Most Minnesota residents know that forensic scientists link suspects with crimes by comparing the DNA in their blood to DNA taken from the samples gathered at the scene. The science behind DNA analysis is extremely strong, but this kind of evidence is far from infallible. Tissue samples collected at crime scenes change hands several times before they are tested, and the technicians who collect them may be the only people taking precautions to prevent contamination.
Chain of custody
Prosecutors who introduce DNA evidence must provide a list of the people who came into contact with the tissue sample. This is known as a chain of custody. When questions about the reliability of DNA testing arise, these people may be questioned to find out how the sample was packaged and transported. This is important because tissue samples that are not packaged in sealed bags can become contaminated, and samples exposed to the elements can be damaged by heat or cold.
Challenging DNA evidence
Experienced criminal defense attorneys may study the chain of custody forms carefully for unexplained gaps or unnecessary delays that could have left tissue samples vulnerable to contamination or degradation. They may also question scientists in court to establish how much they know about the way samples are gathered, stored, and transported. These questions are important because forensic scientists test tiny samples and even a minor transportation lapse could cause a mistake.
When DNA evidence seems to be conclusive and crime scene investigators and forensic scientists appear to have followed proper protocols, an experienced criminal defense attorney would likely suggest pursuing a negotiated agreement. This evidence is not infallible, but it is extremely convincing if it is backed up by solid police work. However, even prosecutors with strong evidence may be willing to make concessions in return for a quick resolution.