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.

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.

THE TRACE ABOUT This story was published in partnership with The Trace, a nonprofit news organization covering guns in America.

Previously, to get DNA off of shell casings, the lab would moisten a cotton swab and rub it over the metal. But their success rate was less than 1 percent. 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 doing that kind of test.

Until 2014. That’s when DNA scientist Shawn Monpetit of the San Diego Police began researching the subject and found a 2011 Netherlands study in which scientists recovered DNA from about a quarter of the casings they 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 San Diego Police Department 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 percent of the soaked samples. They published their study in a peer-reviewed academic journal, Forensic Science International, and the San Diego Police Department 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 of about 30 percent of the casings it analyzes. About 11 percent 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.