HOUSTON -- A college student's checked luggage on a Continental Airlines flight from Argentina contained traces of dynamite, authorities said, in one of six security incidents Friday involving U.S. flights. Federal authorities were investigating why the student, who got off the Continental plane in Houston before it continued to Newark, N.J., had the explosive residue, FBI spokeswoman Shauna Dunlap said. Houston Fire Department Assistant Chief Omero Longoria said the man told authorities he works in mining and often handles explosives. Longoria said federal officials were investigating whether the explanation was true. In other incidents: _A utility knife was found on...