LOUISVILLE, Ky. — Federal agents say tracing firearms and comparing shell casings can help Louisville investigators connect shootings and pursue suspects, sometimes within a day or two of recovering evidence. John Nokes, special agent in charge of the Bureau of Alcohol, Tobacco, Firearms and Explosives’ Louisville Field Division, said the agency works with Louisville Metro Police Department investigators after a gun or casing is recovered. Evidence can be entered into the National Integrated Ballistic Information Network, or NIBIN, which compares markings left on cartridge cases by firearms. Nokes said results are typically returned to local investigators within 24 to 48 hours. ATF also traces a firearm’s sales history, offering a different kind of investigative lead. Nokes said guns can reach the streets through theft or illegal transfers. A firearm recently recovered in Louisville, he said, had been associated with 15 earlier shooting incidents. The work comes amid recent multi-agency operations in the area that recovered nearly 200 firearms. One crackdown resulted in 52 people being charged and 160 guns seized, according to details cited by the agencies. Nokes said ATF seized more than 1,400 guns used in crimes last year. He described the goal as using evidence from one incident to help prevent another, while noting the agency is also focused on machine-gun conversion devices that can make handguns fire automatically. He said the number of shell casings collected at crime scenes and entered into NIBIN has declined, which he views as a sign investigators are targeting people responsible for firearm violence. That measure, however, does not by itself establish how much shootings overall have fallen. ATF is also seeking to keep firearms away from teenagers and prosecute adults who provide them, Nokes said.