TWO THIVES ARRESTED WHILE STEALING

Many of those guns were bought legally in the U.S. and then smuggled over the border, according to the GAO.

"About half were long guns, such as the high-caliber AR-15, preferred by cartel gunmen," NPR's John Burnett reports for our Newscast unit. "Mexican drug traffickers continue to rely on straw purchasers who legally buy the weapons in the U.S., then transfer them to criminal gangs."

The GAO report paints a picture of the challenges officials face as they try to stop the flow of weapons from the U.S. into Mexico, where laws seek to strictly limit the availability of guns to the public.

The report identifies a new wrinkle in the illegal gun trade, citing officials on both sides of the border who say they're seeing instances of unfinished gun parts being transported into Mexico, where they're used to assemble finished weapons. None of those unfinished parts — barrels and receivers; pins and triggers; grips and stocks — are covered by the U.S. Gun Control Act; they're also not subject to rules that require serial numbers, the GAO says.

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