scene with something that is known to see if they are the same. A fingerprint found at a crime scene, for example, is useless without having a comparison standard taken from a suspect. The crime scene investigator must always remember to collect comparison standards when necessary for comparison by the crime lab.
Preventing changes in the condition of a sample after it has been collected requires carefully controlled packaging and transportation. The investigator should ensure that packaging, transportation, and storage procedures are followed to prevent any destructive changes in the condition of samples. (See the Appendix for a quick reference to how specific types of evidence should be packaged).
Many different types of evidence can be hazardous, and precautions should be taken when dealing with dangerous substances. If you do not know the proper safety precautions for a particular type of evidence, defer to someone who does. Note that all biological materials (blood, semen, saliva, etc.) should be considered hazardous. It is best to assume that all biological materials are contaminated with pathogens such as HIV, hepatitis, and others. Protective eyewear, protective gloves, respirator masks, and protective clothing should be worn when dealing with biological materials.
In addition to biological hazards, criminal investigations involve exposure to chemical hazards as well. Clandestine drug laboratories are a prime example. Many forensic chemicals, such as those used to develop latent fingerprints, are dangerous. Be sure to read the labels on all of the chemical supplies that you use, and obtain and read the chemical safety data sheets.
Biohazard was is more common at crime scenes than many realize. Officers often enter scenes where people have been hurt or killed, and blood is a common denominator. A basic forensic principle that whenever two things come into contact, something from one object is always transferred to the other and vice versa. This idea can be generalized to the transfer of biohazards such as blood. If an officer steps in blood at a crime scene, then there is blood on the officers boot. That blood may well be contaminated with dangerous pathogens, and anything the contaminated boot touches is also contaminated. In this way, biohazards can come into contact with people who were never present at the crime scene. Besides the obvious public health nightmare that this situation can create, there is also the specter of departmental liability to contend with. If it can be shown that police personnel negligently or recklessly caused harm to someone, then that person has the basis for a lawsuit.
The cure for all of these potential harms (as with many in policing) is adequate training and good policies that are rigorously enforced. Officers must understand the risks posed by biohazards, and they must be properly trained to mitigate those risks by properly using Personal Protective Equipment (PPE). Officers must also be trained in the proper (safe) disposal of PPE. The use of biohazard bags, sharps disposal containers, and so forth represent a nuisance for officers and an expense for departments, but those costs in officer time and public money pale in comparison to the potential harms that can be caused if the issue is ignored.
While the proper disposal of PPE such as gloves, shoe covers, face shields and the like is important, it is also important to realize that many “tools of the trade” need to be properly cleaned and sterilized or safely discarded. Swabs, pipettes, razor blades and a host of other equipment are routinely contaminated with biological materials. At the end of a long night, it is tempting just to toss those used tools into a kit and forget about them. This creates a real threat to officers from biological contamination, and it also creates a problem that materials from one crime scene may be transferred to a future crime scene via transference from unclean tools and equipment. Because of these dangers, departments need to carefully craft and implement safety protocols for dealing with contaminated equipment. A potential source for good advice (and possibly training) is the crime laboratory.
Is It Evidence?
Once a crime scene investigation draws to a close and the property is released back to the owners, the window for officers to find evidence quickly and irrevocably closes. Investigators must take care to identify and collect any and all items of evidential value. The costs of overlooking a useful evidence item can be very high in terms of the legal outcome of the case. In general, it can be said that it is better to collect items that can later be discarded if they turn out to have no evidential value than to not collect them and lose valuable evidence. The failure to identify and collect evidence at the crime scene is the most commonly cited facet of the evidence identification problem, but the converse can also be an issue. In other words, “overkill” can also be a problem for investigators.
Some departments have adopted standardized checklists of potential evidence, and these can be useful as a reminder of what types of things have the potential to become evidence. If investigators “bag and tag” every item on the list, however, the likelihood of having several boxes of useless evidence becomes quite high. This comes at a cost of inordinate amounts of the investigator’s time that could be better spend performing more fruitful tasks to move the investigation along. In addition, laboratory analysis is expensive and time-consuming. If labs accepted all possible evidence from all crime scenes, the backlog would quickly stretch into years.
The obvious way to eliminate both extremes is to develop the ability to “read” the scene and evaluate the evidence related to the case. Once a narrative of what transpired at the scene takes shape, certain types of evidence can be eliminated because they do not fit the narrative. For example, let us say that a victim is shot by a sniper at long range. It would be senseless for investigators to collect fingernail scrapings in such a case. Since the narrative suggests that the victim never came into physical contact with the perpetrator, there is no logical reason to suspect that any evidence would be found on the victim’s body.
Ensure that the portion of the area or object with the stain has been documented as it was found. When photographing the object:
- Include a scale and an identification label.
- Take one or more location photographs that show the object where it was found.
- Show the relationship of the object to other evidence in the photograph.
Label a container such as a paper bag or envelope with
- your initials and identification number
- the date and time
- case number
- evidence number
- evidence description
Each piece of evidence must have a unique number. This number should correspond to the placard next to the evidence.
The evidence description should include:
- Type of item (e.g., victim’s shirt, glass, carpet fibers)
- Location of the stain
- Whether the stain is wet or dry
- Location of the item at the crime scene
Accelerants and ignitable liquids recognition and collection are best performed by specialized personnel. For scene personnel, evidence may be observed through smell, sight, and sound, and should be recorded in notes.
NEVER attempt to collect any evidence until you have spoken with an accelerant and ignitable liquids investigator or specialist. Ensure the safety of people at or near the scene. Follow the instructions provided by the accelerants and ignitable liquids investigator or specialist with whom you speak.
Blood evidence is present in many types of crimes and can provide important information useful in reconstructing a crime.
Possible substrates with blood stains:
- Entire portable object
- Part of a non-portable object
- Stain on a non-porous surface
Equipment needed for bodily fluid collection includes:
- Paper bags, boxes, and envelopes
- Cotton-tipped swabs
- Paper bindles or other sterile swab storage container
- Distilled water or one-time use sterile water
- Scalpel, utility knife, or scissors
- Clean paper
- Waterproof pen
- Evidence tape
- Protective gloves
- Face protection
Commercial products are also available for crime scene collection of stains Blood and other physiological fluids are fragile, and certain best practices must be maintained. Do not package bloodstained evidence in plastic bags. If possible, collect the entire stained garment. Avoid altering the stain or transferring blood from one portion of the garment to another; do not fold or crumple the garment. Be careful not to lose or contaminate any remaining trace evidence on the garment.
Avoid excessive heat when collecting, transporting or storing blood evidence. Avoid moisture, water or other liquids. Avoid exposing the bloody evidence to strong light, especially UV light. Avoid touching, taking off gloves, or coughing/sneezing over or near the evidence. Describe the stain as a “red stain” or “apparent bloodstain”. Do not label it as blood if the stain has not been forensically identified as such. Mark package with appropriate cautions about contents, such as “Store Frozen” or by affixing Biohazard stickers.
Caution: Leaving evidence exposed at a crime scene can lead to contamination. It may not be possible to dry an item at the scene without risking contamination.
Storing Blood Evidence
Ideally, bloodstained items should be stored in a temperature-controlled environment (between 60–75 degrees, with less than 60% humidity). If stored at ambient temperature: Place the container in a secure, dry storage area. Never expose the container to extreme heat, such as from a heater vent. Avoid exposing the container to direct sunlight.
Semen and Bodily Fluid Evidence
Ensure that the portion of the area or object with the stain has been documented as it was found. When photographing the object: Include a scale and an identification label. Take one or more location photographs that show the object where it was found. Show the relationship of the object to other evidence in the photograph.
Locating Semen Stains
Unlike blood stains, semen stains are not always obvious to the unaided eye at a crime scene. Semen stains are difficult to see under room and ambient lighting conditions. They may appear as a slightly yellow stain on light-colored fabrics or a whitish stain on dark-colored fabrics. Semen stains may also appear “crusty.” Still, many stains will be missed by normal or unaided visual examination; therefore, it is best to collect any item that may have semen stains. Common items to collect are:
- Victim’s clothing, especially underwear of sexual assault victims
- Suspect’s clothing Bedding (e.g., blankets, sheets) where an alleged sexual assault took place
- Tissue paper
- Car seats
Detecting Semen Stains
Detecting semen stains: Items that are impractical to submit to the laboratory (e.g., vehicles, carpets) can be screened using special lighting techniques. All visual lighting techniques are screening tests that can fail to detect semen stains; detection varies depending on the type of fabric or material on which the stain may be deposited. Visual tests will not discriminate between many possible physiological fluids or fluorescent contaminants. A forensic light source is an alternate light source (ALS) that may cause semen stains to fluoresce when viewed through an appropriate color filter. Optimal wavelength is dependent on surface characteristics of an item. Certain surfaces appear to quench the fluorescent reaction. Argon ion laser causes a similar reaction as ALS.
Long-wave Ultraviolet (UV) Lamp
As a precaution, analysts must wear plastic UV eye protection and cover any bare skin that will be exposed, such as hands and arms, during the UV examination or viewing. Semen stains may appear on a dark background. It should be noted that some clothing could fluoresce due to such materials as detergents and food stains.
Collection of Semen Stains
Collection of semen stains: Minimize disturbance, transference/swiping and contamination of the stain. Gloves should be worn during the collection and handling of the swab. Always collect a control sample. Order of preference for collecting dry semen stains should be: Collect entire item bearing the stain. Ensure that the stain will not flake off or become dislodged. Cut the stain from carpet, upholstery or other items that cannot be collected. Moisten a sterile swab with distilled water, swab the suspected semen stain, and air dry prior to packaging.
Label a container that will be used to collect the item. Document the location of the item with photography, measurement, and sketching, where appropriate. Cigarette butts and used beverage cans or bottles are common types of saliva-containing evidence found at crime scenes. In sexual assault cases, consider swabbing the breast or other body areas to collect any potential saliva evidence if case circumstances dictate. Use a dry swab or moistened swab depending upon the circumstance of the stain. Use gloved hands or forceps to collect the item to prevent contamination. Do not lick the envelope flap or cough/sneeze on the sample, or contamination of the sample may occur.
Standard Sample for Body Fluid
In order to compare DNA types from the suspect(s) and victim(s) to evidence analysis results, a standard/reference sample must be collected. In cases involving semen as evidence, it is not necessary to obtain a semen reference standard. Saliva or buccal swab. standards are recommended. Note: Adhere to legal standards for search and seizure, if any, for your jurisdiction.
DNA reference material: Cellular material obtained by buccal swab is sufficient for DNA analysis. The preferred method is to collect buccal or saliva swabs with several clean swabs. Finger-pricking the suspect and placing the blood drops on filter paper, or specialty paper designed for this purpose, is acceptable. This reference material should be handled and processed as evidence.
References and Further Reading
- Michielsen, S. (2019). Transfer of Bloodstains from Textile Surfaces: A Fundamental Analysis. Available: https://www.ncjrs.gov/pdffiles1/nij/grants/252773.pdf
Modification History File Created: 05/02/2019 Last Modified: 02/03/2024
This work is licensed under an Open Educational Resource-Quality Master Source (OER-QMS) License.