A reputed Mexican drug cartel leader was charged in the ambush slaying this year of a U.S. immigration officer in Mexico — a killing that set off a massive search for the assailants on both sides of the border.Julian Zapata Espinoza, an alleged chief with the Zetas cartel, pleaded not guilty in a brief court appearance Wednesday in the killing of Immigration and Customs Enforcement Special Agent Jaime Zapata on Feb. 15. He and another agent, who was wounded, were ambushed in their car by a convoy of vehicles in the Mexican state of San Luis Potosi. . .According to an internal report by the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, three firearms with obliterated serial numbers were recovered by Mexican authorities after the shooting. The serial numbers were raised by U.S. agents, and the firearms were subsequently traced.One of the weapons was "traced directly" to Otilio Osorio Jr. of Lancaster, Texas. Ballistics testing further confirmed the weapon "was used in the murder" of Zapata, the ATF report said. It had been illegally purchased at a gun store in the Dallas-Fort Worth area, and Osorio, 22, pleaded guilty in October to three counts of weapons violations.Lawmakers in Washington who are investigating the ATF's bungled Fast and Furious operation, which attempted to track illegally purchased guns on both sides of the U.S.-Mexico border, have contended that Osorio was known to the ATF as an illegal purchaser. But, they said, the ATF failed for three months to file reports on his activities, and did not do so until Feb. 25, the same day the ATF traced the Zapata murder weapon back to Osorio.