A new method of DNA testing could solve more shootings

Criminologists thought it was impossible to get DNA off of shell casings, but a technique pioneered in the Netherlands is having notable results


This story was published in partnership with Wired.

By Ann Givens, The Trace

Police found 19 spent shell casings scattered in the San Diego street where Gregory Benton was murdered on April 12, 2014. Benton and his cousin had gone to buy cigarettes, a witness later said. As they returned to a family party, two men pulled up in a car behind them. They got out, and at least one of them opened fire.

For San Diego, the new method has been worth implementing. (Photo/Pixabay)
For San Diego, the new method has been worth implementing. (Photo/Pixabay)

Witnesses didn’t get a good look at the men or the car, so when police sat down to review their leads, the shell casings were the best evidence they had. They sent the casings to the San Diego Police Crime Lab, which just happened to be trying out a new DNA testing technique.

Previously, to remove DNA from casings, the lab would moisten a cotton swab and rub it over the metal. But their success rate was less than 1%. This was proving to be a problem for many cities across the country struggling to solve shootings and homicides. Police often find that shell casings they collect from a crime scene are their most valuable evidence. Ballistic testing can offer clues about what kind of gun was used and, sometimes, whether that same gun was used in another crime. But the casings seldom yielded fruitful DNA results, and the San Diego Police Department, like many others, had stopped testing them.   

Until 2014. That’s when DNA scientist Shawn Monpetit of the SDPD began researching the subject and came across a 2011 study in which Dutch scientists recovered DNA from about a quarter of the casings tested using a new method. This new technique required scientists to soak the casings for about half an hour in tubes filled with a cocktail of chemicals that break open cells and release DNA so it can then be isolated and tested. “Think of it like soaking your dishes,” said Kristin Beyers, one of the lab’s supervising criminalists.

In a rare move, the SDPD agreed to fund its own study in 2014. Ten cops and lab workers were enlisted to use ammunition the way a criminal might: They carried some around in their pockets and took some straight out of a package before loading it into a gun and firing. When the scientists ultimately tested the roughly 800 casings they collected, swabbing half using the traditional method and soaking the other half, the lab got “interpretable” DNA samples off about 34% of the soaked samples. They published their study in a peer-reviewed academic journal, Forensic Science International, and the SDPD began using it in 2014 – around the same time they tested the evidence in the Gregory Benton murder case.

The scientists soaked the 19 casings from the Benton case. They retrieved testable DNA from two different people, which they matched with samples in local and state DNA databases. Days later, they brought the two men in for questioning and put them together in a holding cell, where they were recorded.

“Hey homie…my DNA just came back on two of those shell casings,” said one of the men, Emanual Peavy, according to a legal decision in the case. The other man, Lamont Holman, cursed, declaring that they had “no doubt” messed up, the decision said. The two men were later convicted of their roles in the killing.

In the three years since the study, police have sent the lab more than 1,000 casings to test. The lab is now getting usable DNA off about 30% of the casings it analyzes. About 11% of those link to a sample they have in their system – either a suspect or evidence in another case. That would average out to about 30 casings a year that link to a known person or evidence in another case, though lab officials don’t keep a precise count.

These statistics have caught the attention of forensic scientists from all over the country, many of whom have visited the San Diego lab to learn more about the technique. Police departments in San Francisco and Miami say they are researching the method. Jeffrey A. Thompson, commanding officer of the Los Angeles Police Department’s Forensic Science Division, said his lab has already evaluated the soaking method and plans to start using it in the coming months.

“On some shooting crimes, the bullets and casings are the only physical evidence left behind by the suspect,” Thompson said in an email. “A DNA profile match can provide a valuable lead.”

That’s particularly true as more police departments are using software like ShotSpotter that notifies them immediately of the exact location where shots are fired. Prompt notification means cops arrive to a crime scene quickly, before the casings get moved or tampered with. Those casings can hold important clues, especially when retaliation is likely and witnesses go silent, police said.

Still, there are logistical challenges to consider. Many labs submit shell casings to the National Integrated Ballistics Imaging Network, or NIBIN, a database that can connect a shell casing with others that were shot from the same gun. The faster that investigators submit these casings to NIBIN, the faster they can get leads to help solve cases and get shooters off the street.

But many labs are already struggling to enter casings into NIBIN in a timely fashion. In a story last year, The Trace and NBC Bay Area reported that nine out of 10 California labs questioned took at least 20 days to enter shell casings into NIBIN, and most took more than three months. That’s if they get them in at all. The vast majority of casings – about 75% – are never entered into NIBIN.

Testing the casings for DNA would likely delay getting a NIBIN lead. In San Diego, where police now ask for DNA testing on the majority of casings collected in homicides, it takes the lab about eight days from when police pick up a casing off the street to the time they get it into NIBIN. That’s about two days longer than it would otherwise take them.

“The DNA portion does slow down the NIBIN entries,” said San Diego lab director Jennifer Shen in an email. “Although we certainly think it is worth it, as do our customers.”   

But some fear that evidence could grow stale, especially if a lab has a DNA testing backlog. “You shouldn’t spend a month doing that DNA test at the expense of putting cartridges into NIBIN immediately,” says Pete Gagliardi, a former official with the Bureau of Alcohol, Tobacco, Firearms, and Explosives who now works as a private consultant helping police use forensic technology.

Gagliardi suggests one strategy to mitigate this issue: Labs could fast-track DNA analysis in cases where the shell casings are key evidence but decline to do it in other cases, opting instead to get a NIBIN lead back quickly.  

Joseph P. Murphy, commander of the Chicago Police Department’s Forensic Services Division, says that with the number of shootings his department investigates, it’s hard to imagine implementing this technique anytime soon. “Each fired cartridge case would need to be soaked in its own solution; on some cases we have over 50 fired cartridges,” he says. Although shootings and homicides decreased in Chicago last year, about 2,900 people were shot and 570 killed in 2018, according to Chicago police.

Murphy says the Illinois State Police Lab, which processes DNA for the Chicago police, has a current backlog of thousands of cases, which experts say is a common problem. Mechthild Prinz, who heads the master’s degree program in forensic science at the City University of New York’s John Jay College of Criminal Justice, says tests for “touch” DNA have become so popular because of advances in the field that many labs are overwhelmed by requests to do them. (Police are frequently recovering touch DNA – genetic material that can be transferred in just a few skin cells when someone touches an object – from clothing, guns, doorknobs, and other evidence found at crime scenes. It’s harder to get it off shell casings because of their small surface area and their brief contact with anyone’s skin, scientists said.)  

“The amount of manpower it would take to make this potentially run would consist of an entire team of forensic scientists,” Murphy says. With money tight and staffing short at many police crime labs, experts say it can be complicated to start using a new technology – even a promising one. However, the San Diego lab says it hasn’t needed to add staff, and the new equipment it required, mostly larger beakers, cost only a few thousand dollars.

For San Diego, the new method has been worth implementing. “We didn’t want to shy away from the challenge of doing something new,” says lab supervisor Beyers. And the shell casings collected at Gregory Benton’s murder scene? The police ran them through NIBIN and found that one of the guns that killed Benton had been used in an attempted murder two days earlier – a shooting over gang territory. Without DNA pulled from the casings, investigators say both those crimes might have gone unsolved. “It was a spider web of information,” says Monpetit, the DNA scientist. “They suspected it was someone from the rival gang, but who it was inside that gang … I’m not sure they would ever have been able to figure that out or prove without the DNA.”

This report was originally published by The Trace, an independent, nonpartisan, nonprofit newsroom covering firearms policy, injury and death in America. 


About the author
Ann Givens writes about the nation’s law enforcement agencies, from the ATF to local police departments, and the effectiveness of their efforts to curb gun violence.

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