Fourteen FBI agents arrived inHaitithis morning and rushed to a Port-au-Prince boulevard where an outspoken political opponent of President Jean Bertrand Aristide was machine-gunned to death yesterday. The reason for the haste: President Clinton is due to arrive Friday to declare Haiti safe enough for U.S. troops to hand over security duties to a multinational U.N. force. Aristide asked the FBI to investigate the assassination of ultranationalist Mireille Durocher Bertin. A Haitian government source today told TIME contributor Bernard Diederich that Aristide had advance warning of the killing, and offered Bertin protection. Haitian officials insist that the opposition killed one of its own in order to embarrass Aristide and Clinton.