It seems like forensics, once again, will be the key to another high profile police investigation; this one spanning 8 years and one which was, until recently, classified as a cold case. The Chandra Levy murder hit close to home for those living in the District of Columbia when she went missing in July of 2001. Her remains were found a year later near Rock Creek Park. Recent activity in the Levy investigation has brought life back to the investigation and DC Police have identified their suspect to the crime: Ingmar Guandique.
A report from the Associated Press indicated in a recent article printed by NBC13 dated February 22, 2009, that, “a law enforcement official who has spoken to investigators said the break [in the Chandra Levy case] came in part from DNA evidence that was either retested or collected”. This short sentence is the only mention of newly discovered forensic evidence, and it leaves the reader without any real understanding of the evidence or how it could have provided the “break” police were searching for. Furthermore, this short mention of DNA evidence can be misleading to those who do not fully understand the value of the evidence, since it places the bias of guilt on the suspect, and as we all know, suspects are presumed innocent until proven guilty. To fix that, I’d like to discuss the value of DNA evidence and how DNA evidence found 8 years after the fact can still be used in an investigation.
First, let’s very briefly discuss the value of DNA evidence. Every individual has two different types of DNA. The first type is called nuclear DNA (nDNA), it is the type most often referred to when speaking of DNA. It is found in the nucleus of every cell in the body and it is unique to every individual, except identical twins. For example, nDNA can be recovered from skin, saliva, white blood cells (red blood cells do not have a nucleus), hair follicles, and other cells which contain a nucleus. The second type of DNA is called mitochondrial DNA (mtDNA). This DNA is found in the mitochondria of the cell, which are the powerhouses that create energy for cellular function. mtDNA is not unique to the individual, but rather is inherited from the mother, so everyone along a maternal lineage will have the same mtDNA. For example, mtDNA can be recovered from the hair shaft, teeth, bone, and most other cells that do and do not contain a nucleus. Since mtDNA is not unique to the individual, one would assume that it is not valuable, however, as we will soon discuss, mtDNA is very often more valuable than nDNA in a police investigation. In most cases of fresh crimes and crime scenes, nDNA will be analyzed and compared with suspects because it is more specific, easier, and cheaper to test; however, nDNA is very often not available in cases involving longer time frames. Since the cells containing nuclei degrade quickly, and DNA is unstable under many different environmental conditions, nDNA evidence does not last very long. Luckily, mtDNA has a longer lifespan because it is very often taken from samples that do not degrade as quickly, such as hairs, bones, and teeth. So, while mtDNA is not unique to an individual, it is unique to a lineage, therefore, in cases of a missing person or a large disaster, mtDNA can be used to identify a person when compared with mtDNA of a mother or sibling.
In the article quoted above, it is unclear if the DNA evidence in the Levy case is nuclear or mitochondrial, but depending on when the evidence was collected (i.e. fresh vs. old), from what sample it was collected (i.e. skin vs. hair), and what they are claiming about the results of the analysis (i.e. uniqueness vs. lineage), we now have a better understanding of the evidence that investigators are relying on. In any event, it is worth talking about the likelihood of intact DNA being used presently in the Chandra Levy case. In any police investigation where DNA is collected, the evidence is stored in freezers to preserve the evidence for potential analysis later in the investigation. Freezing DNA evidence is the best method to preserve it, as it slows down the degradation process. If, as the article states, the DNA evidence was retested, it is very possible that there was already DNA evidence from the case in storage that is now useful due to new developments. Perhaps when they ran the sample the first time, they didn’t have any suspect to compare it to, or the current suspect’s DNA was not entered in any of the databases that investigators searched in the first time around. If, however, through other investigative leads a suspect has been recently identified, a DNA comparison could now prove to be the key to the case. But, if the DNA evidence was newly collected, we are most likely looking at a situation of the use of mtDNA, since the time frame is so large. As mentioned before, mtDNA can still be used to compare with a suspect, but the conclusions are limited to a shared lineage, rather than individuality.
So the next time you read in the paper that DNA evidence may be the key to break the case open, think back to our lesson on DNA and evaluate which type of DNA is being analyzed (i.e. nDNA vs. mtDNA), what are investigators using the DNA evidence for (i.e. identification of remains vs. identification of a suspect), how likely it is that they will find intact DNA, and is the context surrounding the discovery of the DNA evidence incriminating or circumstantial. An informed reader will be able to understand the evidence and reach his/her own conclusions rather than become disillusioned by the forensic evidence and its perceived value.
For a complete description of the Chandra Levy case and investigation click here to see the Washington Post coverage.