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 some witnesses in cars outside the embassy, which is operating under emergency staffing and was evacuated of even more diplomats on Thursday because of a heightened security alert.

“It’s a cavalcade of obstacles right now,” said a senior American law enforcement official who is receiving regular updates on the Benghazi investigation and who described the crime scene, which has been trampled on, looted and burned, as so badly “degraded” that even once F.B.I. agents do eventually gain access “it’ll be very difficult to see what evidence can be attributed to the bad guys.”…

The Libyan government has advised the F.B.I. that it cannot assure the safety of the American investigators in Benghazi. So agents have been conducting interviews from afar, relying on local Libyan authorities to help identify and arrange meetings with witnesses to the attack and working closely with the Libyans to gauge the veracity of any of those accounts.

“There’s a chance we never make it in there,” said a senior law enforcement official.