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Who Loaded the Gun? Recovering DNA from Bullet Casings

Date Published
August 31, 2016
Jim Dawson

Several years ago an official with the Michigan State Police Forensic Science Division ordered an end to DNA processing of spent cartridge casings. The decision was not a surprise to David Foran, the director of the Michigan State University Forensic Science Program. “Because the success rate was so low — basically zero — the laboratory official asked, ‘Why are we wasting all of this time?’ ’’ Foran said. The question was appropriate. But for the wrong reasons. A likely reason so little useful DNA was being recovered from cartridge casings was that the standard technique used in crime labs are not sensitive enough. It’s wasn’t that DNA testing is a waste of time, but the procedures needed to be updated.

“The standard stuff they are doing isn’t working and isn’t going to work,” Foran said. “So our goal, the goal of the research wing of forensic science in academia, is to do the research to optimize DNA recovery from casings and let [crime laboratories] know what works best.”

Foran is one of several scientists working with a National Institute of Justice research award to improve the recovery of DNA from spent cartridge casings, which are among the most common types of evidence collected in the wake of shootings. Researchers are focusing on how best to recover DNA—swabbing, soaking, or lifting with tape—and within those methods, determining which specific techniques are most successful.

“The research is important,” said University of Denver biologist Phillip Danielson, also researching touch DNA and cartridges, “because guns are involved in a huge number of crimes. Nearly 70 percent of homicides involve handguns, and handguns are used in just under half of robberies and a little under a quarter of aggravated assaults. At least in this country, handguns are ubiquitous.” Danielson noted that most crime labs, with typically limited budgets, must be pragmatic and focus on forensics tests that are most likely to produce results. “Trying to obtain DNA from spent casings with current techniques isn’t pragmatic,” he said. But that may all be about to change.

“We are on the threshold,” Danielson said. “There are two things in forensic science that are working synergistically right now. One is the work we’re doing to improve the actual DNA profile—the data you have to analyze. The other is the progress in the type of statistical tools that are being used to do the comparisons. The statistics are becoming more powerful, and the DNA profiling methodologies are getting better. Together these two things will carry us across that threshold.”

What defines the threshold?

For Foran, the threshold is achieving a success rate that is high enough to make DNA recovery from cartridge casings worth the time and effort of forensic investigators. “Our early work was quantitative, asking how do we get more DNA from the casings,” he said. “We looked at swabbing versus soaking, with the idea that with soaking you can get all of the DNA. But soaking can create corrosion problems, and that can inhibit DNA recovery.” Shaking the cartridges while soaking might help, or it might not. “There are all sorts of variables.”

As part of her research, Rebecca Ray, a student in Foran’s forensic science program, examined the loading and firing order of the casings, caliber size, and the effect of processing the cartridges for fingerprints before extracting DNA. Ashley Mottar, another of Foran’s students, concluded in her master’s thesis on recovering touch DNA from cartridges, that “significantly higher DNA yields are recovered from spent cartridge casings using a double swab method and organic extraction.” She noted that her research “has the potential to provide a strong investigative lead by associating an individual to a shooting incident.”

Foran’s group has improved the DNA recovery success rate to about 26 percent. “That percentage,” he said, “refers to an overall average of a full DNA profile obtained from a series of samples. For some you get zero, for some you get 5 percent of what you want, and for some you get 100 percent.”

From a crime lab’s perspective, an overall average of 26 percent makes the testing worthwhile because, “that would give you a real lead,” Foran said. “The numbers from that percentage of a profile would not give you the DNA numbers that are one in hundreds of trillions (as with a full match), but they certainly would give you one in millions.” That, he said, “makes testing of cartridge casings very useful.”

Heather McKiernan, the lead research scientist at The Center for Forensic Science Research and Education, in Willow Grove, Pennsylvania, said that the overall goal in trying to recover touch DNA from spent cartridges is to “look for places in that process to mitigate opportunities for loss.” In extracting DNA, “there are a number of options available and all of those have different technologies and different opportunities for loss of samples, so you’re not going to extract 100 percent of everything you’ve recovered.”

McKiernan, working with Danielson on an NIJ grant, has examined a host of factors affecting the deposition and recovery of DNA from cartridges, including: the order in which bullets are loaded (could possibly affect DNA deposition on individual casings, but a pattern hasn’t been found); the type of metal a casing is made from (copper may degrade DNA); the chemical elements in gunshot residue (some may reduce DNA amplification); and the effect of heat on DNA when a bullet is fired (studies indicate casing surfaces do not become hot enough to destroy DNA).

McKiernan and Danielson agreed that the sensitivity of DNA testing has improved steadily over the last decade, bringing cartridge casing testing to the threshold of usefulness in criminal investigations. “In the early 2000s, you needed 50 or 60 cells to get a reasonable profile,” Danielson said. “Nowadays, with the high-sensitivity kits, you can get a good DNA profile off of five or 10 cells.”

As the success rate increases, Danielson said, the difficulty becomes not the DNA testing itself, but convincing crime labs to conduct the tests. “Labs have learned over the years that testing bullet cartridges just doesn’t succeed with any appreciable frequency,” he said. “Even though analyzing touch-type DNA has improved over the years with the more sensitive [test] kits,” he said, “at crime labs, old habits die hard.”

Those “old habits” are tied to the long-standing difficulties in analyzing and interpreting low-level partial DNA profiles, which are what prompted the decision in the Michigan State Police forensic lab to stop processing DNA from cartridges. Low-level partial profiles can be time consuming to interpret and the conclusions problematic to report.

Foran, Danielson, McKiernan, and other scientists are now turning to the use of Next Generation Sequencing (NGS) to increase sensitivity and gather more detailed information from DNA. “If you look at the mechanism by which NGS works, it is very clear that there is only one direction that technology is going, and that is toward greater and greater sensitivity,” Danielson said. “With NGS, some instruments can take a single molecule of DNA and sequence that molecule. The only limitation right now is that the accuracy leaves a bit to be desired, but again, we are moving in one direction, toward greater accuracy.”

Foran’s MSU group hopes to overcome the limited amount of DNA typically recovered from cartridge casings by looking at SNPs, or single nucleotide polymorphisms. “The biggest problem,” Foran said, “is that DNA is really degraded (on casings) for two reasons. One is that it is from dead cells, for the most part, and the other is from the heat and other mechanical properties of firing a bullet. We want to analyze the recovered DNA now by looking at SNPs to see if we can get down to really small pieces of DNA, because it is so fragmented, and analyze those pieces. Our goal is to do that via Next Generation Sequencing.”

Danielson believes he and McKiernan will be able to provide crime lab experts with more successful approaches to analyzing cartridges within two or three years. “I would expect that private labs will be the first to offer this to their clients,” he said, “and once people see that you can get results with a reasonable amount of success, say 75 to 80 percent on cartridges, then the higher throughput state labs will not be far behind.”

About the Author

Jim Dawson is a science writer with Palladian Partners, a federal contractor, on assignment at the National Institute of Justice, U.S. Department of Justice.

Date Published: August 31, 2016