Why and when is DNA evidence examined?
DNA evidence is especially valuable for investigating violent crimes such as homicides or sexual assaults because blood, semen or saliva may be left behind by the perpetrator or victim. If the blood found in a suspect’s car contains the victim’s DNA, this is a powerful piece of physical evidence possibly linking the victim to that vehicle. If a perpetrator leaves behind a mask, cigarette butt or empty soda can at the scene, samples of sweat, skin cells or saliva can be collected and the resulting DNA profile compared to samples from the parties in question.
Biological evidence may also be discovered and collected in less violent crime scenes such as vehicle break-ins, but because laboratory resources are limited, the analysis and comparison of DNA evidence is typically conducted in the following types of cases:
- sexual assaults
- missing and unidentified persons
DNA analysis has also been used to help exonerate those convicted of crimes they did not commit. These post-conviction investigations often hinge on DNA evidence as the key information that confirms the innocence of the individual or the guilt of someone else. Many such cases have been successful because the use of DNA analysis was either non-existent or rudimentary at the time of the conviction.
The widespread awareness of the power of DNA analysis and the influence of courtroom television dramas has increased the number of jurors who expect to see DNA evidence in every criminal trial. According to a 2008 study, 22% of jurors expected DNA evidence to be presented in every criminal case [Shelton, NIJ Journal]. In fact, when DNA analysis is not presented, trial lawyers often must present the reasons why it is not presented or why it was not collected or tested. For example, if a family member assaults another family member within their home, DNA of both parties will be found all over the home because they both live there, so DNA may not be helpful in determining guilt as some other form of evidence.
Developing Leads Using DNA
If a case has no suspects to compare the DNA evidence to, the profile of DNA collected at the scene can be entered into the FBI’s Combined DNA Index System (CODIS) so that it can be compared to existing DNA records at the local, state or national level. By doing this, investigators may find a positive match to someone whose DNA profile is in CODIS and thereby identify a person of interest. Investigators can also search other countries’ databases through INTERPOL, an international police organization. This could be beneficial if the investigator has reason to believe the perpetrator was from another country.