When the Stanislaus County Sheriff’s Department in California wanted to purchase new firearms, it sold its used ones to help cover the cost. The old guns went to a distributor, which then turned around and sold them to the public. One of those guns—a Glock pistol—found its way to Indianapolis.

That Glock was involved in the killing of Maria Leslie’s grandson, and the fact that it once belonged to law enforcement makes her loss sting even more.

“My grandson was in his own apartment complex. He lived there,” Leslie said. “He should not have been murdered there, especially with a gun that traces back all the way to the California police department’s coffers.”

Across the nation, it’s common practice for police departments to trade in their old weapons rather than destroy them. Tens of thousands of old cop guns are ending up in the hands of criminals. This week, in a collaboration with The Trace and CBS News, reporter Alain Stephens traces the journey of some of those guns from the police departments that sold them to the crime scenes where they ended up.

Then Stephens brings us reporting from The Gun Machine podcast series from WBUR and The Trace. He explores the reasons why police and other law enforcement agencies have greatly expanded their arsenals over recent decades.