WASHINGTON - Otilio Osorio was just 22 in October 2010 when he purchased a Romanian-made Draco AK-47 pistol in Joshua, just outside Fort Worth. There was nothing remarkable about the sale until the gun, with its serial number obliterated, was identified as one of three weapons used to kill Immigration and Customs Enforcement Agent Jaime Zapata on a Mexico highway four months later. Documents obtained by the Houston Chronicle show that at different points in 2010, two Bureau of Alcohol, Tobacco and Firearms divisions - Dallas and Phoenix - had evidence implicating Osorio well before drug gangsters gunned down Zapata...