Biological evidence can be a vital tool in criminal investigations. It can confirm or refute witness statements, and it can link an alleged victim to a crime scene. In the case of sexual assault, biological evidence is especially critical. However, biological evidence can be contaminated during the process of collecting, documenting, and testing. Therefore, investigators must be aware of the dangers of contamination in order to ensure the integrity of the evidence they collect.

Several types of biological evidence can be obtained from crime scenes, including fingerprints, hair, skin cells, blood, and mucous. Some biological evidence may be apparent by simple observation, but it is essential to conduct a thorough visual search. During the initial phase of a crime scene investigation, investigators use a variety of techniques to collect and document the evidence.

The easiest form of crime scene evidence to gather is a fingerprint. Fingerprints are easy to obtain and they are reliable. Nevertheless, fingerprints can be left behind if they are not intentionally destroyed. If DNA is detected, it can be collected from skin cells, bruises, and even blood.

Another type of biological evidence is the amplification of DNA. This is accomplished when a person’s DNA is amplified by the addition of a nucleotide. This produces several copies of the DNA, and a DNA profile can be generated. For example, an individual can be identified by his or her mtDNA. When mtDNA is found, it can be used in the analysis of unidentified remains.

Other forms of biological evidence include blood and semen. Blood and semen can be difficult to find at a crime scene because they are rarely visible with the naked eye. Nonetheless, they can be located by using a light source that is different from the original. Also, they can be captured with swabs or photographed. These items can then be checked with the DNA database.

Lastly, biological evidence can also be obtained from the laboratory. While many biological screening tests are presumptive in nature, a court order may be necessary in order to retrieve a reference from a person of interest.

Various issues have been raised about the validity of biological evidence. Biological evidence is prone to contamination during the process of collecting, documenting, storing, and testing. Human error can also compromise the reliability of the evidence. Even when the evidence is not contaminated, improper packaging can lead to the growth of bacteria.

Because biological material is easily degraded in warm, humid environments, investigators must take proper precautions to ensure the safety of the evidence. For example, they should always wear gloves when handling biological evidence. They should also avoid storing their evidence in hot areas and should use refrigerated containers to transport liquid samples.

Finally, forensic scientists have developed protocols to prevent contamination. During the initial stages of a crime scene investigation, investigators must properly document all of the biological evidence they collect. A written report and photographs should be included in the documentation. Records should also contain the name and location of the evidence, as well as measurements of the items and a description of their appearance.