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Using DNA to Solve Cold Cases

Modern forensics can aid your investigation



Suzanna Ryan | Friday, July 11, 2008

Nearly every police department in the country, whether large or small, has a stockpile of cases that have remained unsolved for a number of years and have eventually gone cold. Some departments are fortunate enough to have their own cold case units with several detectives assigned to work solely on solving the department's cold cases. If you are an investigator working on cold cases, it just might turn out that DNA analysis may be your best chance for finally solving the case. While DNA evidence is just one of the many types of physical evidence that is typically encountered at a crime scene, it can also be one of the most useful due to its ability to discriminate between and potentially even identify individuals and due to the sheer variety of evidence items that may contain DNA evidence.

Is your cold case evidence being used to its fullest potential?

When DNA analysis first came onto the scene in the late 1980s, the testing methods used required large amounts of pristine evidence sample in order to obtain results. The Short Tandem Repeats (STR) DNA testing used today can yield usable results even when the evidence items being tested are very old and/or partially degraded. This could mean that the stain that was identified 20 years ago as being human blood, but was determined to be too small to even attempt DNA testing, could be a perfect candidate for DNA testing today. Therefore, even if the biological evidence in your cold case file has previously been tested by a forensic laboratory--it may well be worth testing it again. This is especially true if older testing methods were employed. Prior to DNA analysis the typical testing methods employed by forensic laboratories involved conventional serological techniques--think ABO blood typing and blood protein typing (anybody out there remember secretor status testing?). Even though less information was ultimately obtained by these tests, the blood and secretor tests often required larger amounts of material than today's DNA analysis methods!

In addition, you should be aware that even if DNA testing was previously used, the results obtained might not be compatible with the Combined DNA Index System (CODIS) and therefore should be re-analyzed using STR typing. A quick phone call to your local laboratory should be able to clear up whether a DNA profile has been uploaded into CODIS or not. If your report lists results in the form of RFLP (Restriction Fragment Length Polymorphism) or HLA-DQalpha/PolyMarker types, then there is a very good chance that that DNA profile is not in CODIS and therefore is not being searched routinely against the greater than 4 million DNA profiles that are in the CODIS system today.

Evidence: collected but never examined

What else has changed in forensic DNA labs over the last 10 to 20 years? More than you may think! Many types of samples that are routinely tested in the modern forensic DNA laboratory would not have even been considered a decade or two ago. However, the good news is that these types of samples may have still been collected. For example, it is, and has been, routine for medical examiners to collect fingernail clippings from victims. In my personal experience, I have worked many cases where the fingernail clippings led us directly to the suspect. It is not at all uncommon for a victim to try to fight back and in doing so, scratch their assailant--especially in cases of sexual assault and strangulation. Even if the perpetrator was smart enough to not leave blood or semen behind, they may have unknowingly left a little bit of themselves behind in the form of skin cells underneath their victim's fingernails.

One word of caution regarding fingernail clippings, especially from cold cases: the contamination quality control procedures that are in place at the medical examiner's office today may not have been followed at the time the crime occurred. How could this affect your case? As an example, a former colleague of mine was working on a cold case and was able to get a male DNA profile from the female victim's fingernail clippings. The male profile was uploaded into CODIS and within days she had the name of the male who left his DNA under the victim's nails. The only problem? This particular male individual was deceased--and had been since 2 days prior to the victims own death! It was later determined the technicians at the medical examiner's office had used the same pair of fingernail clippers on both individuals, thus leading to the mixture of DNA. This example serves to highlight the care that is needed to avoid cross-contamination of samples. This is especially true when dealing with decades-old cold cases. After all, who would have thought 20 years ago that one would need to be concerned with simply touching an item of evidence and leaving their DNA behind?

Hair is another example of an evidence item that may have been collected but never further examined. Even if you have a report stating that hair comparisons were performed, there may be additional information that can be gleaned from the hairs. Speak with the forensic laboratory personnel and ask them to microscopically examine the hairs for the presence of a usable root. It may be beneficial to provide the lab with a copy of the hair examiner's report so that they can concentrate on those hairs most likely to have come from someone other than the victim. It could turn out that all the DNA evidence you need to close your case and put the perpetrator behind bars may be encased on a single microscope slide.

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