In the battle to crack tough homicide cases, some U.S. police departments are using a promising new tool: soaking the shell casings left behind after a shooting and testing them for DNA samples. The Trace and Wired report Wednesday that the previous method for pulling DNA from shell casings—swabbing them with a piece of cotton—had only a 1 percent success rate. But with a method developed in 2011 in the Netherlands, which involves soaking the casings for half an hour in a “cocktail of chemicals,” police departments have been able to pull “interpretable samples” from up to 34 percent of casings. It’s not perfect—the tests will likely add to the immense evidence backlog stymieing many departments, and it could interfere with other types of analysis. But the San Diego Police Department, which adopted the practice in 2014, was able to use recovered DNA to solve a homicide case—and the practice is now spreading to some departments across the country.