Federal investigators are piecing together details of an audacious new trend in drug smuggling: South American traffickers are buying old jets, stuffing them full of cocaine and flying them across the Atlantic to feed Europe's growing coke habit. "The sky's the limit," one Sierra Leone trafficker boasted to a Drug Enforcement Administration informant. In some ways it is a throwback to the 1970s and '80s, when pilots flew drugs freely between Colombia and staging areas near the U.S. border.