The controversial tactic of "letting guns walk" out of gun shops in the hands of suspected straw purchasers was used in Operation Fast and Furious at ATF in Phoenix in an effort to track the guns to major weapons traffickers and drug cartels in order to make criminal cases against smuggling kingpins who had eluded prosecution for years. But the tracking of the weapons was faulty, and many of them wound up at crime scenes in Mexico and the U.S. Two of the guns spotted at one point during Fast and Furious were later discovered at the scene of the killing of U.S. border agent Brian Terry.