Inside the vehicle was a cache of assault weapons, including AK-47s, Ruger .45-caliber handguns and pistols called "cop killers" because their ammunition can penetrate armor.

federal border patrol agents stopped two men driving a car through the border-crossing town of Columbus, New Mexico. Inside the vehicle was a cache of assault weapons, including AK-47s, Ruger .45-caliber handguns and pistols called "cop killers" because their ammunition can penetrate armor.

The U.S. Customs and Border Protection officers ran the guns' serial numbers in a nationwide database and waited. None of the eight came back flagged as stolen or suspect, so the agents let the men go — just a few short miles from the Mexican border, where gun trafficking is fueling a violent and deadly drug war.

At the time, the border guards were unaware that six of the weapons had been purchased by alleged straw buyers in a federal sting and were supposed to be monitored by Bureau of Alcohol, Tobacco, Firearms and Explosives agents trying to bust a major Mexican gun running ring.

The ATF had not yet flagged the weapons in a law enforcement database, and CBP wouldn't alert its sister law enforcement agency to the traffic stop for five months, delays that would prove fateful for both agencies.

The two men in the car turned out to be Blas Gutierrez and Miguel Carrillo, who earlier this month were indicted as part of a Mexican cartel gun trafficking operation that also involved Columbus' mayor and police chief, court records show.

And one of the Ruger pistols from the vehicle turned up at a murder scene directly across the border in Puerto Palomas, Mexico