BENGHAZI, Libya — Sixteen days after the death of four Americans in an attack on a United States diplomatic mission here, fears about the near-total lack of security have kept F.B.I. agents from visiting the scene of the killings and forced them to try to piece together the complicated crime from Tripoli, more than 400 miles away. Investigators are so worried about the tenuous security, people involved in the investigation say, that they have been unwilling to risk taking some potential Libyan witnesses into the American Embassy in Tripoli. Instead, the investigators have resorted to the awkward solution of questioning...