Former police service weapons traced to violent crimes. Here's how it happens.

A new CBS News investigation looks into how guns once used by law enforcement agencies are later becoming evidence in violent crimes. The investigation shows Metro Police and sheriff's offices in Hickman, Sumner, and Rutherford counties all have one gun traced to a crime scene from 2013 to 2017. The Montgomery County Sheriff's Office has two guns that were traced to crimes.

The ATF says more than 52,000 guns across the country once owned by police were later used in crimes. Law enforcement resold guns to firearms dealers for discounts on new equipment and, in some cases, directly to their own officers, records show. Some guns were later involved in shootings, domestic violence incidents, and other violent crimes.

"It's a legal transaction. So, if I legally sold my old used patrol car and somebody uses that in the commission of a crime, is that our responsibility? I would say no," said Stanislaus Couty, CA Sheriff Jeff Dirkse.

Many police departments resell their weapons while holding buyback events, which they say are important to take guns off the street. Records from 67 agencies across the country showed they've collectively resold more than 87,000 firearms over the past two decades.

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