About nine times a day over two decades, a gun used in a crime has been traced back to its original owner: a law enforcement agency. 

A joint investigation by CBS News, The Trace, and Reveal from the Center for Investigative Reporting has found at least 52,000 such incidents and identified more than 140 police agencies that sell or trade in their guns, allowing dealers to then resell them. 

Here's a look at the key findings of the investigation. You can read and watch the complete investigation here.

Resold or traded-in police guns are ending up in the hands of criminals 

Law enforcement agencies are selling and trading their old duty weapons — often to cut costs when upgrading. A side effect: tens of thousands of those guns have wound up in the hands of criminals. 

They've been used in shootings, domestic violence incidents, and other violent crimes, according to records obtained from the Bureau of Alcohol, Tobacco, Firearms and Explosives and hundreds of U.S. police agencies. 

Internal ATF records show at least 52,529 police guns turned up at crime scenes since 2006, the earliest year data is available from the government. 

CBS News journalists surveyed state and local law enforcement agencies coast to coast and found at least 145 agencies resold their guns between 2006 and 2022. That's about 90% of the agencies that responded. 

Police sell their guns even while holding buyback events to get other guns off the street 

Many of the police agencies that resold or traded in their weapons were the same ones who routinely hold gun buyback events they say are aimed at reducing the number of guns on the street. 

The Philadelphia Police Department boasts on its website of having collected 825 guns in buybacks since 2021. 

But records obtained in the CBS News investigation show the agency resold at least 886 of its officers' former duty guns over the past two decades. 

The Newark Police Department staged a buyback in 2021, offering $250 for each firearm. People turned in 146 guns. 

"Without question, 146 fewer firearms on our streets means less gun violence, fewer gun violence victims, and less risk of suicide or death," public safety director Brian O'Hara said in a YouTube post. 

Five years earlier, the Newark agency resold more than five times that number of guns — nearly 1,000. One ended up in Pittsburgh, where police seized it from a convicted felon in 2019 after he allegedly fired more than a dozen shots in a neighborhood and then led officers on a foot chase. 

A Newark Police spokesperson said the guns had been traded in as a cost-saving measure under a previous administration.  

The data behind this investigation is data Congress voted to keep secret 

In 2003, Congress passed the Tiahrt Amendment. Named after the lawmaker who introduced it, Tiahrt bars the ATF from letting the public see most trace information about guns used in crimes. 

ATF cited the Tiahrt amendment in rejecting a public records request filed in 2017 by our reporting partners on this project, Reveal from the Center for Investigative Reporting. 

Reveal sued. In 2020, the Ninth Circuit Court of Appeals ruled ATF had to release some of the summary statistical information. 

The limited records released during that litigation showed more than 52,000 guns used in crimes had been traced back to law enforcement agencies. A small sample of the underlying data showed at least 800 different agencies' old firearms ended up at crime scenes. 

Some police agencies go a different way and destroy their old guns 

Federal law enforcement agencies are legally required to destroy their used guns. State and local agencies make their own decisions. 

Most sell or trade them in — but not all. 

In Seattle, police stopped trading in handguns around 2016. 

"If we're selling them out, we just don't know where those guns could end up," Police Chief Adrian Diaz said. "We don't want to contribute to the problem." 

Indianapolis Police Chief Christopher Bailey told CBS News that his agency has historically traded in its weapons, but he would consider changing that policy after a recent shooting death involving a former police duty gun sold by a sheriff's office in California. 

"I don't want any weapon that we owned to end up being used violently against another person," Bailey said. 

After CBS News Minnesota showed Minneapolis police officials our findings, Police Chief Brian O'Hara said his department would change its policy. 

"I don't want us to be in a position where a weapon that was once in service for the police department here is then winding up used in a crime, or in an act of violence against a person, or even to shoot a police officer," O'Hara said. "So going forward, we're not going to be selling any weapons at all."

You can read and watch the complete investigation here.

In:
• Guns

John Kelly is an investigative journalist and the Vice President of Data Journalism for CBS News and Stations.