The National Institute of Justice (NIJ), in its guidelines National Best Practices for Implementing and Sustaining a Cold Case Investigation Unit, defines a cold case as a case, “…such as a violent crime, missing person, or unidentified person, that has remained unsolved for at least three years and has the potential to be solved through newly acquired information or advanced technologies to analyze evidence.”
Written by: Ken Doyle, Promega
However, there is no standardized definition of a cold case, and it varies from agency to agency. The NIJ report stated that the country was faced with a cold case crisis. While the document made it clear that determining the number of US cold cases was in itself a challenge, it cited a study that showed conservative estimates tallying over 240,000 unresolved homicides in 1980–2016, with approximately 6,000 cases added to this accumulation of unresolved cases annually. (Stein et al., 2017) The NIJ publication laid out guidelines for cold case investigation units, including determining the scope of the problem, as well as designing, implementing, operating, and supporting a cold case unit.
Although there are several factors affecting the ability of an investigative agency to process cold cases—such as budgetary and management constraints, legal restrictions and availability of suitable evidence—it’s clear that improvements in DNA analysis technology hold the biggest promise for providing a path forward. The NIJ Solving Cold Cases with DNA program, administered between 2005 and 2014, provided funding assistance to state and local law enforcement agencies across the nation to resolve cold cases. During the funding period, its efforts supported DNA analysis in 14,371 cases out of 34,289 where biological evidence was found. Although the report acknowledges the difficulties facing cold case units, it concludes that, “At the very least, bringing a cold case up to today’s technological and investigative standards gives the case a better chance of being resolved.”
Lisa Mertz, at the Office of Chief Medical Examiner (OCME), New York City presented a talk at ISHI 30 titled, “What Was Once Cold is Now Hot: Solving Cold Cases at the New York City OCME”. Her presentation cited Forensic Genetic Genealogy as holding the most promise for helping solve cold cases. Mertz expresses her excitement at the interim DOJ policy. “[FGG] still has a way to go to be fully accepted by the public,” she says. “However, the interim policy can only help in its acceptance.”
Rachel Oefelein, a quality assurance manager at DNA Labs International, agrees. “First and foremost, it provides an initial framework for guidelines that law enforcement and DNA laboratories can look to when considering forensic genealogy as an option.” She hopes that the policy will also guide the allocation of federal funding. “Federal funding could aid both smaller and larger jurisdictions alike in obtaining the resources necessary to access this groundbreaking technology,” Oefelein says.
Mixture Analysis
FGG is far from the only tool at a forensic investigator’s disposal, however. One of the biggest challenges to the analysis of crime scene samples is that they often contain a mixture of DNA from multiple contributors. Resolving the DNA profiles, or “mixture deconvolution” to obtain meaningful data can pose a problem for many forensic laboratories.
In one example, the New York OCME solved a cold case from 1981 where the victim was found strangled to death in the Bronx. In 2009, a fingerprint taken from evidence at the crime scene resulted in a match to a suspect. The only other physical evidence consisted of fingernail clippings. DNA obtained from the fingernails revealed a mixture comprised of the victim and a male donor, who was later confirmed to be the same man implicated by the fingerprint match. Another cold case, involving sexual assault and murder in 1999, was reopened in 2011 with additional testing of vaginal swabs that revealed the presence of a male donor in the mixture. In this case as well, the male DNA profile was later confirmed by analysis of exogenous DNA from the victim’s fingernails.
“The DNA mixtures in these cases were analyzed using mixture interpretation guidelines developed and validated at the OCME,” Mertz says. “With the use of these guidelines, we were able to develop a male DNA profile that was suitable for entry into the CODIS database.”
STRmix™ is a popular tool for analysis of DNA mixtures and is widely used at DNA Labs to revisit cold cases. The software was developed by the Institute of Environmental Science and Research and Forensic Science South Australia. In a nutshell, STRmix™ combines biological modeling with advanced statistical analysis to build likely DNA profiles from a mixture sample. It then grades each profile based on the likelihood of matching the evidence, through a process known as probabilistic genotyping.
“It’s magic!” Oefelein says, when asked how probabilistic genotyping works. She explains that the method “allows today’s forensic scientists to examine profiles that would have previously been deemed inconclusive, assess a profile as a whole, and develop profiles of persons of interest from mixed DNA samples that can then be searched against a database.”
Still, the analysis can be daunting to investigators who are unfamiliar with it.
“With anything new, you have to wrap your head around the concept,” Oefelein says, “and that is not always instantaneous.” She recommends both individual and group training when adopting the software. “There are numerous transcripts available online from STRmix™ admissibility hearings and trials,” she adds, which are valuable not only for understanding the method but for forensic scientists who may be called upon to explain the method in court. Oefelein believes that the method will only grow in popularity, as more and more evidence samples rely on mixture interpretation. “Probabilistic genotyping has raised the bar for how forensic analysts assess DNA profiles,” she says.
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