The now-defunct Fast and Furious program began in 2009 and was run by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). Agents allowed low-level weapons purchasers to cross the Mexican border in an attempt to expose trafficking routes used by Mexican cartels. But the agency ultimately lost track of some weapons, including two found at the scene of the killing of a border patrol agent in 2010.

The ATF acknowledged that weapons from the operation would likely continue to turn up at crime scenes.

“ATF has accepted responsibility for the mistakes made in the Fast and Furious investigation and at the attorney general's direction we have taken appropriate and decisive action to ensure that these errors will not be repeated,” the bureau said in a statement to CNN.