Gathering DNA Evidence
Physical evidence is any tangible object that can connect an offender to a crime scene. Biological evidence, which contains DNA, is a type of physical evidence. However, biological evidence is not always visible to the naked eye. DNA testing has expanded the types of useful biological evidence. All biological evidence found at crime scenes can be subjected to DNA testing.
Identifying DNA Evidence
Remember that just because you cannot see a stain does not mean there are not enough cells for DNA typing. Further, DNA does more than just identify the source of the sample; it can place a known individual at a crime scene, in a home, or in a room where the suspect claimed not to have been. It can refute a claim of self-defense and put a weapon in the suspect’s hand. It can change a story from an alibi to one of consent. The more officers know how to use DNA, the more powerful a tool it becomes.
Crime Scene Integrity
Protection of the crime scene is essential to the protection of evidence. Safeguarding and preserving evidence is fundamental to the successful solution of a crime. Remember, while documenting evidence at the crime scene, to include descriptions of whether evidence was found wet or dry. An example of this documentation would include blood spatters.
The risk of contamination of any crime scene can be reduced by limiting incidental activity. It is important for all law enforcement personnel at the crime scene to make a conscious effort to refrain from smoking, eating, drinking, littering or any other actions which could compromise the crime scene. Because DNA evidence is more sensitive than other types of evidence, law enforcement personnel should be especially aware of their actions at the scene to prevent inadvertent contamination of evidence.
Chain of Custody
The chain of custody of evidence is a record of individuals who have had physical possession of the evidence. Documentation is critical to maintaining the integrity of the chain of custody. Maintaining the chain of custody is vital for any type of evidence. In addition, if laboratory analysis reveals that DNA evidence was contaminated, it may be necessary to identify persons who have handled that evidence. In processing the evidence, the fewer people handling the evidence, the better. There is less chance of contamination and a shorter chain of custody for court admissibility hearings.
Transportation and Storage
The first responding officer may be called upon to transport evidence from a crime scene. As with any evidence, the officer should ensure that the chain of custody is maintained. In addition, they should be aware that direct sunlight and warmer conditions may degrade DNA, and avoid storing evidence in places that may get hot, such as the trunk of the police car. To best preserve DNA evidence, store in a cold environment. Any probative biological sample that has been stored dry or frozen, regardless of age, may be considered for DNA analysis. Nuclear DNA from blood and semen stains more than 20 years old has been analyzed successfully using polymerase chain reaction (PCR). Samples that have been stored wet for an extended period of time should be considered for testing only using PCR and may be unsuitable for DNA analysis. Mitochondrial DNA analysis has been performed on very old bones, teeth, and hair samples.