On Friday, the Department of Alcohol, Tobacco, Firearms and Explosives (ATF) decided to attempt to piece together approximately 125 unsolved cases. They did this by firing roughly 150 firearms that were confiscated during narcotics investigations in Los Angeles over the last two years. The firearms included a combination of handguns, shotguns and long rifles.

After firing the guns, ATF agents collected shell casings. They will analyze the casings and upload the information into a national ATF database. This data could provide investigators and detectives with key pieces of evidence involved in shootings around the country.

“Each firearm leaves a very distinct marking on the back of a shell casing, it’s like a fingerprint,” Chris Bombardiere, who supervises the ATF Crime Gun Intelligence Center in Los Angeles, told the Los Angeles Times. “Once they start going in we have a 24- to 48-hour turnaround to where we start linking these firearms to other crimes that occurred where shell casings were picked up.”

According to the ATF, 76,534 shell casings from crime scenes and 130,004 shell casings from test-fires of retrieved guns took place during 2015. Of those, 7,866 were direct matches.

The Los Angeles Interagency Metropolitan Police Apprehension Crime Task Force is the agency responsible for seizing and test-firing this set of guns. According to the Task Force’s Supervisor, Brian Rose,  criminals don’t hold onto a gun for long periods of time, which means there’s a higher likelihood that confiscated firearms were previously used in a crime.